When a 12V solar panel is rated at 100W, that is an instantaneous rating, if all of the test conditions are met, when you measure the output, the voltage will be about 18 volts and the current will be 5.55 amps. Since watts equals volts times amps, 18 volts x 5.55 amps = 100 watts.

We sell a lot of solar panels for DIY off-grid solar projects. Generally, when we design a solar power system, we start with your loads, what you are trying to power, and from there you figure out what size solar panel you need. But now, we are going to look at it the other way around, what can you power with a 100W solar panel

What can I do with that power? Well, first of all, I need to store it in a battery so that I can use it later when I need it. So, I’m going to use at least a 10 amp charge controller to manage to put the power into a deep cycle battery that can be charged and discharged on a regular basis.

What size battery do I need?  I have my 400 watt-hours(Estimated power generation/day) that I’m making, and I’m putting it in a 12-volt battery. Because watts divided by volts equals amps, 400-watt-hours divided by 12 volts equals 33.3 amp hours. Even though I’m putting it in a deep cycle battery, most batteries still don’t like being drained down more than halfway, so I’m going to make sure I get a battery that can hold at least twice as much power I will be using, so I’ll only use half of the power in it. 33.3 amp hours x 2 = 66.6 amp hours.  The amount of power a battery can store changes depending on the temperature of the room it’s in.  I’m also going to be converting the DC power from my battery to AC using an inverter, and I’m going to lose about 10% of my power through that conversion, so 41.3 amp hours / .95 = 88.3 amp hours.

 But trust me on this one, the sun doesn’t shine every day here. Not by a long shot. So I need to figure out how many days without sun that I need to store the power for to get me through those sunless days. Let’s say I need it to last me a day without sun. 

Great, I’m going to get myself a  deep cycle battery, that’s 100Ah 12V. 

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