Renewable Energy Engineering Forum

Solar PV System Estimating

April 30th, 2008

Cy writes in with a question:

Chris,
I have a quick question and I know there are a lot of variables so just an estimate or approximation is okay. If I wire three panel together in series and buy a controller how long will it take to recharge a marine battery that is at 11 Volts
Cy
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Dear Cy,

For a back of the letter calculation I believe is about right….. if you have 3 – 15 watt panels then 45 watts instead of 160 watts used below, so would then take about 4 days from dead, and most controllers won’t work from dead, this one will!!! and supports marine apps and the outdoors, so you should be fine….
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Dear Cy,

At 11 volts you have a very dead battery….like you said many variables and many assumptions…

Let’s assume 110 amp hour battery,
assume it holds 1100 watts, and
assume it is 90 % efficiency at battery charging including the controller and wiring

(1100 watts / .9) watt needed to fully charge your battery, or 1222 watts needed

then take your panel watts and add them together

then estimate for the sun available to charge them….

say 80% full sun (a pretty sunny day) and 10 hours of available sun

let’s assume 160 watts (as rated) of panels and no load on the batteries

160 watts * .80 sun * 10 hours/day = the amount of watts you can get in one day, in this case 1280 watts available per day…

so in effect less than one day to charge your battery if all the assumptions above were true…

adjust for your parameters, watts, amp, hours, available sun and you’ll get a closer answer….

Hope this helps, I did it pretty fast…..

Chris

Entry Filed under: Renewable Energy, Solar Electric Photovolatic (PV)