Battery runtime calculator

Enter the battery size and the power you want to run. See how long it lasts.

Volts times amp-hours, divided by 1 000, if only Ah is given.

The total watts of the appliances running at once.

How much of the capacity the battery allows you to use. Check its datasheet.

Power lost in the inverter. Check its datasheet.

Estimated runtime

7 h 12 min

Energy you can actually use
3.6 kWh
Runtime in hours
7.2

Real runtime is shorter when the battery is old or cold, or the load rises. Use the manufacturer’s figures for exact values.

Estimates only. Not financial advice.

How the battery runtime calculator works

Last updated 10 November 2026

During an outage, how long you stay powered depends on the battery, the inverter and what you run. This calculator gives a simple estimate.

How it works

  • Usable energy = battery kWh × usable share × inverter efficiency.
  • Runtime in hours = usable energy ÷ load in kW (watts ÷ 1 000).

Worked example

With the starting figures:

  1. The usable energy is 3.6 kWh.
  2. At the load shown, it runs for about 7.2 hours.

Why runtime matters

During load shedding or an outage, how long you stay powered depends on the battery, the inverter and what you run. This calculator gives a simple estimate from the battery's capacity and the load you want to run.

What you enter

  • The battery size in kilowatt-hours, or in amp-hours and volts if you need to convert
  • The usable share of the battery, since most batteries should not be run flat
  • The inverter efficiency, because some energy is lost as heat
  • The load in watts for the things you want to run

How the estimate works

Usable energy equals the battery size times the usable share times the inverter efficiency. Runtime in hours equals the usable energy divided by the load in kilowatts, which is the load in watts divided by one thousand. The result is shown in hours and minutes.

Why real runtimes differ

  • The load changes. A fridge cycles on and off, and a kettle or geyser can drain a battery very fast.
  • Old and cold batteries hold less than new, warm ones.
  • Solar panels recharging during the day extend the runtime. This tool does not include them.
  • The label capacity is not usable capacity, because batteries protect themselves by stopping early.

Planning a backup system

Add up the watts of the items that must keep running, such as lights, a router, a laptop and a fridge. For sizing the inverter that runs them, use the inverter sizing calculator. For what a system could save, see the solar payback calculator. This is an estimate, not a design.

Things to watch

  • The load changes. Fridges, kettles and geysers can drain a battery far faster than lights.
  • Old and cold batteries hold less than new, warm ones.
  • Solar panels recharging the battery during the day extend the runtime. This tool does not include that.

To size the inverter for these loads, use the inverter sizing calculator. Read the full disclaimer.

Questions people ask

How do I convert amp-hours to kWh?

Multiply the voltage by the amp-hours and divide by 1 000. A 48 V, 100 Ah battery is 4.8 kWh.

Why is usable capacity less than the label?

Batteries should not be fully drained. The usable share depends on the battery type and settings. Check the datasheet.

Inverter sizing calculatorThe inverter size in VA for the appliances you want to run together.Electricity usage calculatorWhat each appliance uses and costs, so you can see what drives your bill.Solar payback calculatorHow long a solar system takes to pay for itself, with rising tariffs.

Estimates only. Not financial advice. Read the full disclaimer.