Electronics & Engineering

UPS and Battery Backup Runtime Calculator

How long will a battery bank run your load?

By TheCalcTools Updated Formula checked and tested

Your numbers

Your results

Stored energy (Wh)
Stored energy (Wh)
—
Usable energy delivered (Wh)
—
Runtime (hours)
—
Runtime (minutes)
—

UPS and Battery Backup Runtime Calculator by TheCalcTools

How the UPS and Battery Backup Runtime Calculator works

Enter the battery bank voltage and capacity, the load in watts, the inverter’s efficiency and how much of the battery you are willing to use.

Stored energy is volts times amp-hours.

Only the usable depth of discharge, after inverter losses, reaches the load.

The formula

Wh = V × Ahruntime = Wh × DoD × efficiency ÷ load

Worked example

Using the default values shown in the calculator:

Battery voltage (V)12
Battery capacity (Ah)100
Load (W)300
Inverter efficiency (%)90
Usable depth of discharge (%)50
  1. Stored energy = 12 × 100 = 1,200 Wh.
  2. Usable = 1,200 × 0.5 × 0.9 = 540 Wh.
  3. Runtime = 540 ÷ 300 = 1.80 hours, or 108 minutes.

Result:

Stored energy (Wh)1,200
Usable energy delivered (Wh)540
Runtime (hours)1.80
Runtime (minutes)108

Frequently asked questions

Why limit the depth of discharge?

Lead-acid batteries last much longer if you use only about 50% of the capacity. Lithium iron phosphate batteries can often use 80–90%.

Does the runtime change with load?

Yes, and not always linearly. Heavy loads reduce the usable capacity of lead-acid batteries (Peukert effect).

Sources and further reading

Battery Life CalculatorWork out battery runtime from capacity and current draw Electrical Power Calculator (Single and Three Phase)Work out watts, kVA and kvar from volts, amps and power factor Transformer Turns Ratio CalculatorFind the secondary side of an ideal transformer from the primary values

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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.