Amp-Hour Formula?

pabloesguapo
pabloesguapo Solar Expert Posts: 116 ✭✭
in Off Grid Solar & Battery Systems #1
Is there a simple formula I can use to guesstimate how my appliances are going to draw from my battery bank? Something SIMPLE, please. We can factor in inverter efficiency, but let's leave out solar recharge during daylight hours, temperature, barometric pressure, moon phase (a joke. I kid...) and other unknowable variables?

I'm running a 48 volt battery bank. I have 1200 amp hours total, 600 would bring it down 50%. I run everything post inverter (Outback VFX3648) at 120v AC.

A nice round example: My Killawatt tells me that the appliance is using 1,000w of electricity (120v AC) every 24 hours. How many amp hours off my batteries is that?
27 Kyocera panels, 6,500w
24 CG2 6v batteries, 48v, 630Ah
Midnite Classic 150 & Classic 150 Lite in "follow me" mode
(2) Outback fx3648 inverters
Generac ecogen 6kw backup generator
Mate3s
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Amp-Hour Formula?

    I am not sure if you are using 1,000 watts * 24 hours or 1,000 WH per 24 hours.

    The answer to both is, roughly,
    • 1,000 watts * 24 hours * 1/0.85 inverter efficiency * 1/50 volts average battery voltage = 565 AH used @ 48volt battery bank
    • 1,000 Watt*Hours (per day) * 1/0.85 inverter efficiency * 1/50 volts average battery voltage = 23.5 AH used @ 48volt battery bank

    Note that average battery bank voltage during discharge (or if the sun is up charging) is important for AC inverters... They are "constant power" devices:
    • Power = Voltage * Current

    Since you have a constant AC load, as the battery bank voltage falls, the current rises.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • SolaRevolution
    SolaRevolution Solar Expert Posts: 410 ✭✭
    #3
    Re: Amp-Hour Formula?

    To keep it simple:

    1000w (Hours?) / .9 (90% inverter efficiency) = 1111 Wh / 48 volts = 23.1 amp/hrs every 24 hrs.


    Using the numbers of 90% for inverter efficiency and 48 volts nominal instead of 49v+/- is somewhat pessimistic but it is better to err on the conservative side when considering battery capacity.

    -Alex
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  • pabloesguapo
    pabloesguapo Solar Expert Posts: 116 ✭✭
    #4
    Re: Amp-Hour Formula?

    "...the total cumulative appliance power consumption in KW hours." (Helps if I dig out the manual) So, the meter reads 1.0. 1,000 watt * hours?

    Taking a few assumptions and averages into account...
    WH/efficiency/battery voltage = Battery AH used?

    (1,000/.9 = 1111) / 48 = 23.148
    (2,000/.9 = 2222.222) / 48 = 46.296
    (3,000/.9 = 3333.333) / 48 = 69.444
    and so on?
    27 Kyocera panels, 6,500w
    24 CG2 6v batteries, 48v, 630Ah
    Midnite Classic 150 & Classic 150 Lite in "follow me" mode
    (2) Outback fx3648 inverters
    Generac ecogen 6kw backup generator
    Mate3s
    · Share on Twitter
  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: Amp-Hour Formula?

    Yep--the Kill-a-Watt meter gives Watts, VA, Amps, Volts, etc. in "native" units.

    But displays kWH (1,000 WH) for energy used. So 2.2 kWH = 2,200 WH.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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