ECN Forum Forums General Topics of the Trade General Discussion Area Solar Power: The Basics AND Business of it.

I was using C&D's runtime charts for UPS12-400MR which are high quality deep-cycle AGM batteries used in industrial UPS systems, but cost $300-400 a pop. And unfortunately forgot to divide by 6 electrochemical cells per jar even though I know better, doh! (72 cells = 12 jars.) Serves me right for trying to do the math in my head. I was assuming 12 hours of run-time. Call it 12 batteries, and chop my numbers by a factor of 6. In winter months in northern lattitudes, night lasts a lot longer than day, and you would need larger solar panels and additional batteries to compensate. And that's of course not considering any type of electric heat. You'd need an entire neighborhood worth of roofs to get enough solar power to heat a home in New England in the winter.

Cheapest I'm seeing 85Ah AGM batteries online is about $150. You DO get what you pay for- I clicked a link for one of the cheaper ones, which claims 85Ah on the title, but then in the fine print says you only get 36 minutes run-time at 75 Amps. In this case, the 10 hour run time is slow enough we probably would get close to 85Ah from it, but that sort of deep cycling is very hard on batteries, and there's no word as to the # of cycles. I *know* the $300 C&D above will perform as advertised, and will do so for several hundred deep cycles before performance degrades to 80%.

You just can't beat a generator for this type of backup power, though- even if you have to keep a couple cans of fuel on hand. For the price of just one battery, you could buy an awful lot of gasoline. AND you can do it on rainy days as well as sunny. What happens, for instance, to your 12 hour battery bank during a blizzard that knocks out power lines, coats your panels in a foot of snow, and is accompanied by clouds that blot out the sun for 2 or 3 days? At roughly 1 battery per hour of 800W load, that adds up.