Battery Life Calculator

Battery life in hours is the stored energy divided by the power the load draws. Amp hours give the charge, volts multiply that into watt hours, and the load in watts sets how fast it drains.

CapacityThe nameplate capacity in amp hours, quoted at the twenty hour rate.Pack voltageThe nominal voltage of the pack in volts.LoadThe power the load draws, in watts.Depth of discharge(%)The share of the capacity you are willing to use before recharging.

Runtime in hours

Runtime at your depth of discharge: 1.6
Stored energy in watt hours: 1,200
Load current in amps: 50
Peukert corrected runtime: 1.0024

Values update as you type. This calculator covers the single scenario its formula assumes — see Common Mistakes for what it leaves out.

The formula this calculator uses

runtime hours = amp hours x volts / watts

Ah: The capacity in amp hours
V: The pack voltage in volts
W: The load in watts
DoD: The usable depth of discharge

How to check the result by hand

1
Convert capacity into watt hours
Multiply the amp hour rating by the nominal voltage. A 100 amp hour pack at twelve volts stores 1200 watt hours.
2
Divide by the load in watts
1200 watt hours divided by a 600 watt load is two hours, which is the ideal runtime over the full capacity.
3
Apply the depth of discharge
At eighty percent usable capacity the same pack delivers one and a half hours, because twenty percent of the charge is held back.
4
Check the discharge rate
Six hundred watts at twelve volts is fifty amperes, which is half the pack's rating. Lead acid loses usable capacity at that rate, which the Peukert figure reflects.
5
Add inverter losses if any
If the load runs from an inverter, divide the runtime by the inverter efficiency. At ninety percent efficiency the two hour figure becomes one and a half hours even at full discharge.

For worked examples, common mistakes and the limits of this formula, read the full How To Calculate Battery Life page.