How do I compute the panel wattage to battery AH size?

simagic
simagic Solar Expert Posts: 100 ✭✭✭✭
in Solar Beginners Corner #1
I previously posted a thread mentioning confusion in computing panel wattage to battery AH size. Upon searching this very informative site, it boils down to this as to what is typically "typed" into the thread as the answer for this computation ( I'm using a 10% charge as an example and 800 watt array also as an example as i copied and pasted it from somewhere here on this forum) My question is how do you interpret that "typed" info into real and actual math......... Here's what would be typed here in this forum.......

((( 800 watt array * 0.77 rate of charge * 1/14.5 volt charging * 1/0.10 rate of charge = 425 AH@12volts )).

I can certainly read that someone has typed that info and "come up with" 425 and I absolutely understand what all the numbers mean. What I don't get is how do I write this down on a piece of paper to come up with the 425. ...........Is that 800 (multiplied by).77, ........then the total of that (multiplied by) the sum of 1 divided by 14.5.......then that total (multiplied by) the sum of 1 divided by .10.... ( I'm basing that upon thinking that (*) means multiply and (/) means divide the number after the (/) into the number before the (/), as in 1/14.5 meaning divide 14.5 into 1......I can't come up with 425 that way...... What I'm 'actually" trying to compute is, .....if i have a 500watt array feeding my 220ah battery then what is my charge rate?? Soooooo, when this is answered could you use the 500watt panel as the example....I'm thinking this might be helpful to a lot of us 'out here" and not just me........thanks
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  • Rybren
    Rybren Solar Expert Posts: 351 ✭✭
    #2
    Re: How do I compute the panel wattage to battery AH size?

    The math works out to:

    800 times 0.77 = 616

    616 divided by 14.5 = 42.48

    42.48 divided by 0.10 = 424.83

    In your case, 500 * 0.77 * 1/14.5 * 1/0.X (Charge Rate) = 220

    when we solve for X, we find that the charge rate is ~ 12%
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  • simagic
    simagic Solar Expert Posts: 100 ✭✭✭✭
    #3
    Re: How do I compute the panel wattage to battery AH size?

    Sorry to be so friggin thick....but 500 (times) .77 = 385...........385 divided by 14.5= 26.55........26.55 ( I'm good up to here).... Now you say """* 1/0.X (Charge Rate) = 220. When we solve for X, we find that the charge rate is ~ 12%""".. ......... How did "WE" solve for X. I don't get the math to "solve for" X . Did you just keep on putting in and trying different percentages ( 10,11,12,13) til it came out to approx 220
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  • Rybren
    Rybren Solar Expert Posts: 351 ✭✭
    #4
    Re: How do I compute the panel wattage to battery AH size?

    It's 26.55 divided by X = 220

    so...

    X = 26.55 divided by 220

    X = 0.12068181818181818181818181818182

    or roughly 12%
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  • simagic
    simagic Solar Expert Posts: 100 ✭✭✭✭
    #5
    Re: How do I compute the panel wattage to battery AH size?

    Thank you.......... Thank you
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #6
    Re: How do I compute the panel wattage to battery AH size?

    OK, I am the guy that types it out that way to read like a sentence from left to right (by the way, 0.77 is not a rate of charge, it is a derating factor for the solar array derating and charge controller efficiency:
    • 800 watt array * 0.77 panel+controller derating * 1/14.5 volt charging * 1/0.10 rate of charge = 425 AH@12volts

    The proper way to write it would be:
    • (800 Watt array * 0.77 panel+controller derating) / (14.5 volt charging * 0.10 rate of charge) = 424.8 Amp*Hours at @ 12 volt battery bank

    For a 500 watt array and 220 AH battery bank with a 12 volt battery bank:
    • Rate of Charge = 500 Watt array * 0.77 p+c derating * 1/14.5 volts charging * 1/220 AH battery bank = 0.1207 = 12.1% rate of charge
    • Rate of Charge = (500 Watt array * 0.77 p+c derating) / (14.5 volts charging * 220 AH battery bank) = 1/220 AH = 0.1207 = 12.1% rate of charge

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: How do I compute the panel wattage to battery AH size?

    A few observations:

    1). It's a backwards way to do it. Normally you size the battery to handle the loads and then the array to recharge the battery.
    2). You are unlikely to see peak current at the Absorb Voltage level as the battery is mostly charged then. This is why I use nominal Voltage instead as that is the point when maximum current matters most. This ends up allowing a smaller array to battery ratio.
    3). If you're using a PWM controller instead of an MPPT type the current from the array will only be the Imp of the panels in parallel, not derated Watts divided by charging Voltage.
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