By Greg Illes
So you’ve decided to “go solar.” Congratulations! No doubt you’ve read a lot about panel types and inverter technologies, and maybe received a few quotes. You have probably become familiar with the basic arithmetic of energy management by now, too.
The advice you’ll typically see is to size your panels according to how much power you use each day and, of course, this makes eminent sense. If you’re using 40 amp-hours a day, then you need panel output adequate to recharge those forty amp-hours. If you figure five hours of good sunshine in the middle of the day, then your panels will have to produce eight amps, or about 100 watts.
8 amps x 5 hrs = 40 amp-hours
So that’s it, right? A 100W panel and you’re good to go? Not really.
There are several reasons why the calculation, while entirely valid, is not so simple. Let’s review:
Sunshine Availability — If you’re in the shade or it’s overcast, you’ll get less output from your panels. Even my shade-tolerant amorphous panels drop by 50 percent or more with heavy shade.
Panel Angle — All panels have their power ratings at 90 degrees to the sun. This is never achieved in real practice, even with panels that can be tilted. Even if you get them aimed perfectly at 10 a.m., the sun keeps moving. At a 45-degree angle, you’ll get about 30 percent less power output.
Charge Acceptance — Batteries will not necessarily accept all the available power. As they become more fully charged, acceptance declines. So not all the panels’ power will be absorbed. This physical limitation can only be compensated by more aggressive charging (more power) when the batteries are in a discharged state.
The bottom line is that a typical solar application might need two or three times as much power rating (and sunshine) as is actually used by the rig. This is because all the inefficiencies add up to only getting 1/2 or 1/3 of the rated panel power actually into the batteries.
To be safe, be conservative. Figure an efficiency factor of no more than 50 percent for how much panel power you’ll need. If you’re still uncertain, make sure your system is designed for expansion so that you can add a panel or two if needed. The photo shows my motorhome, using flex panels of 68W each. After going through the learning curve, I ended up going from two, to four, to eventually the six panels shown.
Don’t despair: Despite the uncertainties, you’ll love your solar system and won’t ever want to be without one again.