How Much Solar Do I Need On My RV?


Notice, that’s “need” not “want”.  There’s a difference.

It’s going to be a good day for solar power!

This post grew out of a well-received seminar I’ve been presenting at RV shows.  And that seminar itself grew out of my responses to lots of email questions from our readers that went more or less like this:

“How many solar panels do I need to run my air conditioner?”

Now, I’m not an electrical engineer (I’m aerospace), and I don’t work in the solar power industry.  More importantly – I’m not a solar power salesman.  What I’m about to walk you through is a process and some (hopefully fun) analysis that will help you properly size a solar energy system for your RV without wasting money on stuff that’s very cool but you probably don’t need.

This doesn’t mean I haven’t wasted my own money on cool stuff that I don’t need!  Just to give you an idea of where I’m coming from, here’s a quick rundown of the electrical system on our Class B Winnebago Travato, Lance:

  • Lithium Batteries: Lance currently has a 420 amp-hour Lithium Iron Phospate (LiFePO4) battery from Lithionics.  The battery self-manages, and has thus far self-maintained.  We’ve never wanted for battery capacity since installing it.
  • Solar Panels: We currently have 300 watts of solar panels on the roof.  These are run through the factory-installed combiner, and are integrated through a 40 amp solar charge controller from Zamp.  We started off with one 100 watt panel.
  • Second Alternator: The coach and chassis electrical systems on our coach are completely separate.  There’s a second alternator from Nations Alternator which powers the house system and charges the Lithium battery.
  • No Generator: We got rid of this because it was extra weight and we just didn’t need it.  Most times, even when hookups are available, we don’t bother to plug in.
  • Inverter: We have a Xantrex Freedom SW 3000 inverter charger which takes the place of our generator.  This is the second inverter we’ve had in this coach.

Along the way and while tearing the rig apart multiple times, I’ve learned a thing or two, and that’s what I’m going to share with you in this post.  So now, let’s get back to that question.  Grab yourself a coffee or something – this is going to get lengthy.

 

How Solar in Your RV Works:

“How many solar panels do I need to run my air conditioner?” implies a basic misunderstanding of how solar energy in your RV works.   For example, this is NOT how it works:

At least, that’s not how it works if you want air conditioning on a partly cloudy day.  The solar panels are actually just a part of a much larger system, and a simplified diagram of that system might look like this:

There are really two important concepts to pick up from that diagram above.  The first is that solar is just one of several potential energy sources that you’ll find on a typical RV.  In addition to solar, you might have a generator, shore power, and a vehicle alternator.  All of those will put electrical energy onto the “mini grid” that you’ve got in your RV.

The second thing to note in that diagram is that it’s all about the battery.  All of the sources putting power onto your grid can be either “on” or “off”.  For example, solar power is “off” at night.  The alternator is “off” when you’re not driving.  And so on.  The only thing that’s there for you all the time is your battery.  The battery is what stores the energy produced by the other sources so that you can have power once the sun goes down, once you’ve turned off the ignition, or during generator quiet hours.

These two concepts are important to understand, because once you start thinking of your RV’s electrical systems as a mini grid with some storage; you’ll be able to define an objective for your solar energy ambitions.  If you start making tech purchases without defining an objective first, you can drain your wallet (and  your batteries) in a hurry.  Don’t ask me how I know this.

 

Our Objective for Solar Power in the RV

In as simple terms as I can put it, this is what we’re trying to accomplish:

We want our solar energy system to recover the energy we’re using day-to-day, but no more.  You see, there’s a key difference between home solar energy systems, and RV-based ones.  In an RV, you can’t sell your excess energy production back to the power company.  All you can do is store it in your battery.

But once your battery is full, it’s full!  You can’t fill your battery over 100%.  So if you build your RV solar energy system bigger than you need, you’ll wind up with a lot of wasted potential.  (That’s actually a nerdy joke, because the Volt is the SI unit of electrical potential.  Give yourself 50 bonus nerd points if you got it!)

Besides wasted potential, you’ll also have some wasted money from building your RV solar power plant too large.  And I don’t know of too many people who like wasting money.  So now that we know we want to “right size” our solar power investment, we need to figure out how to balance that equation.  We’ll start with the left side of the equation: charging sources.

 

Charging Sources in Your RV

If you remember from the schematic, there are several potential charging sources in your RV.  Yours may or may not have all of these:

Alternator: If you have a motorhome, or a towable RV with a beefy umbilical connection, you can count on driving or running the engine to provide you with power.  The amount of power you’ll get from this varies according to the size of your alternator and what other electrical loads you’re running.  On the high end, a dedicated second alternator like ours will produce around 180 amps of power when it’s running.  At the other end of things, a stock alternator on a smaller vehicle may only have 40 amps of power left over to charge batteries.

Even if you don’t have a beast like this, your alternator is a powerful source for charging your batteries.

Shore Power:  Here, obviously, you’re tying into the larger electrical gird which (hopefully) doesn’t shut off.  But shore power runs at 120 volts and your battery stores at 12 volts.  To convert things from one voltage or another, your RV will employ a “converter” (not a terribly creative name) or an inverter/charger.  These devices have various capacities.  A stock RV converter may be able to deliver 40 amps of charging to your batteries, and a high-end inverter/charger like ours is self-limited to charge at 100 amps.

Generator:  If your rig has a generator, this too will charge your batteries.  They typically do this by providing energy to your converter or inverter/charger (not by connecting to the batteries directly).  In addition to the converter’s capacity limit, the generator itself has a capacity limit.  This can be only 2000 or 2500 watts for smaller generators.  So if you’re running the air conditioner, two TVs and a blender with the generator, there might not be much left to go into the batteries.

Solar Power:  Solar panels are sold in varying sizes, but a 100 watt panel is a commonly found size – particularly for RVs.  That panel is rated at 100 watts at peak efficiency – meaning on a 78 degree cloudless day at solar noon on the equator during the equinox.  I don’t know too many people who RV under those conditions.  (Come to think of it, I don’t know if those conditions actually exist outside a laboratory.)  But for the sake of argument, if we assume the best case, that 100 watt solar panel can provide just over 8 amps of energy for charging.  With 300 watts on our own rig, this means at peak, I can expect to generate 25 amps of charging if I’m lucky.

Here’s what’s really important about all that.  RV solar is the least powerful of the charging sources typically available. We’ve just reviewed the charging sources you have on an RV, and seen capacities from 180 amps all the way down to 8 amps.   Using our rig as an example, what this means in real-life is that I will generate as much stored energy in one hour of driving as I will with over 7 hours of peak solar generation.  (180/25 = 7.2)

Some Usage Scenarios

This doesn’t mean solar power doesn’t have a place in your RV.  Maybe it does.  I’m not trying to dissuade you from purchasing solar panels, and I’m certainly not implying that solar panels aren’t cool.  But I do think that before you dive into an expensive solar project, you should give some thought as to how you use your RV.  If you don’t use your rig in a way that lends itself to a solar solution, you may want to rethink things.  Let’s look at some typical RV usage scenarios and I’ll try to explain.

“I use my rig almost exclusively at RV parks or campgrounds with electrical hookups available.”

In this case, I’d argue you have little use for solar power.  Even a basic 40 amp converter will provide more charging than five 100 watt solar panels, and it will do it night and day, rain or shine.  Adding solar in this scenario won’t get you anything except lighter pockets.

“We do more ‘touring’ than ‘camping’, and so we drive most every day.”

This actually applies to Stef and me.  In this case, the driving will charge your batteries more than solar ever will.  You’ll likely find that your batteries are full or near full most of the time from driving.  This is another scenario where the addition of solar won’t really add much.

“We do a lot of ‘boondocking’.  Parked, away from utilities, and not moving the rig most days.”

THIS is the ideal use case for Solar energy in an RV.  If you’ve got no hookups, and you’re not driving, solar power starts to look a whole lot better.  Yes, you can run the generator (if you have one), but people generally don’t like the noise and the fumes if they’re avoidable.  Solar power can help you avoid exactly that.

You’ll need to consider how you use your own RV, and what charging sources you have available.  From that knowledge, you can make an informed decision as to what role RV solar can play in your rig.  Assuming you’ve done that, and you’re going ahead with solar power, there are some common terms we need to understand, so let’s look at that next:

 

Watts, Amps, and Amp Hours

Solar panels are typically rated and sold in Watts.  Electrical loads are also typically rated in Watts (you can usually find the wattage stamped into any electrical appliance).  But RV batteries are typically rated in amp-hours.  Since we’re trying to equate things from an energy in = energy out perspective, we need to be able to convert things easily.  Fortunately, the equation is pretty easy:

Watts = Amps * Volts

Volts are the unit of potential, and amps are the unit of current.  Their product is power.

If you’ve read this far, you probably know that you have two kinds of power in your RV.  12 volt DC from the batteries, and 120 volt AC from the grid or generator.  Watts are how you can equate them.  A Watt is a Watt is a Watt.  The voltage might be different, and also the current, but using Watts will get you from one to the other.  So, for example, a current of 1 amp at a potential of 12 volts is 12 watts.  A current of 8.33 amps at a potential of 12 volts is 100 watts.  So remember the 100 watt standard solar panel?  When it’s feeding your 12 volt RV battery, it’s producing a current of 8.33 amps.

(For the rest of this article, I’m just going to assume 12 volts for a battery.  Yes, I know if varies by battery type, state of charge, etc. etc.  I’m just keeping the math sane.)

Battery capacity is typically expressed in amp-hours.  A Group-31 battery might have a capacity of 100 amp-hours .  This just means you multiply the two together, like this:

amp-hours = amps * hours

So a current of 1 amp flowing for 1 hour will produce 1 amp-hour of charge.  And that 100 watt solar panel – at peak efficiency – would produce 8.33 amp-hours of charge in one hour.

It’s important to get this down because with RV solar, what we’re really talking about is using it to charge the RV batteries.  This kind of math tells us how much.  But that’s as tough as the math gets in this post.  So if you’re not a math person, and you’ve made it this far, breathe a sigh of relief and let’s move on!

 

RV Solar Power System Components

I know this seems like a lot of background before we get to the answer, but this is stuff you need to know.  Some of these things might even save you money.

Batteries

There are two main types of battery chemistry that you’ll find in RVs today.  The first of these is lead-acid.  These batteries have been around forever, and though there have been new ways to dress it up (AGM batteries, Gel cells), they’re all basically the same chemistry for the purposes of our discussion.

A typical AGM battery

The other type of battery that’s becoming popular in RVs is the lithium-ion battery.  These use a completely different chemistry from lead-acid batteries, and they require a different charging profile.  They’re also much more expensive than lead-acid batteries.  There are lithium battery chemistries that are safer than others, and those are the ones that belong in your RV.

But it’s far more likely that your RV has some type of lead-acid battery than a lithium one, so that’s where we’re going to focus our examples.  Sadly, batteries don’t come with a “Gas Gauge” to tell you they’re 5/8ths full.  It’s sad because that’s exactly the information we’re going to need.  So to get that info, you’ll need to use a State of Charge Chart.  They look something like this:

While I don’t know exactly what you’ve got in your rig, I’m willing to bet that somewhere in your RV there’s a display panel that will tell you the voltage of your batteries.  Using the state of charge chart is simple: you just find your voltage, and the chart will give you the percent full your batteries are… ish.  So if your battery voltage is 12.1 volts, the chart will tell you that your batteries are between 50 and 60% full… ish.

(It’s most accurate to read your battery voltage when the batteries are “at rest”.  That means with no charge being applied, no current being drawn, and having sat that way for a half hour or so.  But if you can’t generate those conditions, it’s OK.  It’s a pretty inexact science anyway.)

One more piece of wisdom about lead-acid batteries and we’ll move on.  To get the longest life out of your batteries, it’s best to observe the “50% rule”.  This means that you shouldn’t discharge these batteries below 50% if you can avoid it.  They won’t blow up or anything if you discharge them more.  But you will find yourself replacing them sooner.

Solar Panels

There’s a lot of technology around solar these days, so let’s get some basic terminology and options out of the way up front.

Monocrystalline vs Polycrystalline

When you’re browsing for solar panels, you’ll quickly realize they come in two types.  Monocrystalline and polycrystalline.  Does it matter which kind you get?  Maybe.  They actually look different, too, so let’s start with that.

This is a picture of polycrystalline solar panels.  Polycrystalline panels are about 13-16% efficient.  And they’re the less expensive of the two kinds.

These are monocrystalline panels.  You can tell the difference because the monocrystalline panels have a typical square-ish/stop sign appearance.  They’re made from a more pure silicon, and so these panels are 15-20% efficient.  Since they’re more efficient, that means they can be physically smaller and you still get the same output.  They’re also more expensive.

We actually have both kinds of panels on our RV right now.

I’ve played around with them a bit.  While monocrystalline panels are theoretically better in low light conditions, and better in high heat conditions, I can tell you that I’ve tried and I can’t discern a difference in our RV.  Perhaps if I had a solar installation covering a couple acres, those small differences would start to add up to something you can measure and take to the bank.  But in an RV, your install won’t likely be big enough for you to tell.

What I DO notice about monocrystalline panels is that they are physically smaller for the same output.  If you’ve got limited roof space available (like we do on our class B), then this can be a big benefit.  But if you’ve got the room, a 100 watt monocrystalline panel and a 100 watt polycrystalline panel will give you the same 100 watts.

Money Saving Tip:

Unless you need the reduced size of a monocrystalline panel to fit on a cramped roof, save yourself some money and use the cheaper polycrystalline panels.  There’s no moral superiority in the monocrystalline panels, and your toaster won’t know the difference.

 

Flexible vs. Flat Panels

Recent years have seen the advent of flexible solar panels.  These panels claim to be lighter (they are), more aerodynamic (they can be), and easier to mount to your rig since you don’t have to drill holes (also true).  But in my book, the big advantage of flexible panels is that they can conform to curves.  They’re also more expensive.

Traditional flat panels on the other hand are less expensive and have typically longer warranties (25 years vs. 10).  They’re more durable, hence the longer warranty.  Flat panels are also mounted off the surface, which allows cooling airflow underneath and improves efficiency.  They can also be tilted to point more directly at the sun.  On our own RV, I’ve only used flat panels.

But – straight flat panels sticking off the roof of a vintage Airstream would look terrible!  So with that in mind, here’s another…

Money Saving Tip:

Unless you have a unique situation that calls for the aesthetics of a flexible panel, flat panels are a more durable and less expensive way to get the job done.

 

To Tilt or Not To Tilt

(If you’re that vintage Airstream reader, you can skip this section.)

OK. This is an Air Force base and not an RV, but the idea is the same.

If you’ve got flat panels, you may have the option to tilt them to point them directly at the sun.  The tilting is usually accomplished with some sort of hinged rack and support system.  Pointing panels directly at the sun is more efficient – but how much more?  Well, the answer involves trigonometry, but I’ll spare you that and skip ahead to the answer:

If a panel pointed directly at the sun is producing 100% of its possible energy, then a panel 25 degrees off axis from the sun is producing 90%.

Yep.  90%.  For a 100 watt panel, that amounts to less than an amp under typical conditions.  It’s not much.  And in order to maintain peak efficiency – you would have to climb a ladder several times a day and jockey panels around.  Your reward for that effort would be a whopping extra 6 minutes of TV.

Meh.

Money Saving Tip:

Don’t worry about tilting or rotating your panels.  On an RV-sized system, it’s more trouble than it’s worth.  On a utility-company sized system, the gains are worth the expense and effort, but on the roof of your rig, you’ll likely never notice.

 

The Solar Charge Controller

Typical solar panels put out somewhere between 16 and 20 volts, depending on a lot of things.  Your RV batteries are nominally 12 volts.  A solar charge controller is basically a voltage and current regulator that keeps your batteries from overcharging.  Every RV solar installation has one.  There’s more than one way to accomplish this regulation, and so there are more than one type of solar charge controller.

PWM Controllers are more or less the standard ones available today.  It’s the kind we have in our RV, in fact, that picture above is the model that our Travato shipped with.

MPPT stands for Maximum Power Point Tracking.  These controllers represent the ultimate in efficiency at 94-98% (meaning, most of the energy from the panels finds its way onto your RV’s “grid”).  MPPT controllers are also better at dealing with a low state of charge, long wire runs, or really cold days.  Unfortunately, that efficiency comes with a matching price tag.  So if none of those special conditions apply to you, you can guess where this is going.

Money Saving Tip:

Unless you’ve got a huge system, long wire runs, dead batteries, or like to RV in the dead of winter, just go with a PWM controller.  The more expensive MPPT controller won’t likely be worth it.

 

Estimating Solar Output

So with that math and terminology out of the way, let’s flesh out the left-hand side of our equation a little more.  That’s the input side.

You can buy a 100 watt panel, but you won’t get 100 watts out of it.  The reason for this is that there are a LOT of factors that can impact the energy recovery of a solar power system.  Here are just a few:

  • Time of Day
  • Panel Tilt
  • Weather/shade/sun/clouds
  • Dirt in the air
  • Dirt on your panels
  • Efficiency of components
  • Temperature (Contrary to intuition, solar panels work best at cooler temperatures. A 100 watt panel at room temperature is an 83 watt panel at 110°.)

So with all those things affecting the solar energy output, how are you supposed to get a handle on how much energy you’ll get?  Well, if you like to take the easy way out, you can just go with this rule of thumb:

A 100 watt panel will generate 30 amp-hours per day

It’s a rule of thumb, not a perfect calculation, but it’s pretty useful nonetheless.  The number will be higher in the summer, or further south.  The number will be lower in the winter, or further north.  But if you like to work in nice round numbers – 30 is your number.

If a rule of thumb isn’t good enough for you (and I don’t blame you), there are other tools you can use.  Google “solar position calculator” and you’ll find all sorts of tools that will tell you the azimuth and elevation of the sun at any point on the globe on any given day.  Some of these tools are even pretty fun, and it’s easy to waste a full day playing with them (trust me, I know).

But by far the best tool I found is actually put out by the US government (I know!  Right?!), and it’s free.    The National Renewable Energy Lab has an online calculator that will predict how much energy you can recover with a given sized solar energy system, at a given location, on a given day.  It uses historical weather data and lots of math to give you a simple answer to the “how much energy will I get” question.  You can find it here:

http://pvwatts.nrel.gov

To check it out, I used our own RV.  I entered a system size of 300 watts of fixed panels, mounted horizontally flat, at Phoenix International Raceway (where I gave this seminar last).  It told me I could expect a total of 439 kWh per year from such a system.

But I tend to think of my RV energy usage in days instead of years, so I broke it down.

439 kilowatt hours * 1000 = 439,000 watt-hours per year

439,000/365 = 1203 watt-hours per day

1203 watt-hours /12 (volts) = 100 amp-hours per day

100 amp-hours per day /3 panels = 33 amp-hours per day per panel

 

Which is remarkably close to the 30 amp-hour per day rule of thumb.  Since we were estimating in Phoenix, the 10% increase should be expected.

But the NREL website goes beyond that!  It will allow you do download the data – day-by-day, hour-by-hour, so you can predict your energy output on any given day.  On the day we were in Phoenix (February 23), I downloaded the data and the NREL website predicted I would generate 91.6 amp-hours of energy.  I compared this to our actual output, which was 78.8 amp-hours for the entire day.  That’s about 14% low, but certainly still in the ball park.  The difference could have been attributed to clouds, perhaps I didn’t park exactly level (though I certainly try) or any number of other factors.

The NREL website is good enough that I’ve given up trying to calculate solar output in any other way.  I either go with the rule of thumb or jump right to their calculator if I’m contemplating a trip to Alaska or something.  I recommend you do the same.

 

The Load Side of the Equation

Now that we’ve got a decent idea of how much energy we can expect to come in from an RV solar energy system, let’s turn to how much energy will be going OUT.  There are a few ways to do this, but most of them are no good.

Approach #1:

If you’ve poked around the internet on this topic for a while, you’ve no doubt come across energy calculators.  These present you with a number of appliances or other electrical loads and you estimate how long you’ll run each load.  From there, it creates an “energy budget” for you.  They look like this:

These suck.  Every single one of them.  Don’t use them.

You’ll usually find these “helpful” calculators on the websites of merchants selling solar panels.  That should be somewhat of a red flag.

The main problem is that – let’s be honest – you have no idea how long each day you run your hair dryer, or how many watts it uses!  Sure, maybe if you look you can find that it’s rated at 1500 watts, but does it really run at 1250 watts?  I don’t know!  Well, jeez, we’d better be safe and estimate high then, huh?  And my toaster?  How long does it run?  Well, that depends on if I want one piece or two, and how dark I want my toast.  I’d better put down 30 minutes just in case everybody wants four pieces of toast.  And the stereo?  Well I don’t run it every day, but I might.  I’d better put down 4 hours just to be safe…

You see where this is going.  Garbage in – garbage out.  These calculators, without fail, will cause you to overestimate your electrical needs and buy more solar panels than you really need.   Don’t go there.

Approach #2:

This approach is slightly better, and something I’ve actually done myself, but I don’t recommend it.   It involves taking actual measurements in your own RV.

To do this, you’ll need a way to measure current, both AC and DC, and a lot of patience.  I did this on our own rig, and you can find the results in this post I made on calculating the 12 volt loads in our RV.  But again, I don’t recommend this because it’s tedious, time consuming, and you’ll still wind up estimating how long you want to run your toaster.

Approach#3:

This is BY FAR the best, most accurate, easiest, and most fun way to measure your electrical loads.  You just go camping and do your thing.  I call it “camping in the name of science”.  Stef doesn’t buy into this, but that’s what I call it.  Here’s how to run your camping experiment:

  1. First – go do your normal camping thing. It’s important to have as typical a trip as possible.  Don’t try to conserve.  Don’t try to use extra juice.  Just be yourselves.
  2. Once the experiment starts, no generator usage. If you have solar panels on your rig already, unplug them.  If you have a motorhome, don’t turn on the engine.  The idea here is to only draw down your batteries – not charge them as well.
  3. You want to start timing the experiment at nightfall, and you want to start with full batteries. So run your generator up until sundown.  The reason for this is simple:  Solar power doesn’t work at night.  And if you don’t have enough power to make it through the night – you have a battery capacity issue, and all the solar power in the world isn’t going to help you.
  4. Start timing once the generator is off, the sun is down, and the rig is unplugged. Use the rig normally.
  5. Keep tabs on your battery. You want to know how long it takes to drain your batteries down to “empty” (keeping in mind the 50% rule if you have lead-acid batteries).

That’s it!  Way more fun than crawling around your rig with a multimeter.  Here’s how an example might work out.

If your typical RV day involves getting out of your rig – then do it during your experiment!

Let’s say you have a 200 amp-hour battery bank.  They’re lead-acid batteries, so you don’t want to discharge them any more than 50%.  At day zero, you start at nightfall with a full battery bank.

At the end of 24 hours, your voltage is down to 12.4 volts.  That’s still about 80% according to the state of charge chart, so you continue on.

At the end of the second day, your voltage is down to 12.2 volts.  60% according to the state of charge chart, so you keep going.

At the end of the third day, you’re down to 11.9 volts.  That’s 40% on the state of charge chart, so you stop the test and turn on the generator.

So – in three days, you used 60% of your battery capacity.

Your battery bank is 200 amp-hours.  60% of that is 120 amp hours over three days.

Dividing that by three, you used approximately 40 amp-hours per day.

DONE!

 

Putting it all together

So now, you’ve got a handle on the supply side of the equation, from either the rule of thumb, or from the NREL website.  You’ve also done an experiment to determine exactly how much energy you use on a typical day.  So let’s translate that into how much solar you need to equip your rig with.

Continuing with our previous example, we’ll assume we use 40 amp-hours per day of battery capacity.  We’ll also assume that we’re good with the “rule of thumb” of 30 amp-hours per day from a 100 watt panel.

40 amp-hours per day = 30 amp-hours per panel  per day * X panels

Divide both sides by 30 and you need 1.33 100 watt solar panels.

Now, I’ve not seen a 133 watt solar panel for sale.  But I have seen plenty of 160 watt panels.  There are a lot of starter kits that include them, like this one (I’m not affiliated – just found you an example).  So, from our camping experiment, we’ve determined that a 160 watt panel will keep our batteries topped off most days, and that kit fits the bill.

 

Parting  Thoughts

A couple thoughts on the solar power thing before I wrap up.  Even after you’ve gone through the experiment and analysis above, there are still some questions to ask yourself before you invest in RV solar.  For example:

  • Will I ever be staying in one place for three days with no movement and no hookups? In the example above, even without solar, the RV didn’t encounter a battery capacity issue until some time into the third day.  If you never stay put for three days, solar power in that situation isn’t strictly necessary, as the alternator will charge you up as you drive to your next destination.
  • Do you have the propane and holding tank capacity to match? In other words – does it do you much good to have 100% full batteries with 0% fresh water?
  • And finally, were there simple conservation steps you overlooked? Could you replace incandescent bulbs in your RV with LEDs?  That would save a lot of energy and tip your equation.  Good, old-fashioned conservation may get you where you need to go without the investment of time and money in RV solar.

So there you have it.  I hope this hasn’t come across as a “Solar Grinch” piece.  We’ve got plenty of solar panels on our own RV and we think it’s awesome.  We’re in favor of clean, renewable energy wherever we can get it.  But I’m not in favor of wasting money, and often I hear of people maxing out solar capacity on their brand new rig without a clearly defined need for the expense.  I don’t want that to be you.

(It can be me though.  This stuff is like toys to me… 🙂 )

Cheers!

 



James is a former rocket scientist, a USA Cycling certified coach, and lifelong fitness buff. When he's not driving the RV, or modifying the RV (or - that one time - doing both at once), you can find him racing bicycles, or building furniture, or making music. In his spare time, he works for a large IT company.


    105 thoughts on “How Much Solar Do I Need On My RV?

    1. Ken

      My needs are quite different from all I see above. I have a small truck camper, I’m never anywhere near hookups, and it’s always cold. I run a few lights while cooking, and my furnace all night… period. I’m thinking that a single 100 will keep me up and running indefinitely. I’m totally new to this, and apologize for the moronic question, but “if you don’t ask, you’ll never know.”

      Reply
      1. James - Post author

        How large is your battery bank? And how depleted is it at sunset?
        The procedure and the math are the same, regardless of what kind of rig you own.

        Reply
        1. Ken

          Thank you for writing back. I hope I’m not a nuisance. My battery bank consists of the trolling motor battery out of my boat. I’m guessing that you’re suggesting that I use some sort of battery tester (a volt meter? … to assess the battery condition each evening? What numbers would I be looking for? As I mentioned before, this is new to me. All I know is that my first attempt at this was a failure… I bought one way too small ( an 18 I think)… It left me out in the cold

        2. James - Post author

          Yes. Most coaches have a battery voltage meter of some sort. If yours does not, you can test it with a multimeter, or there are any number of plug in options if you have a 12v outlet.
          Once you can measure your battery voltage, then you can follow the steps in the post to determine how much solar you might need.

          Also – you’ll need to know the amp-hour rating of your battery.

      2. Michelle

        Thanks for taking the time to write this article! It’s really helped make some good financial decisions for me. A question I have (since I’m new to travel trailer) is what do u need to be able to use my outlets if I’m only using battery? I want to do the “camping” test you recommend but right now if I’m not hooked up to shore power I can’t use the outlets for the hair dryer or toaster. Is that what an inverter is for?

        Reply
        1. James - Post author

          Yes, that’s what you need an inverter for.
          To run a hair dryer (which are all about 1500 watts), you’ll likely need a 2000 watt inverter.
          Those can use up some pretty serious battery power, so do have that installed professionally.

    2. Harrison Clark

      A detail about the NREL PVWatts model will explain why your day in Phoenix AH did not agree with PVWatt’s prediction for that day. The PVWatts model uses 30 years of data for each location in its data base and creates a typical year that will provide the energy obtainable there. This typical year does not contain 365 average or smoothed days. It contains a hodge podge of days that reflect the variations that can occur from day to day. If you look close at any month of daily kWH predictions from PVWatts, you will find they vary considerably, especially where and in months where weather variations are common. So, any particular day from PVWatts is not relevant. About the best you can do is compare 30 days of actual results at the location with the total for that month from PVWatts. Those should correlate fairly well. The real day to day variations you experience will not track the PVWatts day to day variations other than your minimum day of the month might look somewhat like the minimum day of the month from PVWatts (likewise the max day). A place like Phoenix where the weather and atmospheric conditions (many such conditions affect solar production but are not visible to the eye) are fairly consistent from day to day will track better, but still, one should compare a month from PVWatts to a month of actual experience to ascertain whether field performance matches the PVWatts projection. And, of course, even monthly totals can each vary by ten percent or more from year to year (variations in annual production vary much less but still as much as three to four percent. The PVWatts manual details these characteristics of the model.

      Reply
      1. James - Post author

        Interesting. I obviously agree that the date from PVWatts can’t be assumed to be any kind of guarantee. Still, it’s interesting, and in the case of Phoenix, it at least got us in the ball park.

        Reply
    3. frater secessus

      “But once your battery is full, it’s full! You can’t fill your battery over 100%.”

      True, but you can run opportunity loads. Heat water, dehumidify, run computers and charge devices, whatever.

      Folks who use most of their loads in the daytime can likely run smaller, lighter, and less expensive banks. IMO, of course.

      Reply
    4. Tomas Sarmiento

      What was the name of the company that manufactures the mounting rack for the solar panels on your Travato? I have an Itasca Viva and would like to look into mounting solar panels.

      Reply
      1. James - Post author

        Our roof rack is by Fiamma. But I think any roof rack would make a suitable base for mounting solar panels. There’s not much special about the rack itself.

        Reply
    5. Pete

      Have you considered having a small, somewhat quiet generator (like a Honda 1000 watt unit, 39 lbs) to provide enough power, along with batteries, to run the AC for long periods? The engine generator could also provide this function, but some may not like running the vehicle engine overnight.

      Reply
      1. James - Post author

        We could do that easily by plugging into the shore power outlet. But we’ve never done so. We’d have to find someplace to carry and store that generator. The best place for that is underneath the vehicle where the generator I just removed used to live. If I wanted that sort of a solution (and it IS a valid solution), I wouldn’t have removed our generator to begin with.
        We make a habit of not staying anywhere that we need to run the generator (or engine) all night anyway. If we have to be someplace like that, we find a way to plug in.
        Having just gotten back from supporting a race though, I can tell you that the engine is fine to idle for extended periods. No issues to report with that!

        Reply
    6. TED PHILLIPS

      Dear Lance – I am planning to live in a 2018 59g full time – before i buy i would like to possibly order some mods from the factory ?? who do you deal with ?? I am in San diego Ca thanks – I would like to do the raised bed maybe not as high as yours – All the electrical mods and a couple small changes – thanks TED

      Reply
      1. James - Post author

        Lance is our van. He doesn’t have a login on our website. 😉

        But if you’re interested, his modifications were either done by me or the Winnebago factory. I’m not for hire. But Winnebago has a custom service. They may be able to help?

        I’d start with a call to your local dealer.

        Reply
        1. Tracy

          Maybe Ted is a “different type” of nerd and he really was asking Lance for advice. Just a bit of nerdy
          humor from a nerd wannabe!

    7. Sandra Cole

      This article was easy to comprehend and very helpful. I sleep with a Cpap machine without a humidifier. Would I be able to run it with solar power or is it a matter of running it off my batteries and keeping my battery charged?

      Reply
      1. James - Post author

        This one:
        “is it a matter of running it off my batteries and keeping my battery charged”
        is correct.

        Well, unless you sleep during the day when the sun is shining… 🙂

        Reply
    8. Bobbie

      I am a total newbie at owning a trailer. I have been reading and reading and the bottom line seems to be that I cannot run an air conditioner in my travel trailer at the same time as anything else. Being as I am attempting to live in the trailer more or less full time, (I do go to work everyday nine to six), I am dismayed at this apparent reality. I live in Redding, CA and this summer we have had numerous days where the temperature was over 105, and up to 111. It is not possible to be in the trailer for more than about five minutes without air conditioning. So I hoping that adding solar would help me go off grid. But now I am not at all sure it can be done.

      Reply
      1. James - Post author

        If you can’t run the AC while running anything else, I’d say solar probably isn’t going to help you much.
        It sounds like you’ve got some serious capacity limitations on your rig. Solar won’t get you over the hump.

        Reply
    9. Jim Eguchi

      I knew nothing about solar energy before reading your site, and now having read it I feel like I know enough to get started. It is breathtakingly clear, to the point without unnecessary mumbling, and the flow of it was such that I was able to read from the beginning to the end without much interruption.

      I have a 5 acres property in the middle of desert near Joshua Tree national park in southern California, and on it there is a 1000 sq ft cabin, an Airstream, and a small separated guest room of about 500 sq ft. I want to come off the grid, and I luckily ran into your site.

      Thank you for sharing.

      Just curious, have you installed solar panels on your home?

      Reply
      1. James - Post author

        Hi Jim!
        Glad you liked the post. We haven’t installed solar panels on our house. Part of the problem is that it faces the wrong way to be really effective. And the other part of the problem is our multiple old maple trees.
        But if we did install solar on the house – that’s a different consideration because you can sell excess power back!
        Your property sounds awesome. Good Luck!

        Reply
    10. Luc

      I really like your resource management approach in this article. There is one detail that is missing for me. Did you collect data about the fuel needed to recharge with the alternator? I am gathering data before buying our first RV and wandering about the actual cost of the different energy sources.

      On the road = $ for the fuel to recharge the battery
      Shore power = $/night at campground
      Boondocking = $ Generator + $ Solar power + $ Battery capacity

      We are planing to buy a Class B as a second car and hop from one campsite to the other. I think the best approach would be to have a inverter charger with something like 120 amp-hours of battery capacity for a night off grid.

      Reply
      1. James - Post author

        Hi Luc –
        A number of folks have asked over time about any additional fuel required to charge through the alternator. In my mind, it’s pretty negligible when you’re using the van. 1 horsepower is 750 watts (ish), and we’re talking about a 300HP engine. I don’t think it notices the extra load.
        But running the engine at idle would consume fuel just to charge the battery. That’s a bit different. I don’t know exactly what the fuel consumption at idle is for the ProMaster, but I’d then have to compare that to the generator (which was drawing from the same fuel tank when I had it). Those are all small numbers though – and we almost never idle just to charge the battery, so I haven’t done the math.
        If all you want to do off grid is run a few things (NOT the air conditioner), a 120AH battery should do you just fine!

        Reply
    11. Scott Hallenberg

      This is terrific and just what I needed to get started. I have 2 6v in a vintage airstream and am looking for a portable system. I’m likely going to start with a 100W panel as I think it will be sufficient in most cases and may add an extra panel should we find that it does not keep up. Normally I would be inclined for the overkill approach. This really help me weigh may options more thoughtfully. Thanks!!!!

      Reply
    12. Russ

      James,, thank you for talking electrical english on this article…
      I have so many questions,,,, but, the one to start with is:
      If I want to convert a small vehicle like a Nissan NV HIghtop,, or equivalent,,, (for just me, right now),,,and , you may have made sense to not need solar after all,, but, just alternator(s)…. where does a second alternator mount to?
      Then, later ,, I would like to ask about figuring the amount of power needed,, for example:
      2 burner stove
      blender (breakfast) short time @ 3.2 amp
      countertop water distiller (580W running 4 hours each day)
      small frig///feezer
      2 laptop computers (running all day)
      cell phone charging
      small TV ( on most of the time)
      maybe an exhaust fan ( to take heat and humidity out)
      That’s all the power I can think of.
      Thank you for this blog, and all your help to the RV community
      Russ,, just a 65 your young guy wanting to try the boondocking life… and will at least be driving small distances each day.(around town,,, shop, workout, checking out places, etc.)

      Reply
      1. James - Post author

        I’m not sure where a second alternator for a Nissan NV would mount. Nations Alternator would be a good place to start looking. It seems they have one available.
        As far as figuring out how much battery capacity you need, the data you’ve given is an example of the kind of thing you could try plugging into one of those electrical calculators I never use.
        Alternatively, you could just take the wattages from each appliance, multiply, and come up with the watt-hours and go from there.

        Reply
    13. Mike

      Hello James. We just bought a 2017 k and are going to do a lot of boondocking in Alaska. It came with a 100 watt panel. I would like to add a secong 100 watt panel. If you were going to buy one today, what brand and model would you buy? Thank you!

      Reply
      1. James - Post author

        I’m actually not that brand-sensitive when it comes to the solar panels. But I’d get a polycrystalline flat panel. Probably the least expensive one I could find.

        Reply
    14. Arvind Mallya

      A very well written article with lot of real resaearch on panels, controllers, load conditions and a decision on what we need based on our usage. As an electrical engineer and now a Travato owner, I had all these questions and most of the time I received a layman’s answer. Thanks for all your groundwork it certainly helped me understand the complexity of the power management we need to do in our RVs.

      Reply
    15. Ronald William Worth

      Good informative article, easy to understand without being confused by complicated math that engineers love.

      Reply
    16. John Brundage

      Great post. But you still haven’t answered the question, “How many solar panels do I need to run my air conditioner?” My wife wants to know. LOL!

      Reply
        1. Eric

          James – Great article! I appreciate the pragmatic approach. My only issue is, I read this hoping for you to answer the A/C question. I understand it depends on the appliance load so will vary. You could have mentioned what loads you had used over your 3 days of testing. Did you run A/C? Microwave? – Thanks

        2. James - Post author

          I didn’t answer the air conditioner question because it can get kind of silly. 🙂
          To generate the stored electricity needed to run our RV air conditioner all day, you’d need over 340 square feet of solar panels (ish), plus 3600 amp hours of battery capacity, and guaranteed perfect sunshine. I’m not saying it’s impossible… but it pushes the limits of practicality for a mobile installation.

          But to answer your other question: when I do a load test, I don’t think about it. I don’t want to color the results by trying to use or not use a certain appliance or feature. I recommend everyone do the same.

        3. Eric

          James – Thanks for the reply. I guess I should have been clear on what I was looking for with the A/C question. You have a 3000watt pure sine inverter, so I assumed when you boondocked you ran the A/C off this and your battery bank, not directly from the PTV. Plus you have an independent house alternator….so do you need a generator? I could see keeping a small portable unit to supplement battery charging once below 60% or so.

          I ask because I would like to remove my large generator and replace with solor/inverter/batteries solution and keep a 1000-1500watt gen on stand-by for long boondocking with maybe poor sunlight.

        4. James - Post author

          Ahhhhh!!! Now this makes more sense.

          You are correct in most of what you wrote. When boondocking, we do run the AC off of our inverter and battery bank. In theory, we could turn on the vehicle, and run the AC off the vehicle alternator indefinitely, but we’ve never put that to a test. We no longer have a generator, having removed it when we did the inverter install. You can see the details of all that in this post.

          If you didn’t go the second alternator route, then absolutely yes, what you describe would be a very workable solution. A small generator (2000 watts or so, like the Hondas we see everywhere) would be a good backup strategy. Totally sane, and an easy way to get into this without having to buy an enormous battery bank.

          Sorry for the confusion. But you’re right on track!

    17. Dirk Leverant

      You Guys Rock! Seriously. The information you post is always well thought out and still accessible to people like me, who are new to the RV world. Thank you for this, and for your many other pieces on YouTube. As an aside, my wife and I were on the fence about getting an RV, and watching your videos pushed us over the edge. So far, this is a good thing. Keep the information (and entertainment) coming.

      Reply
      1. James - Post author

        Jump on in! The water’s fine!
        Seriously though, glad we’ve had a positive influence.
        Hopefully you get your RV soon and we’ll see you out on the road!

        Reply
    18. Alexander

      As a relative newcomer to RV life I must say this has been the best and most informative article I have yet come across. 5 ***** James, excellent.
      Prior to happening upon your article, I found that the more I searched, researched and read up, the more confused I became, what with MPPT regulators, controllers, Inverters, Amps, Watts and Volts not to mention panels, batteries and installation, . . . had me in a tizzy.
      Have planned for a lot of off-grid adventures and laying up for days or even weeks at a time in France, Italy and especially Greece so therefore a Solar installtion will be essential for lady wife and myself. Things are starting to make sense now and you have helped make the decision process a whole lot simpler for us and I would like to sincerely thank you for that.
      Still have some research to do yet. Would like to get more advice on Lead Acid batteries over AGM deep cycle and what size/ capacity Ah batteries to use. Also, what wattage Inverter (pure sine) to install. Enjoying the learning process 🙂 Many thanks from Ireland.

      Reply
      1. James - Post author

        Well thank you for the compliment!
        I’m glad that this article is proving to be useful for people – around the globe even.
        Your proposed RV destinations sound fantastic, and we hope you get your rig set up exactly as you need.
        Cheers!

        Reply
    19. Phil S.

      Here’s another potential case for solar battery charging – NASCAR race RV camping! RVs are often parked side by side dry camping with the generator exhaust typically pointing into the next RV’s patio space. The noise and fumes are not very welcome. Sometimes exhaust extension stacks are used to direct the fumes up overhead, like trucker pipes – but the noise still persists. RV campers may be parked in one place for as long as a week. There are site restrooms (portable and fixed) available (sometimes showers too), plus three types of tanker trucks often circling the campground (I bet you can guess what services they provide) – water, propane and waste tank pumping. We’ll be using propane for the refrig and portable grill that runs off of the low pressure supply from the RV propane tank.

      As a recently retired EE and NASCAR fan, my wife and I plan to visit several NASCAR tracks in our new class B+ Pleasure-Way. I figured that two panels would keep the LED lights, TV, microwave, coffee maker and my wife’s hair dryer powered pretty well without need for the generator, from the 2000W inverter, even if we have a cloudy/rainy day or two. We’ll see how well that plays out. The only generator use I anticipate is for air conditioning, and hopefully not much of that. Our home NASCAR track is Phoenix, so we’re used to hot temps as long as there is some shade available (RV canopy or venue shade).

      Food may be the only real limiting factor here – and sometimes there is an on-site grocery store for that!

      Reply
      1. James - Post author

        Yep – what you describe sounds like a situation where solar power would definitely come in handy.
        In fact, I’d recommend exactly that as your “test camp”!

        Reply
        1. Phil S.

          James – thanks for endorsing my NASCAR RV camping solar power test scenario. I can’t help but peek at the solar charging status panel anytime we’re stopped and disconnected from shore power – great entertainment for me! Do you know where I could find (or can you publish on this Post) a State of Charge chart for Lithium Ion RV batteries. Ours is a 200 amp-hour unit that is now standard for Pleasure-Way RVs. The Go Power status panel does provide an indication of %charge, but it’s based on an AGM profile (no Lithium setting) – so I don’t know how accurate that may be.

          Thanks!

        2. James - Post author

          Hey Phil – A State of Charge chart for lithium batteries is problematic because they have a very flat discharge profile. Meaning: the voltage stays relatively constant until you’re 90% discharged or so, and then it falls off a cliff. So by the time you notice a change, it’s too late. You can see a good chart (and read a good explanation) here: http://batteryuniversity.com/learn/article/how_to_measure_state_of_charge
          To get a really good read on the state of charge of a lithium battery, you’d want to install a shunt-based battery monitor. On a Pleasure-Way, I believe the batteries are inside the coach, so it shouldn’t be terribly difficult. (But don’t try it if you’re not sure you can do it safely.)

          And sometimes I have Stef get up and read me numbers from the monitors while I’m driving… so you’re not alone… 😉

    20. Wade

      James – great article. One item I am curious about that has an indirect relationship to this topic is regarding the multistage chargers. With AGMs for example there are various recommendations of using 3 stage or better chargers to assure longevity of the battery. There are chargers and inverter/chargers running on 110volts out there that meet this spec but I don’t think that an alternator does. I expect that most solar charge controllers do support 3 stage or better charging.

      So then, does heavy reliance on an alternator to charge the battery bank actually result in reduced battery life as compared to other methods that use a smart charger? Or does your system incorporate smart charging for all 3 options? (Solar, alternator, shore power)

      Reply
      1. James - Post author

        You are correct in that most vehicle alternators don’t support multi stage charging of house AGM batteries, and that solar charging systems do. This is one of the reasons to go with a separate alternator, as we did. Our second alternator supports a proper charging profile for lithium batteries.
        Whether you rely on it for charging or not, a standard engine alternator will present 14.something volts to the batteries the entire time it is running. You’re right in thinking that this doesn’t lead to optimal battery life.

        Reply
    21. Alain

      On re-reading this blog, I got to thinking about the usage side of the equation and how many of our new toys use battery power. Mylène’s new ebike has a big battery that requires hours to recharge fully. Same for our trolling motor battery. Ideally you don’t want to use stored house power to recharge these in the evening, so a bit of over capacity on the production side is a good idea and have them charge during driving or sunlight hours.

      Reply
      1. James - Post author

        Yes, it makes sense to pay attention to the toys.
        If your typical trip involves charging those up – then by all means, include them in your field test. The math will work out.

        Reply
    22. Tracey Y

      So if you were doing it over again now what would you setup be? Would you have 3 panels, 1, no panels? I assume you stick with Li, the alternator and inverter.

      Reply
      1. James - Post author

        Well, I’d definitely leave the inverter/alternator/battery setup as it is now.
        As far as solar, that’s a tough one. The electrical loads on Lance are always changing! And that will probably continue for a while.
        For example, by changing out our refrigerator for a 12 volt model, I’ve increased our electrical load.

        If I had no solar and I were going to stop here and add solar, I would follow exactly the procedure outlined in the article. Do a load test and work from there.

        Reply
    23. Waynne

      Ditto on all the comments about this outstanding article James! I’ve been trying to explain this all to my wife for hours. Now I’m just going to have her read your article. Well done! Thank you!

      Reply
    24. James

      I don’t know where I read it so I can’t footnote it. But when I was doing my own research I read somewhere that you should pitch the panels at the same angle as the latitude you are at. And, of course face them south.

      I did not know they work better cold. Thank you for that info.

      And thank you for bringing up tank capacity vs. battery capacity. We have 3 kids so our water is our main concern when we don’t have hookups.

      Solar for us would be wasted unless I could teach the wife to drive a trailer and follow her with a fresh water tanker.

      All the rv blogs tell you, you will be amazed how much water you really use. And, it’s true!

      Reply
      1. James - Post author

        So true – I think people get lost in the electrical aspects of things, and overbuild relative to their other systems. I know for us, water is a limiting factor as well.

        Reply
    25. DT

      Excellent post James. Am soaking up all the info around and about to spec up my own Sprinter based conversion and this is the best, most objective info I’ve read. Chapeau sir. Keep up the good work both.

      Reply
    26. Sandy DePottey

      Nice article, well written for us non tech types I was able to follow easy peasy. Have a new Travato with solar and it seems like a pretty slick setup, so that I am pondering adding solar to my old class A. I rarely have shore power and use the generator for everything and thinking it would be nice to actually use the 2000W inverter,( that I never use) to power stuff and keep the batteries charged up with solar. with the class A there is plenty of roof but it would be good to do some math and not go over board on the solar. A little bit of microwave and some fridge (not compressor) would be sweet. Thanks

      Reply
      1. James - Post author

        If you’re not using your inverter, I might first investigate adding battery capacity before I go adding solar.

        Reply
    27. Ed Marks

      Thanks for the well written informative article James! Solar seems cool, but I appreciate your approach and making sure that any install matches your intended use and overall power management system. Well done!

      Reply
    28. Patrick (Pat) Shubird

      Very nicely done, Sir, even for an aerospace engineer. 🙂 (I’m an electrical engineer, BTW.) To piggyback on the first poster’s comments, I have often had thoughts about all the wonderful electrical upgrades I could do to our Travato. However, I do have those rare moments of restraint when I stop and think about our actual usage habits. My wife and I use our motorhome frequently, but we stay almost exclusively at full-hookup campgrounds. So, do I have an actual need for lots of gadgets to capture and store energy? No, I really don’t. Would I love to have and play with all the gadgets? Heck yes! Thank you, James, for indulging your inner Geek – that way I can save money and just live vicariously through you. Woo Hoo!

      Reply
      1. James - Post author

        Hey – if I’m saving you money, then I’m calling that a success!
        (There’s probably no saving me though…)

        Reply
    29. Kelli11

      Wow, this is SO helpful! As a brand-new (and clueless) Travato (2018) owner, I’ve been dreading spending hours reading through the 47 system manuals and roaming the internet trying to understand the power system. Your article has saved me many hours and given me the bottom-line info I need. Thank you, thank you, thank you! Even though my eyes glaze-over the calculations, it’s so cool to see the joy you take in the nerdier aspects of these systems. Bravo James!!!

      Reply
      1. James - Post author

        Glad this is helpful to you.
        I expected a lot of comments from knowledgeable folks debating one aspect or another of the article. But they probably already really know their solar.
        I really wrote the piece for people like yourself! So I’m glad to hear it’s helped you out. 🙂

        Reply
    30. Jim Weeks

      A very interesting and informative article. I understood most of it, I think. My question is: If I buy a standard Travato 59k and go on your 3 day boondocking scenario (not running engine or generator), won’t I need to buy additional solar panels and an inverter in order to power my coffeemaker, toaster and refrigerator?

      Reply
      1. James - Post author

        Well, the “do I need an inverter” question can really be a separate one from the solar question.
        Basically, if you want to run standard household appliances without running the generator, then yes, you need an inverter (but not necessarily more solar).
        If you normally run the generator to power these appliances, then I’d suggest just running it briefly if you can’t live without the K-cups. After you’ve made your cup, kill the generator and back to the experiment.
        The charging you’ll get while running the generator for coffee will skew the results somewhat. But if that’s reality for you, then that’s what it is.

        Reply
    31. Gary Reysa

      Hi James,
      Good stuff.

      One exception to the tilt does not matter much is winter camping in northern states.
      In our MT location in Dec, output goes from 1.2 KWH/sm-day with 0 degree tilt up to 2.6 KWH/sm-day for 60 degree tilt (using PVWatts).

      Gary

      Reply
      1. James - Post author

        Thanks, Gary. Far north in December would obviously be an outlier.
        (But if you’re RVing in Montana in December… hats off to you! And I thought Stef and I took winter camping seriously!)

        Reply
    32. Terry Lee

      Correction: 0.6 gal LP contains 54,798 BTUs (not 91,330 BTUs), which is still equivalent to 16,000 Wh.

      Reply
      1. James - Post author

        Shame on me for not checking the math before approving! lol.
        Thanks for the correction.

        Reply
    33. Tom Cahill

      If you boondocked for an extended period (I get that is not what you usually do) how long do you think your battery would last with your normal demands on it? If your battery was “dead” how long would it take to charge (while staying in place) with your 2nd alternator? Again, assuming your normal demands on the battery. How much does your furnace fan tax your battery overnight in cold weather? Your post is great. I was thinking of asking you previously to do an “RV Electrical Systems for Dummies” Thanks for anticipating that request.

      Reply
      1. James - Post author

        Lots of questions. Let’s see if I can answer.
        With our 300 watts of solar and moderate sun, we could boondock until we ran out of water. If we unplugged the solar panels, we could still make it several days. But I have not checked this since installing our new compressor fridge. That post is coming in the future. Our alternator could charge our completely dead battery in about three hours. And the furnace overnight is not very noticable on the battery (we have a Truma Combi).

        Reply
    34. Paul A. Jackson

      Hey James, as always informative and fun! Am an electrician/geek so love the tech, but have a saying that the trade seems to use a lot, “better looking at it than for it”. In my experience damn the money buy big! Best controller, more panels and get out there! Will you be reviewing or looking at any of the technology built into the Roadrek/Hymer offerings? Would love to hear your take on EcoTrek/VoltStart and some of their Solar Options. THX

      Reply
      1. Keith Bradshaw

        Excellent article James. I’m commenting in part for the poster interested in Hymer / RoadTrek products. Your article didn’t touch on the scenario of the new RV buyer ticking off options on an option sheet. We are guilty of ticking a lot of options!

        We recently bought the 2017 RoadTrek Zion with 400 ah lithium (2 200 ah EcoTrek modules), 280 amp second alternator, and 300 W solar. RoadTrek indicate in their docs that the phantom loads on their EcoTrek modules are 6 amps, which admittedly is CRAZY, and we didn’t know under AFTER our purchase. Even the service tech who introduced us to our RV quoted the 6 amp figure from his factory training course. While I’d like to think they have improved this and their docs are out of date, let’s assume it’s still true. From the 30 ah/day/panel estimate provided, we would have recovered 90 ah/day total from the solar panels offset by 144 ah in EcoTrek phantom loads. On a recent 6 week trip, I don’t think the van ever sat still for more than 2 days. We never ran low on battery power.

        James you make a great point that if you move every couple days with lithium batteries and a large alternator, even if it’s just a grocery run, you likely don’t need solar at all. In our case, I’m thinking the solar is saving us from the phantom loads but otherwise was totally unnecessary.

        I think the heart of this article, for those buying a new RV or retrofitting, is consider the electrical system as a whole. While our system isn’t perfect, I still think a large alternator combined with lithium batteries is a great solution for those that tour and boondock. My add would be to investigate the phantom loads of any system BEFORE you buy!

        I’d be curious James if you know what kind of phantom loads your system has (losses to battery management / disconnect relays).

        Reply
        1. James - Post author

          I’ve heard about these HymerTrek phantom loads before, and that’s just NUTS!!
          We have NONE. I did a big experiment before where I measured all the loads in the RV using our shunt based battery monitor. There was less then an amp of undefinable load.
          That has not changed with the addition of our new system. If anything, it’s decreased as we no longer need to leave the propane solenoid open and there’s no auto gen start.
          We have no heavy duty disconnect relays. There may be one internal to the Lithionics battery (because I CAN shut that off), but it appears to draw milliamps, if anything, and it does not affect the rated capacity of the 420AH battery. I still have all 420AH available to me. (actually more, but that’s another topic.)
          The only load that might be considered phantom would be if we left our inverter on when we didn’t need it – but simply we don’t do that.

          Regardless of all that, you make a good point about the new buyer checking off options! The only way they could know this kind of thing would be if they asked some actual owners for opinions and advice beforehand. That’s actually one of the things I recommend new buyers do in our “RV Buying Process“. But if they were still unsure after that, I think the way Winnebago does it makes a lot of sense. The basic solar option gets you one panel, all the wiring, and a combiner box on the roof. If you decide to add more panels later, it’s super easy.

          Interesting comment, Keith. Thanks for joining the conversation!

    35. Scott Baldassari

      Excellent Job! Thank you, I plan to refer folks to this article often..

      If I could change anything (you knew that was coming 😉 ), I would put the USAGE SCENARIOS at the TOP of the Article, with a larger font, in Bold, and maybe a different color text, like bright red… and BIGGER. Did I say BIGGER?

      As cool as solar power is, it is definitely NOT the end all be-all many folks expect it to be. People often get hung up in details before they even know if they NEED solar power panels at all. Almost without fail, every time I ask someone why they want solar, the answer is; “to run my air conditioner”, “to replace my generator”, or, “so I can boondock as long as I want” – ALL of which solar, on a van, will NOT DO.

      With a little more questioning, it invariably turns out they fit into your first two categories of USERS: They make use of campground hook ups OR they drive at least a little every day, in which case, solar is of very little benefit. So really, most folks don’t even NEED to CONSIDER Solar unless they just like cool new stuff, and spending money (or it’s a reasonably priced factory installed option on a new rig).

      Folks that actually DO boondock for days on end in a B Van without driving, can make use of solar to top off their batteries, but should probably be more concerned about having a healthy battery bank to begin with. A larger battery bank could easily last as long as your water supply withOUT solar panels…

      Solar IS cool. But get it to top off a good battery bank suited to YOUR energy use, not to run MORE, or additional items you can’t already run comfortably off your batteries. Conservation is more beneficial and cost effective than Solar. Solar is the LEAST powerful, and slowest charging method at your disposal.

      …and don’t even get me started on big fancy inverters and Keurig Coffee Makers in a B Van….

      Reply
    36. Terry Lee

      It is so refreshing to see an objective analysis of solar for RVs. There are so many zealots who claim solar is a “no brainer.” It is not. (Disclosure: I don’t have it.)

      A couple of additional points:

      1. Camping in shade – I seek shade when camping in the summer. Solar offers little benefit.

      2. The Peukert effect offers me on average about an at least 20% capacity bonus based on typical draw of 30 Ah/day, e.g. my 220Ah Lifeline AGMs give me about 130 Ah down to 50%. When necessary (but rare), an hour of generator gets me another day, empty water and full waste tanks not withstanding.

      3. Battery capacity contained in LP
      I have rarely seen this calculation, but here it is. Generators are very inefficient electricity producers. My generator at half load (15 amps) consumes 0.6 gal/hr LP while producing 1800 watts (15×120). The 0.6 gal of LP contains 91330 BTUs, equivalent to 16000 Wh, while producing only 1800Wh of electricity for an efficiency of 11%. Still, my 10.4 gal (usable) LP tank contains the equivalent of 31,200 Wh ((10.4/0.6) x 1800) of “product” electricity, compared to 1560 Wh (130Ah x 12v) stored in the battery, or 20X as much. (The above ignores charger inefficiencies.) This is even 6X as much as Lance’s incredible 5040 Wh (420 Ah drawn to 0% SOC).

      Thanks again for an excellent summary!

      P.S. I can’t help asking how much you’ve spent, dollars and hours, in upgrading Lance. I know, I know … because you can.

      Reply
      1. James - Post author

        I didn’t even mention the contradiction between camping in direct sun to get full solar… and then needing extra air conditioning because you’ve parked in full sun. Good point.
        If you really have a 10.4 gallon propane tank – we’re envious! We have less than 2 gallons typically available.

        And as far as how much I’ve spent on RV mods. In terms of either time or money… I don’t want to know! And for exactly that reason, I don’t keep track. 🙂

        Reply
    37. SkagitStan

      Excellent post!
      As an EE I *had* to find a few things to quibble about.

      Our Ford Transit conversion has about 400AH of AGM, and 560W rated solar, with an MPPT controller, so it’s a fairly large system. No generator. Our use is remote boondocking, for multiple days at a time, mainly in one location. Then moving on and repeating.

      Since we’ve built from the ground up in a stripped van, it really wasn’t practical to figure out our loads empirically, as in your #3.

      So we designed for likely worst case power. Figured that was better and cheaper than under designing, then needing to rip out and rebuild. We did this potential solar overkill for one main reason that you didn’t really talk about…we live in the Pacific NorthWet, and expect to camp anytime during the year. We can expect our available sunlight to be wildly variable, from season to season, and trip to trip. We estimate that on cloudy winter days up here, we might be able to get about 10% of rated solar. For a much shorter day than Phoenix.

      For example, we can expect our compressor fridge alone to pull about 60AH/day, assuming it’s a ~5A load, and running at ~50% duty cycle.
      A 560W panel array at 10% on a cloudy 8 hours/day is 56W*8hr/12V = only 37AH, so we still are operating at a net loss. But at least we’ve extended our stay time significantly over battery alone.

      We chose the MPPT controller for similar “overkill” reasons. It is much better that a PWM on low light days at getting all the available energy from the panels. On bright days, the difference is not as significant.

      Of course, if we travel down to California in the summer, the panels will easily keep the batteries fully topped off, and at the same time directly power all the loads.

      Finally, we stayed with AGM for a couple of reasons:
      I think that the cost difference doesn’t yet make up for the Li charge density and weight advantages. Hopefully, when our AGMs die, Li cost will be more reasonable.
      Also Li still has some teething problems, like low temperature charging limitations…with, IMO, rather kudgy work-arounds.
      That’s about it.

      Thanks very much for your blog. It’s given us a TON of useful info as we’ve built our Annie the Van.

      Reply
      1. James - Post author

        Interesting point. I hadn’t considered the case of the self build. You really can’t just “go camping” in that case!
        I wonder – if you were to take the data from the NREL website, as calibrated for your system, and compare it to your actual solar yield – what would the difference be?
        If you do this, please chirp back and let me know the results. I’ve wintered in Seattle before… some days it seems like there’s about 4 hours of daylight…

        Reply
      2. Kyle

        SkagitStan,
        I am wondering how that system has worked out for you so far? I am about to start a van build and so I dont know my power usage yet. How many days have you been able to boondock in the worst of conditions? Do you run out of food, water or propane before you run out of battery? Thanks for any insights you could provide me with.

        Reply
        1. SkagitStan

          We’ve not taken Annie on any long outings yet, mainly because I haven’t finished the upper cabinetry and doors yet. That makes packing for a longer trip awkward. So we haven’t hit any limits based on propane, food or water.

          But, the solar and batteries have been able to keep up with our electrical demands, even if we are camped in deep shade for a day or two. Each trip to date, the solar has been able to get the batteries fully charged once we came out of the shadows and into partial sun. In the deepest shade, over 2 full days/nights, the battery SOC dropped to about 85%

    38. Alain

      Really great article. I wish I had it a couple of weeks ago, since I’m presently evaluating the electrical usage of our new setup, because I suspect there is maybe a problem. I used your approach, fully charged the battery until sunset using my shore hookup. It was only showing 12,4V after more than 30 hours of charging so I tend to think that already is telling me something. I turned off everything I could think of to get rid of phantom charges, except for the 2 compressor fridges. These were to be my baseline loads, since my main concern their proper functioning. I set up my GoPro on time lapse to record the voltage meter, but since I am as much a novice on GoPro’s as I am on RV’s I didn’t use the right setting and it’s battery dies after 2 hours of recording (if you read the book “The Martian”, you’ll know where I got the inspiration for that…LOL). The next morning I got up before sunup and the battery was reading 11,3V So after about 8,5 hours of use with no charging, my batteries dropped from 78% to 10%. Even if the battery was 100% initially, it would have dropped to 32%, way below the target of 50% Which is why the RV is now back to the dealership for verification. Luckily the owner of the dealership is a fellow engineer, so I can easily discuss this and not get a run around by somebody who doesn’t really understand.

      Reply
      1. James - Post author

        I’d say back to the dealer is the right response. That’s a compressor fridge, right? If that’s the case, they yeah – just running it overnight shouldn’t kill your batteries. Something else is wrong.

        And about the GoPro – their batteries just suck. They last about 8 minutes in use, or 36 hours in a turned-off device. When I did the time lapse video, I had to plug the GoPro in.
        (Don’t even get me started on GoPro. You’ll get an earful!)

        Reply
        1. Alain

          Right, both are compressor fridges. The “beer” fridge close to the door so you can just reach in…LOL. A total draw of 4,4 A when both are running according to the specs. I believe the batteries total 190 Ah. For 8,5 hours, at 50% duty cycle, they should only have pulled 18,7 Ah. And since its rather cold these days, I think the 50% duty cycle is probably overly conservative.

          I also went to the max of 380 watts solar panels available to compensate for our northern cloudy grey low efficiency days later in the year. I believe reading somewhere that the Travato has the basic 180 A alternator. I’m suppose to have the 220 A option, so a bit more power on that end but nothing like your extra 180 A.

    39. D. J. Heaton

      James,
      Excellent article. I wasn’t going to read as I feel pretty informed on solar, Ah, volts, capacity, etc., but read all as interested in how you presented…and very well done. I know subj. was solar not lead-acid battery, “wet” v.s. AGM v.s. gel, but I think a review on characteristics and advantages of gel & AGM over wet lead plate batteries would be a good review for you to consider.

      Reply
    40. Interstate Blog

      In the shortest possible way, let me re-emphasize a lot of the other points as they relate back to this one statement:

      “We do more ‘touring’ than ‘camping’, and so we drive most every day.” – In this case, the driving will charge your batteries more than solar ever will.

      ^^ IF your alternator is suitable (in some cases it need not be a second alternator, but it danged well better be properly-sized or else there is likely to be failure and grief), and IF your batteries are lithium.

      Especially given the myriad ways in which Class B RVs come OEM-equipped, perhaps the best way to clarify this for less-technical folks is to use the example of my husband and me. Our electrical upgrade sequence went like this:

      (1) We started out with a pre-owned stock 2007 Airstream Interstate which had one conventional Lifeline coach battery and no solar. We quickly found out that the MB Sprinter-installed alternator had no hope of keeping that coach battery charged, no matter how many hours we drove each day. Between the pathetic alternator and the resistance of conventional batteries to accepting charge quickly, it would never happen. The only way to charge the Lifeline was via shore power, and that was unacceptable to us because we are not campground people. We are primarily boondockers.

      (2) Not having a coherent long-term plan, we then DIY-installed 300 watts of solar, a controller, Trimetric monitor, etc. PRESTO! Lifeline battery issues totally solved. The solar was able to supply a smaller amount of charge over much longer periods, and we never had the battery run too low. Boondocking bliss ensued, UNTIL…

      (3) Six months or a year following solar installation, we decided that one Lifeline was no longer meeting our needs. I am self-employed and wanted to be able to function more like Technomadia (in fact I paid them for a consultation on remote connectivity) and I had to be able to run a full-sized, multi-monitor off-grid computer out of our rig. That required lithium. Given that my husband and I are both employed and “our time is not our own” as the saying goes, it took several months of free time – countless hours – to DIY-retrofit the lithium system. PRESTO! All computer issues totally resolved and I’m off to the off-grid working races with no more power limitations. Plus I can now run my hair dryer (important while working on the road when one must be well-groomed), microwave, etc. EXCEPT…

      (4) During the lithium retrofit, my husband also upgraded the alternator to a model that had more than enough capacity to serve both the Sprinter and the lithium battery recharge function. And lithiums have very low internal resistance, so this is entirely different from trying to charge a stubborn Lifeline – this charging happens far more efficiently. So, all of a sudden, our former-problem-child alternator starts blowing our solar out of the water (she said as she pounded her head on her desk). I no longer need the solar for my work – in a worst-case boondocking scenario, I just need to idle the engine for a while and let the alternator do its work. But we didn’t know that back when we first started planning solar improvements. Lithium was just beginning to appear in the market and, at that time, in the very beginning, we couldn’t see a path forward to DIYing it.

      So my husband and I ended up becoming those poster children to whom James referred above, the poster children who spent a few thousand extra dollars on solar only to later not need it. It looks cool, it’s fun to play with, I guess it’s a good back-up charging system of sorts, but it was a boat-load of money and work that we could have skipped if only we had known then what we know now. Retrospectively, I wish we’d put the money and effort somewhere else on the van because there are additional projects that we really want to do.

      Moral of the story: Know exactly what you want to accomplish in advance, and plan for it ruthlessly. We are two years further down the road now vs. when my husband and I started out on our electrical system improvements, so this determinative process should now be easier for people.

      Reply
      1. James - Post author

        Ouch!
        We previously had Lifeline AGMs in Das Bus, and we never had any problems with the stock Sprinter alternator giving us a good charge in a reasonable amount of time. Maybe we were just lucky, or maybe I had hacked something the right way, or maybe there’s something on the Airstreams that we didn’t have.
        In any case – we completely agree that it’s best to have a plan in place before embarking on knee-jerk upgrades!

        Reply
        1. Interstate Blog

          We were under the impression when we first bought our rig that the OEM alternator could successfully charge one lousy Lifeline coach battery. But in practice (ground truth always reigns supreme), we were getting only about 5% per hour recharge from it, even at fairly low states of charge (because remember that conventional battery charging rates are not linear), and even with a brand-new Lifeline (i.e., not damaged from running it too low). Therefore it would take about 5 hours of driving to raise it 25%, which was ridiculous (I’m oversimplifying the non-linear part). Given that we were using around 25% per each night of boondocking, we’d need to average a minimum of 5 hours driving per day, each and every day, to safely sustain the battery above its 50% usage threshold… that was not happening. Hence our solar retrofit.

    41. Jerry Fern

      This is the best basic solar primer I have ever read.
      Precise and to the point.
      You must have been a teacher in your previous life.
      Thank you for all the videos and info you publish.
      Very informative.

      Reply

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