# Amp-Hour Formula?

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?

• Super Moderators, Administrators Posts: 29,489 admin
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
• Solar Expert Posts: 407 ✭✭✭✭✭✭
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
• 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?
• Super Moderators, Administrators Posts: 29,489 admin
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