Battery calculators · FREE TOOL

Battery Runtime Calculator

Estimate how long a battery can run a steady AC or DC load, including usable capacity, inverter losses and idle draw.

1

Your inputs

Edit the example
Core inputs
Ah
V

Use the nominal rating on the battery, not its charging voltage.

W
Planning assumptions
%

Editable planning assumption. Follow the battery maker’s limits.

%

An assumption, not a measured equipment specification.

W

Battery-side watts, added after the inverter’s conversion losses.

Example estimate · editable inputs

Estimated runtime

8.64 hours

Nominal energy
1,200 Wh
Usable energy
960 Wh
Battery-side draw
111.11 W
Conversion loss
11.11 W

Constant average load. Real runtime depends on battery condition, temperature and equipment cutoff.

BEHIND THE RESULT

How the calculation works

Runtime (h) = nominal Wh × usable fraction ÷ (AC watts ÷ inverter efficiency + battery-side idle watts). For a direct DC load, runtime = usable Wh ÷ load W.

Use measured average watts for a cycling appliance. Lithium battery runtime and 12 V battery run time use the same energy balance; chemistry changes which usable fraction is reasonable.

A worked example

A 100 Ah, 12 V battery holds 1,200 nominal Wh. At 80% usable and 90% inverter efficiency, a 100 W AC load runs for 8.64 hours. Adding 10 W of battery-side idle draw reduces this to about 7.93 hours.

What this estimate can’t tell you

A constant-load estimate does not model voltage sag, battery aging, temperature, Peukert effects, cutoff behavior or changing inverter efficiency. Nominal capacity does not guarantee runtime.

Sources & methodology

Formulas are derived from the units shown. Read our methodology or report a correction. Sources reviewed 2026-11-01.