Battery Backup Calculator

Estimate battery runtime hours for a backup or off-grid load.

Runtime
— hours
At the given load.

Usage Tip

Do not discharge lead-acid batteries below 50 percent; lithium tolerates deeper cycles, so size capacity to the chemistry.

THE MATH
watt-hours = amp-hours × voltage
usable Wh = Wh × efficiency ÷ 100
runtime = usable Wh ÷ load
A battery stores watt-hours equal to its amp-hours times its voltage. Real-world losses cut the usable amount, and runtime is the usable energy divided by the load.
Enter the load in watts, the battery capacity in amp-hours, the battery voltage, and a system efficiency (about 85 percent for an inverter setup).
The result is approximate runtime.
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The calculators and tools on Formula Factory are provided for general guidance and informational purposes only. Results are estimates based on standard formulas and the values you enter — they do not constitute professional engineering, electrical, or architectural advice. Always verify calculations with a qualified professional before making decisions for any safety-critical, code-compliance, or commercial application. Formula Factory makes no representations or warranties as to the accuracy or completeness of any result, and accepts no liability for errors, omissions, or any outcomes arising from reliance on this information.