Battery runtime is one division: usable energy divided by the load. The trick is that “usable” is less than the label says — you never drain a battery flat, and the inverter loses a slice on the way to your appliances. Here is the method, with the two factors that decide it.
The formula
Runtime = usable energy ÷ load, where usable energy = battery capacity × depth of discharge × inverter efficiency. 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.
Backup Runtime Calculator
Enter your battery size and appliances for the hours it runs during an outage.
Depth of discharge (DoD)
How much of the battery you actually draw before recharging. Draining a battery fully shortens its life, so you never use 100%.
| Battery type | Usable depth of discharge |
|---|---|
| Lithium (LiFePO₄) | 80–100% |
| Lead-acid / AGM | ~50% |
That difference is why a 5 kWh lithium battery outlasts a 5 kWh lead-acid one by nearly double.
Why inverter efficiency matters
The inverter turns the battery’s DC into the AC your appliances use, and loses some energy as heat doing it — a typical inverter is 90–95% efficient, so only that share of the stored energy reaches the load. Lower efficiency means shorter runtime.
kWh or amp-hours?
Use whatever your battery label shows. Many lithium batteries are rated in kWh directly. If yours is in amp-hours (Ah), you need the system voltage too: kWh = Ah × volts ÷ 1,000. A 100 Ah battery at 48 V is 4.8 kWh.
Why your real runtime is shorter
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 watts when they switch on; and real loads cycle on and off rather than staying constant. Build in a safety margin rather than planning to the last minute — and to size a bank for a whole night, work backwards with the battery sizing calculator.