Step 7: Batteries
But of course, I am not using all of my electricity all at once, all during the peak sun of the day. At mid-day I am likely to be a t work, and if I am home, I certainly am not using any lights. That means in order to make use of those 225 watt-hours, I need to be able to store them somewhere.
Here is where an independent system really varies from a utility-intertie system.
The intertie system takes all the power generated and not used at midday, and pumps it back into the grid, so that your neighbors are actually using the solar power from your roof (and indirectly paying you for it, via the utility company, which takes a cut).
Then at night, when the sun isn't shining, you draw your power from the grid. If you work during the day, you technically aren't using solar power at all. Its more like you are leasing roof space to the power company for them to generate solar power for the grid, and then they give you free power later to compensate.
An independent system stores the excess daytime power in a battery bank, which you then draw from at night (or when its cloudy).
This means you need to have a big enough battery bank to store all the power your panels are generating.
In my case, I want a battery bank of at least 225 watt-hours, so that if I don't use any electricity during the peak sun hours on any given day, I can store all of it and none gets wasted.
For longest battery life, they should never be drained below 50% capacity, so its better to get twice the calculated minimum capacity.
Batteries are generally measured in amp-hours, not watt-hours, so dividing 225 watts by 12volts gives me 18.75 amp hours of minimum battery reserve. Considering battery life, I should have twice that much, or 37.5 amp hours, so that the batteries never get 100% drained. Ideally I'd like to go even higher than that, to have plenty of reserve for cloudy days - say, enough for 5 days in a row of minimal sun light. 37.5 x 5 days = 187 amp hours.
I started out with two 115 amp hour batteries, for a total of 230 amp hours. I just recently bought an 8D battery, which have about 220 amp hours, bringing my system to a total reserve of 450 amp hours, so I should have more than enough to supply power to my 12v power draws for even a week of cloudy weather.