Inverter & Battery capabilities ??

Arkansasoffgrid
Arkansasoffgrid Solar Expert Posts: 117 ✭✭
in Off Grid Solar & Battery Systems #1
Whats the formula for calculating the max inverter output with a given set of batteries?
Ex. My 4400w inverter with 8-6v GC2s@215ah ? Compared to 8-6vL16s @440ah ?
8-420AH US Battery L-16s, Midnite Classic 150cc, Whiz Bang Jr, Magnum MS4448PAE inverter/charger,  4590w Canadian Solar panels. Honda EU2000i generator.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Inverter & Battery capabilities ??

    A quick rule of thumb for Flooded cell batteries (AGM, LiIon can do much better) based on C20:

    1,000 Watt inverter -> 100 AH @ 48 volt battery bank
    1,000 Watt inverter -> 200 AH @ 24 volt battery bank
    1,000 Watt inverter -> 400 AH @ 12 volt battery bank

    Note the above work about right for maximum solar array size (i.e., a 1,000 Watt array on a 100 AH @ 48 volt battery bank).

    The detailed calculations I like to use (again for flooded cell):

    C/20 -- Average discharge rate (5 hours per night, 2 nights, 50% max discharge)
    C/8 -- Maximum continuous discharge rate
    C/5 -- Maximum short term discharge rate (run a microwave, power saw, minutes to an hour)
    C/2.5 -- Maximum surge discharge rate (starting well pump, etc.)
    8-6v GC2s@215ah ? Compared to 8-6vL16s @440ah?

    So, for a C/8 discharge rate:

    8*6 volts * 215 AH * 0.85 inverter eff * C/8 discharge rate = 1,097 Watts max continuous
    8*6 volts * 440 AH * 0.85 inverter eff * C/8 discharge rate = 2,244 Watts max continuous

    Seems to work fairly well... Always looking for feedback from others.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Ethan Brush
    Ethan Brush Solar Expert Posts: 235 ✭✭
    #3
    Re: Inverter & Battery capabilities ??
    Arkansasoffgrid wrote: »
    Whats the formula for calculating the max inverter output with a given set of batteries?
    Ex. My 4400w inverter with 8-6v GC2s@215ah ? Compared to 8-6vL16s @440ah ?

    BB provided good rules of thumb. Only thing I will add is that you can often add in your array wattage if you do your heavy stuff on sunny days. For example, I have a "very small" battery bank and a "Very large" solar array, partly because I want to be able do laundry, run power tools, and even light welding "right off the array" so to speak. Likely not everyone is as flexible as me ;)
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  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    #4
    Re: Inverter & Battery capabilities ??

    Hi, I think BB gave a pretty comprehensive answer to your question.

    In my opinion it comes down to Peukerts Law and voltage sag.

    1) Peukerts Law: For FLA batteries discharging at a higher rate than C/5 means you are losing a significant portion of their capacity. For AGM batteries this happens when you discharge at a rate higher than C/3.

    2) Voltage sag: Ultimately this determines the useful capacity your batteries have for you. If a 24v inverter shuts down below 21.6v this is all you can get from your batteries if that is what you're going for. Many FLA batteries can keep supplying useful power way below this voltage if you are discharging them at a very high rate of say C/1 (100 amps for a 100AH battery), but the inverter won't be able to make use of it. AGM batteries hold their voltage higher, but will also not give you everything they have above the inverter cut-off voltage. The best way to get the most out of your batteries is to select a size that enables you to discharge at an average rate of C/20 and note let the voltage fall below 2v per cell (24v for a 24v battery etc.)
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: Inverter & Battery capabilities ??
    CALLD wrote: »
    The best way to get the most out of your batteries is to select a size that enables you to discharge at an average rate of C/20 and note let the voltage fall below 2v per cell (24v for a 24v battery etc.)

    Did you intend to type "not let..."?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    #6
    Re: Inverter & Battery capabilities ??
    BB. wrote: »
    Did you intend to type "not let..."?

    -Bill

    Hahaha, yes thanks for the correction Bill;), I'm gonna blame Autocorrect for that one...!
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