Finding the right panel
Now that you know how to convert between power (P) in watts, current (I) in Amps, and voltage (E) in volts, you are well on your way to having enough information to buy your first panel.
As we determined previously, I need my panel to generate an output of 1.89 Amps. I need to take my battery up to 13.7 volts for it to be considered fully charged. By using the Power formula, P = I * E, I can now plug in the numbers.
P = 1.89 * 13.7 = 25.89
So, I need at least a 25.89 watt panel. Truth be told, I could get away with only a 25 watt panel. And that’s just what I did. Kind of. But more on that later.
Why can I get away with a 25 W panel? Simple…. I turn stuff off when I don’t use it. That means, when I have to go get a drink of water, use the bathroom, take a shower, grab something to eat, etc., I turn the radio off. Simply removing power when the radio is not in use, even for a short time, will save us over the course of a day. That 0.89 watts will end up being something that we really do not need to worry about.
However, this 25.89 is actually the minimum size we want. Remember I noted earlier about 75% efficiency on a cloudy day? We really want to add 25% to the panel size to account for this. So, 25.89 + 25% = 32 Watts. Given this buffer, I would say a 30-35 watt panel would be just the ticket.
But wait, there is still more!
We can hook up a panel directly to a battery and let it charge. However, this is a really bad thing to do for a number of reasons. First off, batteries are not cheap. We want them to last a good long time. I have a set of 85 Ah gel cells that are currently on their 5th year of service. Why? Because I take care of them. They are around $200 each new. I want to be able to get as much use out of them as possible.
The second, and most important reason, however, is because batteries are dangerous. Overcharging a battery will kill it by causing gas to vent, and the liquid gel inside to end up evaporating, but it can also cause the battery case to melt which can lead to an explosion or fire.
We must therefore use a charging circuit on our battery bank!
There are quite a few chargers out on the market. Some are rather simple and some are super complex with all sorts of signal outputs so you can do remote monitoring of the battery pack, etc. These do work just fine. What I prefer to use though is a charger that came in kit form from CirKits.
The model I use is the SCC3, which is a 12V charger that will handle 20 A worth of solar panels. What I like about this controller is that it will pulse the battery when the battery is done charging. This pulsing seems to help keep the plates of the battery clean, thus helping the battery hold a charge after years of service.