When sizing a battery bank for an off-grid home there are any number of variables that can come into play, making for a very thorny problem if you think on it too hard. Unfortunately, some of these variables—such as the amount of power your home will use each day—are often little more than guesses. After living off-grid for 16 years, I still don’t know the average daily energy usage for our 1,600 square-foot home, although I’m confident it’s between 7 and 9 kWh, most of which goes to power an electric fridge and freezer, two computers, and a 1 ½-horsepower well pump. You can—and should—do your best to arrive at an energy-usage range before buying batteries, and for this task you might find the worksheets in my book, Power with Nature, helpful. Just be sure to err on the high side in your calculations.
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Next, you will need to decide how many days of zero sunlight you want your battery bank to carry you before you have to fire up the backup generator. One or two days are realistic, but beyond that it gets expensive.
So let’s say you want a battery bank that will supply 9 kWh/day for two days of zero sunlight, for 18 kWh of stored power. The next question is, how far are you willing to discharge your batteries? The accepted limit is 50 percent, and if we go with that number, we’re up to 36 kWh.
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We still need to add in system inefficiencies, however, which run roughly 15 percent, so 36×1.15=41.4 kWh of battery capacity needed to supply 9 kWh/day for two days. How many batteries does it take to get there? Sixteen standard L16s will just about do it, or six Trojan IND23 industrial-grade 4-volt batteries, or…
The possibilities are too numerous to list. Just remember that each series string must add up to the system voltage, and the fewer series strings the better.
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This article was originally published in The NEW PIONEER™ Fall 2015 issue. Subscription is available in print and digital editions here.