Battery Runtime Calculator: calculate how long a battery will run your equipment, or work out the battery capacity you need for a required runtime. Enter your battery, inverter and load details to estimate usable backup power.
BATTERY & INVERTER BACKUP CALCULATOR
Calculate battery runtime or battery size
Estimate how long a battery and inverter can support a load, or calculate the battery capacity needed for a target runtime.
Private calculation: values are calculated in your browser. Backup Power Calculator does not receive or store your calculator inputs.
Estimate only: actual runtime varies with battery chemistry, age, temperature, inverter idle consumption and load behaviour. Check manufacturer specifications before purchasing or connecting equipment.
Battery runtime and inverter backup calculations
A battery runtime calculation answers one of two practical questions: how long will my existing battery and inverter run this load?, or what battery capacity do I need to run this load for a chosen number of hours? This Battery Runtime Calculator handles both.
For runtime, enter the battery voltage, battery capacity in amp-hours, the load in watts, the inverter efficiency and the percentage of the battery you expect to use. For battery sizing, enter the load and required runtime and the calculator works backwards to estimate the nominal watt-hours and amp-hours required.
How battery runtime is calculated
Battery capacity and appliance power are different measurements. Battery energy is measured in watt-hours (Wh), while the load placed on the inverter is measured in watts (W). A 12V 100Ah battery has a nominal energy capacity of about 1,200Wh before allowing for usable capacity and conversion losses.
The Battery Runtime Calculator first estimates the battery energy available for use, then allows for inverter efficiency. It divides the resulting usable AC energy by the load in watts to estimate runtime. When sizing a battery, it reverses that process and rounds the requirement upward rather than presenting the theoretical minimum as a buying target.
Why inverter efficiency and usable battery capacity matter
An inverter does not deliver every watt-hour stored in a battery to the connected equipment. Some energy is lost during conversion, and the inverter itself uses power while operating. Battery type, battery management settings and the desired depth of discharge also affect how much of the nominal battery capacity is realistically available.
The default efficiency and usable-capacity figures are starting assumptions only. If the battery or inverter manufacturer publishes more appropriate figures for your equipment, enter those instead. Real-world runtime can also change with battery age, temperature, load variation and inverter idle consumption.
Using amp-hours and watt-hours
Amp-hours are useful when the battery voltage is known, but they should not be compared without considering voltage. The simple nominal conversion is volts × amp-hours = watt-hours. For example, 100Ah at 12V is approximately 1,200Wh, while 100Ah at 24V is approximately 2,400Wh.
For comparing battery systems, watt-hours are usually the clearer measure because they describe stored energy directly. The calculator shows both Wh and Ah where that helps explain the result. If you are planning several household loads rather than a single battery-and-inverter combination, you can also use the Home Backup Power Calculator to build a broader backup-power estimate.
Battery Runtime Calculator FAQs
Can I use this as a battery capacity calculator?
Yes. The Battery Runtime Calculator can also work backwards to estimate the battery capacity required. Select Size the battery, enter the load and desired runtime, and the calculator estimates the nominal battery capacity required in watt-hours and amp-hours at the selected voltage.
Can I calculate inverter backup time?
Yes. Select Calculate runtime and enter the battery voltage, amp-hour capacity, load and inverter efficiency. The result estimates how long that battery-and-inverter combination can support a steady load.
How long will a 12V battery last with an inverter?
That depends on the battery's amp-hour capacity, how much of the battery is usable, the inverter efficiency and the connected load. Two 12V batteries can provide very different runtimes if their capacity or condition differs.
Should I use the figures printed on my equipment?
Yes where possible. Use battery and inverter specifications from the manufacturer and a measured or manufacturer-provided load for the equipment being powered. The calculator is a planning estimate, not a guarantee of runtime.
Planning note: battery specifications are normally stated under defined test conditions. High loads, cold temperatures, battery ageing and protective battery-management limits can all reduce the energy available in practice, so allow sensible headroom when the required runtime is important.