Estimate nominal battery kWh and amp-hours from backup load, duration, voltage and usable capacity.
Allow separately for inverter surge, battery aging, temperature and manufacturer limits. Trust level: engineering estimate; verify site and equipment assumptions before purchase or installation.
| Assumption | Value |
|---|---|
| Battery-bank voltage | 24 V |
| Usable depth of discharge | 80 % |
| Inverter and wiring efficiency | 90 % |
Method: battery Wh = load W × hours ÷ (depth of discharge × efficiency)
Inputs used: 500 × 4 ÷ (80% × 90%) = 2777.7777777777774 Wh
Assumptions: Battery-bank voltage Usable depth of discharge Inverter and wiring efficiency
Scope and warnings: Allow separately for inverter surge, battery aging, temperature and manufacturer limits.
U.S. Department of Energy · United States government educational resource; technology concepts are broadly applicable · effective As published · reviewed 2026-09-12
high confidence · Engineering estimate
Supports the distinction between power, energy and storage-system limitations.
Exclusions: Not a product-sizing recommendation or outage-resilience guarantee.
Required load energy is watts multiplied by hours. Nominal battery energy is larger because only the chosen depth of discharge is usable and conversion loses energy. Amp-hours then divide nominal watt-hours by battery-bank voltage.
A 500 W backup load for 4 hours needs 2 kWh at the loads. At 80% depth of discharge and 90% efficiency, minimum nominal storage is about 2.78 kWh, or 115.7 Ah at 24 V.
No. Battery energy and inverter power are different constraints. Size inverter continuous and surge power for the connected loads, then account for battery age, temperature, wiring and manufacturer limits.
Calculations run on your device. Review your results before exporting or sharing them.
List essential loads and estimate their simultaneous watts before choosing hours of autonomy. The depth-of-discharge and efficiency inputs must match the battery chemistry, inverter and manufacturer guidance you expect to use.
The result is minimum nominal energy under the entered assumptions. Add design margin for surge, battery aging, temperature and future loads; compare inverter power independently with electrical load rather than equating kWh with kW.
Published by ToolMitra — methods and editorial approach.