Solar power is a popular subject for those hoping to save money while reducing their dependence on fossil fuels. In areas with abundant sunshine, it is not unusual to see solar panels covering the roofs of home after home. For RVers, who travel in a small, portable home, solar power can be an important part of the electrical supply. Yet many people do not fully understand how a solar power system works, which can lead you to waste money on an oversupply or find yourself stuck without electricity when you need it most. Here is what you should know.
The most important thing to understand is that solar panels are only part of the equation. They are part of a larger system that works together to power your rig.
Solar panels are one of several potential energy sources for your RV. The others include shore power (plugging in to the power box at your campsite), your vehicle’s alternator (which builds up energy as you drive), and a generator (if you have one). Each of these sources may be “on” or “off” at any given time. For example, shore power is only on while the rig is plugged in, and solar panels are only on when the sun is shining.
Your RV has a set of batteries, which store power from all the different energy sources. This allows you to use electricity when none of the energy sources are on. However, it is important to note that once the batteries are full, they’re full. You can’t store more than 100% of your rig’s battery capacity. And unlike a home solar power system, you can’t sell excess power to the electric company. This means that it is very important to correctly size your RV’s solar power system. Too small, and you may run out of power when needed. Too large, and you are just wasting money.
To figure out your solar power needs, first you need to understand the output of all your available energy sources. There is a LOT of math involved, and you will need to read up on the specifics of the power system you have. For example, your alternator may put out as little as 40 amps or as much as 180 amps per hour when the engine is running. Shore power comes in at 120 volts, and then it goes through a converter or an inverter to become 12 volt storable power for the battery. Inverters and converters have capacities ranging from 40 amps to 100 amps per hour. Generators run through the inverter or converter, and have capacity limits of their own.
It is important to note that solar panels are generally the least powerful of all available energy sources. A 100 watt panel (common size for RVs) actually generates 100 watts only at peak efficiency (78 degrees, no clouds, solar noon, on the equator during the equinox—or, in other words, lab conditions, not real world conditions). In this best case scenario, it will generate just over 8 amps. If you have three of these panels, you can generate around 25 amps per hour at best.
This does not mean that you should forget about solar power. Instead, it means that you need to dig deeper to figure out whether solar is right for you and, if so, how much you actually need. If you almost always connect to shore power, or you typically drive several hours every day, solar power may not do you much good. If you boondock, or camp without hookups, for several days at a time, though, solar power may be essential, even if you have a generator.
You will need to understand some basic terminology to properly size your RV’s solar power system:
Watts vs. Amps: Electrical loads and solar panels are rated in watts. A watt is a standard unit of power. RV battery capacity is measured in amps (short for amperes), which are standard units of current. To convert between watts and amps, you also need to know the volts, or units of electrical potential. From there, it’s easy:
Watts = Amps * Volts
Amps = Watts / Volts
Since watts are the standard unit of power, they do not change. You can have different numbers of watts, but a watt itself is consistent. The same is not true for amps, since the amount of current can change, nor for volts, since the amount of voltage can change. In fact, it’s easy to see this voltage change in your RV, since you know that you can bring in 120 volt AC (alternating current) from shore power, or 12 volt DC (direct current) from the batteries.
So now the math is easy to determine how many amps are generated by a 100 watt solar panel feeding power to a 12 volt battery.
Amps = Watts / Volts
Amps = 100 / 12
Amps = 8.33
It is true that resistance, battery charge and type, etc. can all change the numbers a little. But for purposes of solar power calculation, it is safe to assume 12 volts for a battery, 100 watts for a solar panel, and 120 volts for shore power or generator power.
Amp-Hours: To learn how much usable power you actually have, you need to understand amp-hours, or the total capacity of the battery. Amp-hours are typically based on 20 hours of total available use, unless otherwise stated on the battery. To figure out how many amps per hour, simply divide the amp-hours (AH) listed on the battery by 20 (or another number if specified on the battery):
For a 100 AH battery:
Amps = Amp-hours / 20
Amps = 100/20
Amps = 5
So the battery will deliver 5 amps (at usable current, defined as 10.5 volts or higher) for 20 hours. You can draw more amps for less time, but the capacity decreases as the discharge rate increases. A 100 AH battery will only give you about 40 minutes at 100 amps of usage.
You can use amp-hours to figure out how fast your battery will charge as well. For a 100 watt solar panel, we already know that it produces 8.33 amps. In this case, 1 amp of current flowing for 1 hour charges the battery by 1 amp-hour. So 8.33 amps of current create 8.33 amp-hours of charge per hour.
Batteries: Lead-acid batteries are the most commonly used type in RVs. Somewhere in your rig, you will find a panel that shows you approximately how fully your batteries are charged. It is inexact, especially if your batteries are not “at rest” (not draining or being charged for at least 30 minutes), but it will give you a reasonable idea. It’s not harmful to drain lead-acid batteries completely, but to maximize their lifespan, try not to let them go below 50% without a recharge. Newer lithium-ion batteries are more user-friendly for a variety of reasons, but due to costs, they are not yet popular in RVs.
Types of Solar Panels: Solar panels come in two types—monocrystalline and polycrystalline. Monocrystalline panels are slightly more efficient, but they are also more expensive. If you have a small RV with little roof space, monocrystalline panels can be physically smaller for the same output. If you have the space, though, cheaper polycrystalline panels work just fine.
Flexible vs. Flat Solar Panels: Flat solar panels are less expensive and more durable than flexible panels, and usually have a longer warranty. However, flexible panels are lighter, more aerodynamic, easier to mount, and in some cases (such as on an Airstream) more aesthetic. Choose whichever makes more sense for your rig.
Note that flat panels can be tilted to point more directly at the sun, while flexible panels cannot. However, on a small RV solar system, the additional energy you will gain (minutes at best), is really not worth the trouble.
Solar Charge Controller: Solar panels typically provide 16 to 20 volts of electrical potential. An RV battery can handle 12 volts. A solar charge controller is a standard part of an RV solar installation. It regulates the voltage and current to prevent battery damage. The standard controller is fine unless you have an unusual setup or use your RV in extremely cold conditions.
So far, we’ve been working with the assumption that a 100 watt solar panel will generate 100 watts of power. But as we noted in Understanding Capacity, it won’t. There are numerous factors that affect the actual amount of solar power generated, from the time of day to the ambient temperature to the cloud cover to the level of pollution.
Fortunately, experts have come up with a rule of thumb. On average, a 100 watt solar panel will generate 30 amp-hours per day. If you need more specifics, you can find quite a few free solar position calculators online, or you can go with the National Renewable Energy Lab’s free solar power energy calculator at http://pvwatts.nrel.gov. It isn’t perfect, but it is remarkably close most of the time.
The next step is to figure out how much energy you actually use per day in your RV. There are innumerable free energy calculators available online, so you might be tempted to start there. All you have to do is click on each listed appliance and input how many minutes or hours per day you use it. Easy, right? But do you know exactly how long you use the toaster every day? Or the microwave? Or the air conditioner? The best you will get is a wild guess that could be remarkably far off from your actual energy consumption.
A far easier, more accurate, and more fun solution is to actually take your RV out for a camping trip. Make sure your batteries are charged by nightfall, and then remove all sources of power. Turn off the generator, unplug the shore power, and do not turn on the engine.
Use your RV as you normally would. Go out if that’s part of your routine. Watch TV as usual. Run the hair dryer after your shower.
Keep an eye on your battery as you go about your daily RV life. The goal is to drain your batteries to empty (or to 50% if they are lead-acid). When you reach your goal, calculate how much power you used.
As an example with simple math, let’s say that your batteries drain to 40% in three days. That means you used 60% of your total battery capacity. Let’s also assume that you have a total of 200 amp-hours in battery capacity. So you used 60% of 200 amp-hours, or 120 amp-hours, in three days. Divide by three, and you used 40 amp-hours per day.
200 amp-hours * 60% used = 120 amp-hours
120 amp-hours / 3 days = 40 amp-hours per day
Now all that is left is to balance the equation. Remember, your goal is to have the power input roughly match the power output. In this hypothetical example, your power output is 40 amp-hours per day. By the rule of thumb, a 100 watt solar panel inputs 30 amp-hours per day into your batteries.
40 amp-hours per day = 30 amp-hours per panel per day * X panels
40 amp-hours per day / 30 amp-hours per panel per day = X panels
X = 1.33 panels
So you would need 1.33 100 watt panels, or one 133 watt panel to match your solar power needs. You can’t buy a third of a panel, nor can you buy a 133 watt panel. But you can buy a 160 watt panel. That will cover your average daily needs, with a little spare energy in case the weather is awful or you use a bit more power than average on the occasional day. And since we’re using the 50% battery drainage rule, there *is* extra battery power available in an emergency.
Solar power can be an excellent way to generate electricity for your RV. To make the most of your investment, though, it is vital to understand exactly how it works and how much power you need. Taking the time to go through the calculations can save you a great deal of money and frustration in the long run. At Auto Boss RV, we are always happy to help you select just the right solar power system to meet your needs.
Whether you are in the market for a new RV or you need service on your current RV, Auto Boss RV has the experience you need to solve even the most challenging issues. Contact Auto Boss RV today at 480-986-1049 to learn how we can help!