This is a continuation of the Living in a Small RV series. It will be a bit boring for anyone who isn’t interested in solar power, but I wanted to write it like this because I had a tough time finding all of this information tied together.
There are two classes of devices in an RV that need electricity, AC and DC. The DC ones run off the battery and these include things like lights, the water pump, the vent fan(s), and anything you can plug into a 12v socket.
The AC ones are primarily the air conditioner and the microwave. They get their power from either plugging the RV in to a campsite or 120v socket at a house or by running the generator.
The generator uses gas (somewhere between half and a full gallon per hour) from the main tank. Because the generator is too loud and costly to run all the time, I almost never use mine. As a result I never use the air conditioner, favoring a vent fan that pulls air through the windows and uses very little electricity.
A quick primer on electricity which will be focused more on practical application that scientific accuracy:
Think of Watt-hours as a bucket of electricity, and Watts as a hole in the bucket.
For example, my batteries store roughly 2400 watt-hours. That means that I can run a 24 watt light bulb for 100 hours. Or I could run a 200 watt TV for 12 hours.
Amps are just Watts divided by volts. So on a 12v system like all RV DC systems (as opposed to the 120v AC system), 1 amp is equal to 12w.
The best way to work with these numbers is to just multiply amps by volts and work in watts.
You can only store DC power in the battery, not AC power, so if you want to power on of your AC appliances, you need to get an inverter, which just converts DC to AC.
The problem, though, is that devices designed for AC power tend to use a lot of watts since AC power is usually so plentiful. For example, I could only run my air conditioner for about an hour and a half on a full charge IF I had a really expensive high wattage inverter.
Here’s a rough breakdown of wattage:
- Laptop when charging an empty battery – 60w
- Laptop when not charging battery – 20w
- Light bulb – 20w
- Water Pump (for showers, sink, toilet) – 50w (only when actively being used)
- Vent fan – 12 – 35w (depending on speed)
So, just for fun (and because we’ll need it in the next section), let’s see how much electricity I use in a day.
I usually have one light on usually from 10pm to 3am. That’s five hours times 20w. 100 watt-hours.
I have my laptop on for that same amount of time, which is another 100Wh.
The fan is on for 11 hours a day on low and 13 on medium. 130+285 = 415Wh.
Half an hour for the water pump. 25Wh.
The laptop is plugged in for about 6 hours, so that’s another 120Wh.
If we add that all up, I use about 760Wh per day.
How do I get that power? Hmm….
I could run my generator for a short period of time every day. Or if I drove a lot the batteries would just charge from the inverter on the car (the same system that powers your headlights).
But… I prefer to not have to use gas and to have my electricity just magically regenerate itself.
And that’s where the miracle of solar comes in. In an average day in Texas we get an average of 5 hours of full sunlight. That means maybe we get 1 full hour, two half hours, four quarter hours, etc, since the sun is only directly above us for a short period of time. If you live much farther north, you can count on 3.5-4 hours of sunlight.
You can get all sorts of solar panels, ranging from 1 watt to about 225 watts, which is the amount of watt-hours they generate in one hour. Anything more than that and you start having to get two panels and chain them together (which is very easy to do if you need that much power).
So in a five hour day a 1 watt panel can make 5 watt-hours of electricity, which is totally useless. A 225 watt panel can make 1125 watt-hours of electricity.
I need 760 watt-hours of electricity per day, so I need at least a 156 watt panel.
But… 156 watts isn’t going to cut it. It’s important to have excess capacity so that you can save electricity for cloudy days or for days where you use more than expected. There are also a bunch of sensors and stuff that run from the battery and some inefficiency in charging the battery which increase the actual daily requirement.
I went with a 200 watt panel. Maybe a 180 watt panel would have been enough, but I’d rather overdo it than find myself running out of power.
You can’t just hook a solar panel directly up to a battery, though. A 12v panel can actually put out up to 17v in sunlight, which is way too much for a battery. Instead you buy a charge controller which regulates the charge.
There are a few different kinds of solar controllers, but the best ones to buy are the ones that have MPPT technology. The regular kind are fairly inefficient, especially during the winter, so you get up to 30% more power for using MPPT. For a more detailed and technical explanation, read about MPPT here
Of course, unless you’re only going to use power while it’s sunny out, then you’ll need a battery (or two) to store the charge. You generally want to buy a battery that has twice the capacity of your daily solar output. This is for two reasons.
- Frequently discharging a battery below 50% will decrease it’s usable life.
- You want to be able to store extra power as a reserve.
Batteries are rated in amp-hours, which are basically just watt-hours divided by 12. In my case I generate 1000 watt-hours per day, so I need approximately 2000 watt-hours of capacity. Divided by twelve to make amp-hours, that’s 166Ah. Luckily my RV came with two batteries that total 180Ah, so I was all set. Since I also use my RV as my car, I can actually use that extra capacity because the batteries charge as I drive.