what is the Minimum PV watts needed to my battery banks AH size......

rgearhead
rgearhead Solar Expert Posts: 38 ✭
in Solar Beginners Corner #1
just wondering what Ratio there is for PV to battery bank AH size , my little bank is 690 ah'' and if i was to take them down 70% in a day, what size PV would i need to recover in one day ??





thanks....
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  • verdigo
    verdigo Solar Expert Posts: 428 ✭✭
    #2
    Re: what is the Minimum PV watts needed to my battery banks AH size......

    Are you taking them down to 70% or taking 70% from them?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: what is the Minimum PV watts needed to my battery banks AH size......
    rgearhead wrote: »
    just wondering what Ratio there is for PV to battery bank AH size , my little bank is 690 ah'' and if i was to take them down 70% in a day, what size PV would i need to recover in one day ??





    thanks....

    Other than verdigo's legitimate question about whether you mean state of charge or depth of discharge ...

    Without the system Voltage it is not possible to determine the Watts required for charging.

    For example: 69 Amps peak charge current on 12 Volts requires 1200+ Watts using a PWM type controller or 1075 using an MPPT type controller.
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  • rgearhead
    rgearhead Solar Expert Posts: 38 ✭
    #4
    Re: what is the Minimum PV watts needed to my battery banks AH size......

    sorry, It's a 12 volt system, and a tristar Mppt chargers, and also meant used 30% and 70% left in the Bank.....
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: what is the Minimum PV watts needed to my battery banks AH size......

    Well then there's your answer: about 1075 Watts +/- assuming 'normal' usage. Of course this would need to be adjusted to the nearest capacity possible from available panels. A little higher being preferable to a little lower. The 10% rule-of-thumb works well with 25% DOD in most cases and 30% would not be too far different (depending on what your insolation is like where you are).

    Note that a TriStar is only 60 Amps maximum, so one of these would be 'maxed out' under the circumstances. That is not to say it wouldn't work, though.
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  • PNjunction
    PNjunction Solar Expert Posts: 762 ✭✭✭
    #6
    Re: what is the Minimum PV watts needed to my battery banks AH size......

    The other factor is to know your solar-insolation hours. These are different from sunrise-to-sunset hours, where panels typically have very little output early morning and late afternoon. These sun-hours center around noon.

    From what I remember last time I looked, Chicago has about 4 hours insolation in the summer, but the winter is dreadful at something like 2 hours max.

    In winter, you'd have to have a very big system to recover in only 2 hours of solar insolation time. And if you did, that usually means using batteries that can handle huge amounts of charging current, ie pure-lead AGM's, or LifePo4's for example.
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