Backup Runtime Calculator

It estimates how many hours your battery bank can keep a chosen load running during an outage — for example lights, a fridge and Wi-Fi. Enter your battery size and what you want to power, and it returns a runtime like "5h 20m".

Runtime = usable battery energy ÷ load. Usable energy is your total battery capacity multiplied by the depth of discharge and the inverter efficiency. So a 5 kWh battery at 90% depth of discharge and 92% inverter efficiency gives about 4.14 kWh usable; running a 500 W load, that lasts roughly 8.3 hours.

Depth of discharge is how much of the battery you actually draw before recharging. Draining a battery fully shortens its life, so you never use 100%. Lithium (LiFePO₄) batteries are commonly rated for 80–100%; lead-acid and AGM batteries should be kept to about 50%. Picking the battery type sets a sensible default, which you can override.

The inverter converts the battery’s DC power into the AC power your appliances use, and some energy is lost as heat in that conversion. A typical inverter is 90–95% efficient, so only that share of the stored energy reaches your appliances. Lower efficiency means shorter runtime.

Use whichever your battery label shows. Many lithium batteries are rated in kWh directly. If yours is in amp-hours, switch to the Ah option and enter the system voltage (usually 12, 24 or 48 V). The calculator converts it for you: kWh = Ah × volts ÷ 1000.

Check the rating plate or manual — it lists watts (W) or amps (A); watts = amps × voltage. The appliance library gives typical running wattages to get you close, but your specific model may differ, especially for fridges, pumps and air conditioners.

This is a best-case figure. Batteries lose capacity as they age and in the cold, motors and compressors (fridges, pumps, air conditioners) briefly draw several times their running wattage when they switch on, and real loads switch on and off rather than staying constant. Always build in a safety margin rather than planning to the last minute.

Add batteries (or a bigger kWh figure) until the runtime covers the hours you need. As a rough guide, multiply your load in kW by the hours required, then divide by the depth of discharge and inverter efficiency to get the battery kWh needed — or use our Battery Bank Sizing calculator, which does exactly that.