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.


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

    1. 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
    2. 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
    3. 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
    4. 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
    5. 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
    6. 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
    7. 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
    8. 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
    9. 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
    10. 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
    11. 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
    12. 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
    13. 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
    14. 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!

    15. 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
    16. 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
    17. 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
    18. 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.

    19. 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
    20. 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.

    21. 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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