A good question when building solar panels is how much voltage do you want it to put out? It's basically a question of balancing size and power. First though, let's talk about the minimum voltage that you'd need for the 3 main renewable energy system voltages: 12V, 24V and 48V.

A fully charged 12 volt battery doesn't exactly hold 12 volts. Usually the fully charged voltage is anywhere from 12.6 volts and 14.8 volts (depending on the type of battery you have). That means at a minimum your solar panel will need to put out at least 15 volts. Since each solar cell puts out 0.55 volts you'll need at least 27 cells to charge your battery. Since each solar panel should have cells that are multiples (2x2, 3x3 etc) your panel should have at least 28 cells for aesthetic reasons. It would look a little weird to see a panel with only 27 cells.

The same applies to 24 and 48 volt panels. Fully charged 24 volt batteries (usually two 12 volts wired in series) hold between 25 and 30 volts. Your panel should generate at least 30 volts (55 cells minimum). A 48 volt panel fully charged will be between 50 and 60 volts so you'll need to generate that much.

When I build my solar panels I usually try to use the space most effectively. It's not going to hurt your batteries if you pump a little higher voltage into them. Since 2' x 4' plexiglass is fairly cheap that's the size I go for. 36 cells fit quite nicely into that space which produces around 19.8 volts. It really cuts down on wasted material which is what I really like about it.

I also don't really recommend building 24 volt or 48 volt panels. The problem with building panels that big is that if something goes wrong (loose connection or something) you have quite a few connections to go through to find it. It's much easier to make 12 volt panels and simply wire them in series to achieve 24 or 48 volts. That way if one breaks you're only out 72 watts and not 140 (or 280).

So just remember that when you're building your solar panels not to build them to 12 volts. You have to account for the fact that batteries always hold more voltage fully charged than what they're rated. It's best to build 12 volt panels simply because the bigger you go the more complicated it gets.

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