How big is it?

How large should your battery be?

Courtesy of Otmar Ebenhoech

In response to a question posed on the Burning Man alternative- energy email list, Otmar made the following helpful comment. I asked for and received permission to Webify the answer and put it on the Web.

All right, Let's get the tech talk going...

Earlier I was asked a question about how large a battery is required for a solar system. Here's a copy of what I wrote in the hope that someone might make use of it, or at least it will start a barrage of questions. :-)

As for battery size, it's all about figuring how many watt hours you want to use between sunny days. At BM we can probably push it to the limit because it's not the end of the world to run out. If we assume we have enough sun every day, then a simple daily power budget will determine the capacity required of the battery.

For example: Let's say you only use a stereo for 8 hours a night. It could take 1 amp at 120V. (this varies a lot depending on size and volume) so, 1 amp at 120 is 120 Watts (Volts * Amps = Watts) You need 120W at 120V for 8 hours. Multiply this to get Watt hours (120 * 8 = 960 Whr) So, you need 960 Watt hours AC power. An inverter is about 85% efficient (a bit pessimistic) so you need about 1130 Whr out of the battery. Assume 12V battery and the formula "Amps * Volts = Watts" and you see that you need about 94 Amp hours of capacity. Now, a deep cycle battery (one that doesn't mind being run hard) might give close to that for a 60 to 70 lb battery.

So now it's just a matter of figuring out what you want to run, how long, and how much power it needs. Then it's just number crunching to put it all together.

BTW, when using lead acid batteries, capacity is closely related to weight, so it's sometimes easier to figure about how much weight in lead you need and use that as a measure to avoid all the conflicting ratings on batteries.

When looking at battery ratings remember the quote which I believe Edison said "There are liars, there are damn liars, and then there are battery salesmen."

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