Re: How to calculate a battery capacity


Simon Dice wrote:
I'm working on a project in wich I need to have a system running 24hrs
a day , power supply will be solar panels feeding a battery. Can any
one share with me formulas on how to calculate de Amp/hr for the
battery and the power of the solar panel?
Systems will consume average 80watts and need to keep running on a few
clowdy days as well.
Thanks for the assistance!

FYI, it's Amp-hrs (since the units are multiplied), not Amp/hr (which
would be "amps per hour", which is different).

When you divide the amp-hour rating by the battery current (in amps),
you will get how many hours the battery should last. Eg., a 10
Amp-hour battery lasts 10 hours at 1 amp, 5 hours at 2 amps, etc.

As others have said, though, for higher amperages, er, uh, I mean
CURRENTS, the amp-hour value is actually less.

As for the solar panels ... I don't know your specific location, but a
rough calculation gives:

Incident power of sunlight (about 1000 Watts per square meter),
times the efficiency of solar panel (I'll use 12%),
times the fraction of day that panel receives "full sun" (I'll use 6
hours out of 24),
times the fraction of days that are sunny (I'll use 2/3) ***

equals:

(1000 W/m^2) x (0.12) x (6/24) x (2/3) = 20 W/m^2.

The actual, useful power is 20 Watts per square meter. So for an 80W,
24-hour-per-day system, you'd need 4 square meters of solar panel.
That's just a ballpark figure, there are people who know the relevant
numbers better than I do, and you'd need to factor in the efficiency of
charging the battery from the solar panel too.

Hope this helps,

Mark

*** p.s. on the "sunny day" factor: use what is reasonable for your
area. Also, if your area occasionally goes a few days to a week with
rainy weather, you'll need a battery (or several batteries) that can
run for a few days to a week without recharging.

.

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