Going solar? Be realistic about your power needs


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.

photo: Greg Illes
Greg Illes is a retired systems engineer who loves thinking up RV upgrades and modifications. When he’s not working on his motorhome, he’s traveling in it. You can follow his blog at www.divver-city.com/blog.

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.

newest oldest most voted
Notify of
Carson Axtell

I am designing my solar energy system to meet my needs for a minimum of three days of poor sunshine, which basically means putting as many solar panels on the roof as I can fit, and maybe carrying a portable unit, as well. I also intend to carry a small inverter generator of about 800 watts just for recharging the battery bank, and to store all the energy in lithium cells since they seem to be the most efficient and economical long term electric storage setup. (See Will Prowse’s argument for the economics of lithium batteries over lead-acid batteries: https://www.youtube.com/watch?v=Rp8Hspi4BC4)

Gene Bjerke

Rather than getting all bound up in calculations and what-ifs, I just filled the remaining space on my roof (which in a Class B with other stuff up there amounted to two units) and called it done. So far everything has worked out okay. Admittedly, I don’t boondock a lot and I can always turn on the generator if needed.


One thing I have asked about in many places, and the best answer I received was “It depends”, is this. Can you have TOO MANY panels? I mean, if a full battery is a 2 cup measuring cup, you can trickle water into it or hit it with a fire hose and it will never hold more than 2 cups. I will be building a system for my small utility trailer I bought (and am awaiting notice of completion so I can go pick it up), but I am an occasional traveler and will never actually live in the thing like full time RV people do. My power needs will be geared toward 5-7 day trips maybe every 6th week or so, and the consumption will be a CPAP machine, a 12v fridge, a laptop and a 10 inch O2 Cool fan. I will not cook in it because it is so small the smell would linger forever. Plus, part of going to see cities is enjoying their local cuisine.

I had planned on 2 flexible 100w panels (the roof is curved, as are the roof racks where they will be mounted) an MPPT charge controller and a 100ah lithium battery (to keep the weight down). I have a feeling that the math is not as simple as to say “If 1 panel rated at 100w charges my battery in X hours, then 2 panels the same size should charge my battery in half of X.”

I am reading a lot of books and watching a lot of video, but as with all youtube videos that are posted by people who received product for free in exchange for their positive reviews, I take those videos with whole shakers of salt.

Anybody here have any guidance they can offer me?