Batteries wired in series? Add up the voltage.
Batteries wired in parallel? Add up the amp-hours.
Modern RVs aren’t the basic camper vans of the old days, when the only appliances you might need to plug in were a radio, a few lights, and maybe a coffee maker. Modern motorhomes have all the creature comforts you’d find in any suburban home. There are compact versions of all the major appliances you’d want: LED TVs, coffee makers, microwaves, refrigerators, dishwashers, a furnace, even clothes washers and dryers.
But even though these are scaled down in size and use less electricity than full-sized appliances, they can still use a lot of energy. When you’re not at a hookup or running on a generator, that power demand is satisfied by the battery bank, usually called the house or coach battery.
If you’re away from a hookup, the amount of power that the house battery can supply before becoming depleted is limited. You can fire up a generator, but that’s not very enjoyable if you’d rather hear the sound of birds than the rumble of an internal combustion engine.
Also, most RVs and boats use lead-acid batteries, which require some care about how they are discharged. It’s possible to permanently damage a lead-acid battery with a single deep discharge, so it’s important to have a little knowledge about how to properly maintain the charge level of your battery.
Because of these demands, RVers and boaters with a cabin cruiser often turn to solar panels to meet their power requirements. Solar can not only help keep you powered when you’re not near a hookup, but can prolong the life of your batteries, helping you to save money in the long run.
If you only ever go to RV campsites with electrical hookups, it may not be worth it to bother with solar panels, which can involve modifications to your vehicle if you plan to roof-mount them.
But if you frequently go boondocking, especially for days or even weeks at a time, solar panels can be a great idea. They’ll keep you powered while you’re camping off the beaten path, and you won’t have to worry about the noise and fumes of a generator.
Similarly, if you’re a boater with big electrical requirements, solar panels can keep your beer cold and the stereo blasting for long periods of time without you having to interrupt the party by firing up your engine to charge the battery.
If this is you, then this guide will introduce the basics of everything you need to get started.
First, let’s start with the basics of solar photovoltaics, and explain the components that make up a solar system. In this article, we’ll mostly refer to solar systems for RVs, but the basic principles also apply to boats and other off-grid applications.
One of the good things about solar systems for motorhomes, boats, and other small off-grid applications is that they are simpler than solar for your home. Most solar houses are grid-connected, which involves a two-way connection with the electric grid, and a utility meter that can measure that usage. You also need a sophisticated inverter to turn the DC power from your panels into AC power that your house can use.
Neither of those components are things you have to worry about for an RV. There are really only two components that you need to add to your motorhome to make it solar: the solar panels, and a charge controller.
The main part of an RV photovoltaic system are the solar panels, which are sometimes also called solar modules. Solar panels are semiconductor devices that turn sunlight into electricity. When photons of sunlight strike the surface of a solar cell, electrons are excited and flow along the wires of the panel, creating electricity.
The electricity that solar panels produce is direct current. This is useful for an RV, because the house electrical system is based on 12 volt DC power. (This is true even if your RV includes a separate 120 volt AC system, which can be powered either by the batteries via an inverter or a campground hookup.)
This basic photovoltaic technology is the same whether you are installing solar on your home or on your RV. However, while it’s possible for you to yank a panel off the roof of your house and install it onto the roof of your RV, and this might work just fine, there are some differences between residential solar panels and those intended for mobile applications.
These differences mostly have to do with with size and the type of frame, which are affected by the type of technology used. We’ll review these below.
The most common type of solar panels for RVs are based on silicon, just like the solar panels commonly used for homes and large utility installations. Silicon solar panels are either monocrystalline or polycrystalline, which refers to the manufacturing process used to make the primary element in the panel.
Our guide to solar panel specifications goes into greater detail, but the important thing to know is that monocrystalline silicon is generally higher efficiency, which means they turn more of the incoming sunlight into electricity. The tradeoff for higher efficiency is usually higher cost, although this is not a rule. In fact, if you shop around, you can often find a similar price-per-watt for polycrystalline and monocrystalline panels.
Solar panels intended for the home are quite large, usually about 60 inches by 40 inches in size, and produce anywhere from 260 watts up to almost 400 watts, depending on the efficiency. This is too large to either mount on the roof of most RVs or keep in the RV storage bay, which is why RVers usually go with smaller panels in the 100 watt to 150 watt range.
These panels are less than half the size of a residential panel, making it easier to fit these panels around the various vents on your RV roof.
One option with conventional solar panels is to store them out of the way while you’re driving, and then pull them out and place them on the ground once you’ve set up at your campsite. Some panels have frames with built-in kickstands designed for this purpose, or are designed to fold up neatly for storage.
You can also pretty easily make a DIY frame with some PVC pipe. Here are some examples.
One thing different about solar for mobile applications is that thin-film solar is more commonly used than in residential homes. The reason for this is that thin-film solar cells are lightweight, flexible sheets. They are so bendable that you can even roll them up like a sheet of paper.
This property of thin-film solar means that you can simply glue them flush to the roof of your RV or boat. Unlike conventional solar panels that have an aluminum frame almost 2 inches thick, thin-film panels might only be 0.1 inches thick - barely any profile at all.
The nice part of this thinness is that when you mount them on the exterior of your vehicle, the force of wind on the panel is minimal, which means that the proper adhesives are plenty strong to keep them in place - no screw holes in your roof are required. Plus, the thin profile means that there is no added wind noise when you’re driving down the highway.
The disadvantage of thin-film is lower efficiency, sometimes much lower. We surveyed a number of thin-film solar panels on the market, and most produce only about 0.025 watts per square inch. Meanwhile, we found a high efficiency monocrystalline solar panel from SunPower that produces about 0.1366 watts per square inch - more than 5 times as power per square inch. This low efficiency can be a problem if you have high power requirements but your roof space is limited.
Thin-film can also be very expensive per watt, mostly because of lower manufacturing volumes. Thin-film simply isn’t as popular as the crystalline solar used by homes and utilities.
However, there’s one useful niche for thin-film, which is portable solar for hikers. If you’re an RVer who also likes to take off for long hikes and needs electricity to keep your smartphone or digital SLR charged, it might make sense to have a thin-film solar panel that you can use to keep your RV battery charged and also have the option to unplug it and take it with you when you go on a hike.
If you want flexible solar panels, your best bet are a relatively new product on the market: bendable crystalline solar panels.
These panels are made of polycrystalline or monocrystalline solar cells, just like those used in conventional solar panels, but these are designed to withstand significant bending. This type of solar module isn’t as completely flexible as thin-film - for example, you can’t roll them up into a tube, but they are flexible enough to conform to the shape of your roof. They’re also thin enough that they can be glued or velcroed into place.
These panels don’t use glass and an aluminium frame, but are instead laminated between polymer sheets. Because of this, they are much thinner and lighter than a conventional panel. In fact, these panels may weigh only 20% as much per watt of power as a conventional solar module. Lower weight means easier handling, easier mounting, and less weight for your vehicle to drag around.
Finally, because they use conventional silicon technology, these panels tend to be cheaper than thin-film panels. While ongoing advances could one day mean that thin-film solar cells might be cheaply embedded into all kinds of materials, such as fabrics and windows, for now the much more widespread manufacturing of silicon solar cells means that they are more cost-effective.
This kind of flexible solar panel is a great choice if you want to mount the panel on your vehicle directly. They’re light and thin, and so they can be placed unobtrusively on a roof and generate no wind noise. But if you want to use these panels as moveable units - that is, you want to store them when not in use and pull them out only when needed - because they’re so light and floppy, you would need to build a frame to place them onto. A sheet of plywood, velcro tape, and a DIY stand made from PVC pipe would do the trick. But if you plan on using solar panels this way, you would probably be better off with a conventional solar panel.
Before we discuss the next component in your photovoltaic system, it’s a good idea to understand some things about lead-acid batteries.
Your RV has two types of batteries: a starter battery, which provides the energy to crank the starter motor when you turn the ignition key, and the house or coach battery, which supplies power to the living space when you aren’t connected to a hookup.
The starter battery is charged by the alternator, and is used only for the starter motor. It doesn’t supply electricity for the living space. It’s actually a type of lead-acid battery that’s constructed differently than the house battery: it’s designed to supply a very high amperage for a short amount of time, but doesn’t have a large power reserve. As far as a photovoltaic system goes, we’re not going to worry about your starter battery.
Your house or coach battery is what powers the appliances in your RV. In contrast to the starter battery, the house battery is a deep-cycle battery that is designed to store a lot of electricity and supply a steady current for a long period of time. As the same suggests, it’s capable of handling deeper discharges than a starter battery.
All lead acid batteries have the same basic chemistry. The terminals of the battery are connected to plates of lead metal, and the two plates are separated by an electrolyte of sulfuric acid. How that electrolyte is held inside battery differs. There are three basic types:
Flooded batteries hold the sulfuric acid electrolyle in liquid form. This is the least expensive type of lead-acid battery, but this advantage does come with a tradeoff in terms of higher maintenance requirements. One thing to be aware of is that flooded batteries need to be stored in a ventilated area, because charging can result in the production of hydrogen gas, which is flammable.
That hydrogen gas is the result of water molecules splitting into hydrogen and oxygen, so that means you also need to periodically check the electrolyte levels in the battery and top it off with distilled water. Allowing the level to drop too low can damage the battery. Be careful when doing this: you’re dealing with sulphuric acid!
Another thing to be aware of is that flooded batteries must be held upright, or else the electrolyte can spill out, creating a hazardous situation.
Gel cell batteries contain the electrolyte in a thick silica-based gel. The gel is in contact with the lead plates, and electrons can move freely through the gel. Because of this design, the battery can be sealed, except for one-way valves that allow the release of hydrogen gas in the case of overcharging. There are no electrolyte levels to check or water to add, so this type of battery is sometimes called “maintenance free”. These batteries can be stored on their side because there is no liquid to spill. These batteries are about 2-3 times more expensive than flooded batteries.
Absorbed glass mat (AGM) batteries also avoid a liquid form of electrolyte by containing the acid in a fiberglass mesh. Like gel cells, AGM batteries are called maintenance-free because there are no electrolyte levels to check. They are also sealed and can be stored on their side. They are also more expensive than flooded batteries, but are also more popular and usually less pricey than gel cell batteries.
Now that you understand the basics of how lead-acid batteries work, it’s time to review how you charge your battery.
The state of a charge of a lead-acid battery can be measured by its voltage. The voltage level will be highest when fully charged, and will slowly drop as the battery is drained.
To recharge the battery, you have to apply a voltage that is higher than the voltage of the battery. For example, if you have a 12 volt battery, you have to apply an electrical charge that is a couple volts higher than 12v. The optimal voltage necessary depends on the model, so you should consult the manufacturer’s datasheet for this detail. As an example, here’s a snippet from the datasheet for one Trojan battery model:
You can see that there are three charge voltages specified. Bulk charging is used when the battery is significantly depleted. The bulk charge phase will rapidly bring the battery back up to 80-90% full. When the battery is full, float charging is used to maintain the battery level at 100%, because lead-acid batteries will slowly discharge over time, even when not connected to a load. (This is sometimes called trickle charging.)
There is actually another charging state in between these two, which is called absorption charging. This is a slower charging phase that gradually and safely brings the battery up to about 98% full.
The last charge state listed for this battery, the equalization charge, is a special maintenance step that is used periodically to remove a sulphate coating that can build up on the lead plates of the battery over time. Be aware that not every battery is designed to tolerate an equalization charge, especially gel cell and AGM batteries. Check your battery’s manual to find out.
Battery chemistries differ in how they handle the battery being depleted. Lead-acid batteries don’t have a “memory effect” like Ni-Cad batteries do, meaning that they aren’t adversely affected by habitual partial discharge cycles.
Instead, to maximize the life of lead-acid batteries, short discharge cycles are best, and deep discharges should always be avoided. Even a single dischange of more than 50% can permanently reduce the capacity of the battery.
You can maximize the life of your battery by maintaining a high charge level, keeping it at least 80% full in typical day-to-day usage.
This is one of the reasons why solar panels are a good idea for battery systems. The constant flow of charging current from the panels can help maintain a high charge level and prolong the life of your battery.
As you can see, the safe and proper charging of a lead-acid battery is a multi-step process that involves carefully monitoring voltages to ensure that they stay within the correct ranges. Because batteries cost hundreds of dollars, you don’t want to ruin your battery by charging it improperly. That’s the job of the charge controller in your photovoltaic system.
The charge controller takes the electricity from your solar panels as the input and sends out electrical current with the correct voltage to your battery. It will automatically monitor the voltage of the battery as it’s charging, adjust the output as the battery “fills up”, and switch to float (trickle) charging to keep the battery safely topped up without overcharging.
As an additional safety mechanism, some charge controllers include a temperature sensor to ensure that the battery isn’t dangerously overheating while it’s being charged.
There are two types of charge controllers:
Pulse width modulated (PWM) charge controllers are less expensive and more simple devices that manage the flow of power to your battery by sending it in controlled pulses. These are cost-effective units that are appropriate for small photovoltaic systems. They are best used with solar panels that have the same voltage as your battery bank. For example, to charge a 12v RV house battery, you should pair it with a single 12v solar panel, or multiple 12v panels wired in parallel. If you pair a PWM controller with solar panels that have a higher voltage than your battery, the controller won’t be able to use all of the current and will waste some of the generated electricity.
Maximum power point tracking (MPPT) controllers are more sophisticated units that are useful for larger or more complicated solar setups. Rather than sending simple pulses of electricity, an MPPT controller has electronics that determine the optimal voltage and current that is appropriate for the battery’s state of charge. The result is that an MPPT charge controller is more efficient at charging and can extract around 30% more electricity from your solar panels than a PWM controller.
Another feature of MPPT controllers is that you can use solar panels that have a higher voltage than the battery. For example, if you have a 24v volt panel, an MPPT controller will step that voltage down so that it can charge a 12v battery without loss of efficiency.
MPPT controllers can also handle larger loads, so if you are designing for a large solar system, you might need an MPPT controller to handle the higher amperage.
The tradeoff for these benefits is higher cost. For a small system, it may be more cost effective to buy more solar panels and a cheap PWM controller than to buy a more expensive MPPT controller that efficiently uses a smaller number of panels. You should do the math to figure out what the most cost-effective solution is for your setup.
As you can see, while there are some important details to understand about how an RV solar system works, the overall picture is actually quite simple:
The solar panels are wired to the charge controller, which is wired to the battery bank. Simple!
But before you go out to buy any hardware, you need to figure out how many solar panels you want, and this depends on how much electricity you use in your motorhome.
If you want to fully power your appliances with solar, you’ll need to figure out how much power you use. There’s two methods we’ll describe. But first, let’s start with some electrical basics.
Power usage, as far as your battery is concerned, is measured in amp-hours (Ah). Check the label of your battery: it’ll tell you the capacity in amp-hours. One amp-hour of capacity means that the battery will support one amp of current for one hour.
You may have multiple 12v batteries connected in parallel, in which case you can add up the amp-hours to find your total capacity. For example, if you have two 12v batteries with 100 Ah capacity each, wired in parallel, your total capacity is 200 Ah.
But let’s say you instead have two 6v batteries wired in series. In that case, you add up the voltage while the amp-hours stay the same. For example, two 6v batteries with 100 Ah of capacity each, wired in series, equals 12 volts and 100 Ah.
Here’s a visual explanation of series versus parallel wiring:
Once you know the capacity of your battery bank, you can see how long you can operate your appliances without recharging.
For example, let’s say you have a roof vent that is specified to use 3 amps at full power. If you run that fan continuously, after 24 hours it will have consumed 72 amp-hours.
That’s less than the sticker capacity of a typical 100 Ah battery. But remember that discharging a lead-acid battery past 50% will shorten its lifespan, and in day-to-day usage you want to keep your battery charge level at 80% or higher. This means that if you want to run your fan continuously and you have only 100 Ah of capacity, you will need to recharge your battery before the end of the day.
The next step is to calculate how much power all your appliances together use. There’s two ways to do this.
Many RV hookups include a utility meter, just like the one you would find in your home. If you examine it, it will have a readout in kilowatt hours. This tells you how much electricity you use over time.
When you arrive at the campground, plug your RV into the hookup and write down the number of kilowatt hours on the meter.
Then, go about your normal business. Watch TV, fire up the furnace, turn on the lights and fans. After 24 hours, check the meter again. The difference between the two numbers is your kilowatt-hour usage per day.
Now, you have to figure out how many amp-hours that translates into. First, take another look at the meter for a voltage rating. It’s probably a 240 volt meter.
Multiply by 1,000 to convert your kilowatt-hours into watt-hours, then divide by the voltage to get amp-hours.
For example, let’s say that you use only one kilowatt-hour (1,000 watt-hours) in a day. 1,000 watts divided by 240 volts is 4.2 amp-hours.
To find out the load on your battery, multiply that by 20 (because 240 volts divided 12 volts is 20).
In our example, 4.2 amp-hours at 240 volts translates to an 84 amp-hour load on our 12 volt battery.
This method is an easy way to estimate the power usage of your RV without having to manually add up the power usage of each of your appliances, but it’s not perfect. There are line losses to account for, and when you first hook up you may be using some power to charge your house battery. To make this more method more accurate, you can take the measurement over a longer period of time and make sure that your house battery is already charged.
The other method is more work and involves manually tallying up the electricity you use.
Start with your 12 volt appliances. Find out the amperage for each appliance, and then estimate how much time you use it for. If the appliance label doesn’t list amps, it will list watts. To convert that to amps, divide watts by volts. For example, a 1,000 watt coffee maker at 12 volts equals 83 amps.
Next, divide that by the number of hours you use the appliance a day to get amp-hours. If you use that coffee maker 30 minutes a day, that translates to 41.5 amp-hours per day.
Add that up for all your 12 volt appliances, and you get your total consumption in amp-hours.
If you have a 120 volt system, do the same process. But when you’re done, multiply your calculated amp-hours by 10 because your 120 volt system is getting its power from the 12 volt battery.
For example, 8 amp-hours at 120 volts converts to 80 amp-hours at 12 volts.
There’s one last another complication with the 120 to 12 volt conversion, which is inverter losses. Check the specs for your inverter to find its efficiency. It might be 90% efficient, but electricity is also lost due to electrical resistence, especially with DC power. So you might end up getting only 80% efficiency.
So add another 20% to your 120 volt appliance number, which means our 80 amp-hour example becomes 96 amp-hours.
Add up your usage from both your 12 volt and 120 volt systems to get your total electrical usage.
Let’s say that you use 200 amp-hours in a day. Multiply that by 12 volts to convert your amp-hours to watt-hours. In our example, we use 2,400 watt-hours a day.
Now, divide that by how many hours of full sunshine you get to find out how many solar panels that is. Let’s say we get the equivalent of 8 hours of full sunshine a day.
2,400 watt-hours divided by 8 hours is 300 watts.
That conveniently happens to be three 100 watt solar panels, which would work out nicely.
However, you should understand that the label efficiency of solar panels is rarely achieved in the real world. Shading, soiling, and high temperatures make your solar panels less efficient. Read our guide to solar panel specifications for an indepth understanding of this topic, but as a general rule of thumb you should expect your solar panels to achieve, at best, only 75% of their nameplate rating.
That means our 100 watt solar panel will probably only generate 75 watts of power in real conditions, which may mean adding one more solar panel to compensate.
Estimating how much full sun you’ll get does involve guesswork. How many cloudy days will there by while you’re camping? You can still generate solar power on a cloudy day, but not nearly as much. Depending on the answer, you might decide to expand your system with more panels, or live with the fact that you’ll need to either curtail your electrical usage or sometimes fire up your generator.
Once you know how many panels you need, the next thing to decide is between flexible and rigid solar panels. If you’re going to mount them on your roof, flexible panels should be your first choice because of the less hardware involved, lower wind profile, and non-penetrating installation.
Placing solar panels on the ground is also a perfectly good choice - sometimes better, because you can move your panels around to find the best sunlight and while still having the option of parking in the shade. If you’re going that route, you’ll want conventional solar panels and a tiltable bracket.
Renogy makes flexible monocrystalline solar panels at a good price point. While these aren’t the most efficient solar cells available (that distinction belongs to SunPower) these panels are a great balance between quality and cost. Renogy is a California-based company and offer a 5 year materials warranty and a 25 year power output warranty.
The no-drill installation requires adhesives, and looks something like this:
There are a number of adhesives that can be used to attach the panels to your roof.
One nice option is velcro tape, which lets you easily remove the panels. Make sure that it’s water-resistant and rated for outdoor use. Another option is VHB tape, which is very secure but also removeable.
100 watt solar panels are a common choice because they’re small enough to fit around vents and other projections on your roof, and not too bulky to handle if you want to use them on the ground. They’ll fit nicely in most RV storage compartments.
This is a 100 watt monocrystalline panel made by Renogy. It’s 42 x 20 inches in size and weighs 16.5 pounds, so it’s small enough to handle easily. Like most panels in this class, it has a 5 year materials warranty and a 25 year power output warranty. It’s currently selling for $120 on Amazon. (Get current pricing)
If you’re buying one of the Renogy solar panels above, you could get one of their kits that package the panels, charge controller, brackets, and wiring all in one. This can simplify your choices, especially if you aren’t sure of the best charge controller to pair with your panels.
Once you’ve got your solar panels and charge controller, the next thing to do is to hook everything up.
If you refer to the diagram earlier, you can see that all that’s really required is to connect your solar panels to the charge controller, which gets connected to your batteries. If you’re lazy, none of this needs to be permanently installed. You could throw everything, wires and all, into a storage bin and pull it out when you want to set up.
However, most people will at least mount the charge controller near the batteries. Because there are a large variety of different RV configurations, it’s not really possible to give explicit instructions for every situation. However, here are some good products to help you with mounting.
If you’re using conventional solar panels and you want to attach them to your vehicle, there are a couple options available. A common way is to use aluminum z-brackets, but these require you to penetrate the roof of your vehicle with screws. One good alternative is to use drill-free corner brackets. Instead of screws, these are attached to your vehicle using very strong adhesives.
This one is made by Renogy. It’s preferable because it has six attachment points rather than the four offered by other manufacturers.
It’s currently selling for $27. (Get current pricing on Amazon)
The panels themselves are screwed into the brackets, but adhesive is used to fix them to your roof. You might wonder if glue is strong enough to keep the panels from flying off when you’re driving down the highway. There are two extremely strong glues that we can recommended: a polyurethane adhesive made by Sikaflex, and VHB (very high bond) double-sided tape made by 3m. This tape has a tensile strength of 90 pounds per square inch, so it’s not like the duct tape you have in your drawer.
Use the links below for pricing.
Tiltable aluminum mounting brackets let you angle your panels to maximize the electricity you harvest. These are used with conventional solar panels and can be installed on the roof of your vehicle with screws and sealant.
If you don’t want to drill holes into your roof, these brackets also work perfectly well as solar panel stands that you can simply place on the ground.
This model from Link Solar comes in a range of sizes, including lengths that fit 100 watt panels, which are commonly used for RVs. The black knobs make angle adjustments easy, and the whole bracket will fold flat for driving or storage.
The bracket comes in 22-inch, 28-inch, and 41-inch lengths to fit a variety of solar panel sizes. It starts at $46. (Get current pricing on Amazon)
If you roof mount them, they need to be screwed in, and then you seal the hole with an appropriate sealant. A good choice is Dicor’s Lap Sealant, which will adhere to both TPO and EPDM roofs. It also sticks to aluminum, mortar, wood, vinyl, galvanized metal, fiberglass and concrete, so it’s pretty much an all-purpose product. One tube costs $10. (Get current pricing on Amazon)
The standard connector for solar panel wiring is the MC4 connector. They come in male-female pairs that simply plug together.
If you have multiple solar panels in your system, you’ll need MC4 branch connectors to join the individual panels into a common wire that feeds your charge controller. (Search MC4 branch connectors on Amazon)
When dealing with DC power, you want to use a fat cable to minimize power loss due to electrical resistance. 10 gauge (10AWG) is a good choice. Look for 10AWG cables designed for solar, because they come with MC4 connectors built in.
We hope you found this article useful. If you’ve made it this far, you’ll have a learned a lot about solar for mobile off-grid situations. If you want to understand more about solar in general, you can read these other articles: