Notice, that’s “need” not “want”. There’s a difference.
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 525 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.
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 it 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.
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.
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.
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.)
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.
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
- 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:
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.
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.
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.
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:
- 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.
- 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.
- 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.
- Start timing once the generator is off, the sun is down, and the rig is unplugged. Use the rig normally.
- 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.
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.
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.
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… 🙂 )