Last week, “Sean” wrote me an email asking how many solar panels he needed to recharge his RV house battery.
“So I apparently have the wrong battery (deep cycle marine approximately 88 Ah) for my 21’ Coleman trailer but we’ve easily gone 3-4 nights dry camping without any problems. (Propane fridge and any a/c or microwave wave using the generator). Normal use of the water pump, led lights, stove fan and minimal heater use. What would you recommend for solar just to help recharge the battery?”
Sean was asking how much solar power he needed to recharge his 88Ah house battery.
Here’s what I wrote back (edited):
“Hello Sean, good to hear from you! Not sure what you mean by “wrong” battery as it sounds like your current battery is meeting your needs 🙂 Although I suspect if you dry camp a lot you’d get better battery life with a true deep-cycle battery.
“Anyhow, there’s a super rough rule of thumb that for each amp-hour of battery capacity you need at least one watt of solar panel array capacity. That’s not true in many cases, but it’s a good starting point. Your battery is rather small, so I’d recommend at least 100-200 watts.
“The quality of the panel matters just as much as the rating. Cheap solar panels won’t produce much power in the early morning or evening hours, or when partially shaded. If your Coleman RV doesn’t have a solar power system, you might consider an all-in-one “solar suitcase,” which doesn’t require permanent installation, just a battery hookup.
“If you want more than 200W solar, you should probably add to or upgrade your battery. Otherwise, the battery will be too small to absorb the energy generated by the solar panels.
“P.S. I should add that 100-200 watts of solar will EXTEND your dry camping time and help maintain a battery state of charge greater than 50%. It WON’T enable you to dry camp continuously. That’s a much bigger investment.”
Sean responded with a few compliments on my blog, but I won’t brag 😉
But I must confess: The “Rule of Thumb” I shared is often inaccurate. And here’s why: Solar panels are sized on two factors, not one! Solar panel arrays are sized based on BOTH your battery bank capacity AND your load demands.
So why did I recommend a (potentially) inaccurate Rule of Thumb to figure out how many solar panels for an RV? Well, read on!
Solar Panels Based on Battery Bank Capacity
Imagine that your battery is like a water tank, and the solar panel is like a pump. The bigger the solar panel array, the more energy gets “pumped” into your battery.
The “pump” and the “tank” should be matched in size, right? There’s no point in having a giant pump overfilling a tiny tank. Same thing with solar. There’s no point in having 2,000 watts of solar panels generating immense amounts of power, and only a tiny 88Ah battery! Most of the power will have nowhere to go.
That’s the premise behind sizing RV solar panels based on your battery bank. It’s an optimization strategy to ensure you’re not wasting money on excess power.
Solar Panels Based on Load Demand
But there’s a potential problem with sizing your solar panels based on your battery bank. What if the system is too small to keep up with your actual demands? (Not sure what your load demands are? Here are some tables showing common item and appliance amp draw).
Let’s say you want to run your microwave on battery power. A medium-sized microwave draws about 1,200 watts on High.
If Sean has an 88-Ah battery, his battery has about 1,056 watt-hours. Only 50% of that is available because it’s a lead-acid battery.
Factor in Peukert’s law, voltage drop, inverter efficiency losses … and the microwave would probably drain Sean’s 88-Ah battery from full to “dead” in 15 minutes.
So in Sean’s case, his battery can’t keep up with his microwave. But … neither can his solar panels! The microwave needs 1,200 watts; the solar panels can only supply up to 200 watts in ideal conditions (full sun, direct irradiance, no dust on panels, mild ambient temperatures).
So why did I recommend Sean only invest in 100-200W of solar power?
Here’s the Trick to Calculating How Many Solar Panels You Need for an RV
Here’s what I’m saying: There’s a world of practical difference and cost between sizing a solar panel array to recharge your battery versus sizing a solar panel array to continuously power your RV.
Sean told me he uses a generator to run the air conditioner and microwave. Smart man. (I hope it’s an inverter generator!) So he just needs a system to top off his batteries, not because he wants to live off-grid on BLM land. Different strokes for different folks.
Sean also told me he doesn’t use much power when dry camping: lights, range hood, water pump, and furnace.
Of those four, the biggest power hog by far is the furnace fan. A water pump draws more instantaneous amps (usually 7-10), but a furnace fan (around 4 amps for a small unit) runs continuously whenever the furnace is on. Sean said he had “minimal heater use,” and that’s the trick. In severely cold weather, his furnace fan would probably drain his battery in a single night!
Honestly, I’m rather surprised Sean’s getting 4 days/nights out of his 88Ah battery. Unless he’s over-discharging his battery, that’s only 132 watt-hours per day. That’s pretty minimal use. I’m guessing they don’t watch much TV or run any ceiling fans.
(I wonder if he uses his microwave a lot, and so he’s inadvertently topping off his batteries with the generator? Or if he drives from site to site, maybe he’s trickle-charging his battery through the umbilical cord?)
I also suggested maybe Sean should purchase a solar suitcase. If you’re interested in getting into solar but afraid of all the techno mumbo-jumbo, a packable RV solar suitcase is a great way to find out if solar is right for you.