solar watt/battery AH ratio?

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System
System Posts: 2,511 admin
in General Solar Power Topics #1
Is there an optimum solar watts vs. battery amp-hours relationship?

Assume solar watt-hours matches daily use on sunny days.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: solar watt/battery AH ratio?

    yes, there is a kind of relationship, but it's solar charge amps to battery ah capacity in its simplest form even though you could make the watts equivalent as watts=voltsxamps. as to an optimum charge rate that has been and would be debated somewhat. if i had to generally guess, the optimal rate would probably be about 10% and would vary to battery type and even manufacturer so this isn't in stone.
    now in the bigger scope of how i think you mean, you could say that amps consumed by the loads+all efficiency losses+all resistive losses+any maintenance/charge losses all in ahs should=the generated ah of the pvs and/or other generating source(s) over the course of a day. this is in ahs because your batteries at a c10 charge rate want more than 10hrs to charge when there is only say 5 or so hrs of full sun for the pvs to charge with per day.(varies depending on location and time of year) there could be a problem if you take the batteries down to 50% dod and wish to charge them back up in 5hrs as that 5hrs represents a 20% plus charge rate and if your batteries aren't rated for that much of a charge rate then you shouldn't take them below a 75% dod so that a 10% plus charge rate could be utilized. some batteries aren't meant to go much beyond 10% and many do go to the 20-30% range. my sunxtender agm battery is rated at an outstanding 100% charge rate.
    although lengthy, does this help answer your question?
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
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    Re: solar watt/battery AH ratio?

    A couple of good “rules of thumb” are to size a battery bank for ~six days of average use (= three days of autonomy with discharge down to 50% SOC), and to size the PV array to provide a mid-day charge current of ~5% of battery Ah capacity.

    In somewhat conservative and rough -- but easy to remember -- terms, these two ROT’s combine to suggest sizing a PV array at one Watt (STC) per Ah capacity of a 12 V battery bank. For example, a ~500 W (STC) PV array should be a good match for a 12 V x ~500 Ah battery bank.

    HTH,
    Jim / crewzer
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