Over the next few pages we will build up a more-and-more complex system and try to explain things as we go. Ok, for the most basic systems we need two things: 1. Solar Panel(s), and 2. Battery(s). All your purchasing choices revolve around the battery so that is the first thing we need to work out.
First Decision: The Battery(ies).
For our example we will be running a 12 Volt, 6 Watt pond pump for 2 hours each day. A deep cycle 12 volt lead/acid battery has the right voltage. What size one though? The different sizes are all described in Amp Hours. We work this out by, Watts = Volts x Amps, 6 = 12 x Amps. The pump needs 0.5 Amps for 2 hours, or in other words 1 Amp Hour ( 0.5 Amps x 2 hours = 1 Amp Hour). So we need, at least a 12 Volt 1 Amp Hour battery as a minimum.
After reading a bit about batteries you quickly discover if you want your batteries to last a long time, it's best not to drain the below about 50%. So double whatever estimate you have worked out so we hopefully won't need to go below half way. So now we buy a 12 volt 2 amp hour battery which is pretty small.
The Next Decision: The Panel(s)
As you read about batteries you will also quickly realize that a 12 volt battery is pretty much dead at 12 volts and full at about 12.7 volts. So for our 12 Volt battery example we want to not only charge above 12 volts but at about 13.5 volts. So we should look at about a 14 volt panel.
Now we need to work out what wattage panel you need. You need to supply all the amp hours you will use in 24 hours in the period that you have full sun, lets say 6 hours to be safe. For example, we need to use 1 amp hours every 24 hours and we need to generate that in a 6 hour period. So over 6 hours we need to generate 1/6 of an amp every hour. Since Watts = Volts x Amps, 14 Volts * 1/6 Amp = 2.33 Watts and, as always to play it safe (and to make it a round number), lets go for 3 Watts. So now we can go out and get our 14 Volt 3 Watt Panel or several panels that add up to 14 Volt 3 Watts. See our electricity basics page for more information on adding voltages together.
That's the basics. A battery and a solar panel to match it that covers what we take out. Unfortunately though the system needs some management to make sure it is running efficiently and not breaking our equipment. On the next page we will go through this.
A BIG thanks to Roy Montgomery for helping us with some much needed gammer fixes on here.