Here Comes the Sun, Part 2

If your eyes glaze over when you read about Watts, Amps, Amp-hours and math of any kind, you may want to skip to Part 3. That said, this section is not very technical and the nasty arithmetic can be skimmed over. If you really want to learn about RV solar, two places to start your education are Jack Mayer’s overview of RV electrical and solar and AM Solar’s education section.


Because of our residential (electric only) refrigerator, we need a fair amount of electrical power each day. The rest of our trailer actually is pretty efficient. The lighting is entirely LED and uses little power. The water pump, furnace and hydraulic system are all standard for RVs, although if we needed heat frequently while off grid we’d consider supplementing the furnace with a portable propane heater that does not require electricity. We do have some parasitic loads, but the battery monitor shows them as only a few amps (@ 12V). We can use propane to heat water and cook with. The 42″ TV is pretty energy efficient (compared to TVs of yesteryear) and we don’t have to turn it on if we’re short on power. Appliances like the convection oven and air conditioner just aren’t an option when running on solar, so we do have to live a little differently if we’re not connected to “the grid.”

Based on preliminary, limited real-world data that we collected from our three nights of dry camping so far, we’ll need about 250 Ah @ 12V for a typical day. This assumes only moderate conservation and includes recharging computers, an hour or two of TV and Oma’s morning coffee. The 250 Ah will vary somewhat – running our furnace will require more power, and we can skip the TV, limit computer recharging and generally reduce our usage some if necessary, but 250 Ah/day seems to be a reasonable target for planning purposes.

One rule of thumb is that you get about 5-6 hours worth of “peak sun” each day (assuming a sunny day). This is very much an approximation; a sunny day in June can deliver more power, and even a super clear day in December is challenged by the low-on-the-horizon sun and shorter daylight hours. Our panels will put out about 5.5A of current at peak sun. (Our solar controller can actually do a bit better than this, but I’m using 5.5A for planning purposes.) For general estimating purposes, that’s about 28-33 Ah per day per 100W panel (5-6 hours peak sun * 5.5A). Divide 250Ah by the amps per panel, and we need eight or nine 100W panels to meet our needs on an “average” sunny day. More panels are always nice – we can run more stuff if we have more power, and on a cloudy day or if we’re in partial shade, extra panels would help compensate.

Eight to nine panels are a lot of panels for our moderate sized fifth-wheel. And a lot of cost. Our plan was to compromise some – put in a solar controller that could handle ten panels, but only install six initially. That would mean that we may need to run the generator in the morning (while Oma is brewing her coffee), but an hour a day of generator time might make up the difference on a sunny day. And for a single overnight, we could skip the generator all-together – the batteries would not get fully recharged, but that could happen when we plugged into electricity once we reached our destination. Installing more solar panels later would also be comparatively easy and inexpensive if we decided we wanted more.

That was the plan. To my dismay, AM Solar (our planned installer) stopped carrying the 60A solar controller that I was planning to use. Since we were only planning to put six panels in, their 40A system would be adequate (it can handle seven 100W panels), but if we ever wanted to install those 8+ panels, we would have to run additional wires to the roof and add a second solar controller – much more effort than just adding a few panels to an existing controller using existing wiring. I considered going with someone other than AM Solar, but in the end decided to stick with AM Solar because the 40A solution would cover our initial install plans (which we might never expand anyway) and switching to a different installer would probably have delayed our installation to next winter.

We scheduled our appointment four months in advance, which was just in time as their calendar was already filling up. (A fellow Excel owner stopped in when we were getting our installation done and was told that the first opening now is five months out.) Appointment made, we went back to planning the rest of this trip.

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