Battery Runtime Calculator: Estimate Backup Hours from Wh and Watts

Runtime estimates require both available energy and average power demand. Real batteries and inverters lose energy, and actual usable capacity changes with conditions, battery management and aging.

Why a runtime estimate can differ from reality

The model treats average load as steady and combines losses in one usable fraction you choose. Refrigerators and pumps cycle, and motors may draw more power at startup. Temperature, aging and operating conditions can also affect battery output. Do not depend on this calculator for medical or other life-critical power planning.

Example

A nominal 1,000 Wh battery with an assumed 85% usable fraction and 100 W steady load has an idealized 8.5-hour runtime. This is a mathematical illustration, not a product test.

Return to the backup power guide to decide what equipment specifications and safety checks matter before buying.

Two ways the same battery can behave differently

At an illustrative 1,000 Wh nameplate capacity and assumed 85% usable energy, a steady 50 W load would produce an arithmetic estimate of 17 hours; a 200 W load would produce 4.25 hours. These are not empirical test results. An inverter's idle draw can become a meaningful part of a light-load scenario, and a refrigerator's average wattage can hide a high start-up surge.

Read these specifications before treating the result as a plan

• Continuous AC output rating and peak/surge rating.
• Nameplate Wh and actual usable capacity under the planned operating mode.
• Power draw of the inverter itself and efficiency at the intended load.
• Charging input limits, temperature limits and manufacturer operating instructions.
• Battery-health and lifecycle warranty conditions.

For an outage, leave a safety margin rather than planning to use the final watt-hour. Your critical-load requirements and actual manufacturer information matter more than the headline storage number.