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Battery

Battery Backup Runtime Calculator

By HowMuchWatts Editorial Team Reviewed June 30, 2026 Formula and source method

Whether sizing a home backup battery or a portable power station, this calculator estimates how many hours it can run a given load.

Change any number below — your result updates instantly, no button to click.

Your details

Wh

Watt-hours. For kWh, multiply by 1000 (e.g. 1 kWh = 1000 Wh).

W

Combined power of everything you are running.

%

How much you can safely use. Lithium ~90–100%, lead-acid ~50%.

%

Results

Usable energy
—
Estimated runtime
—
Optional next step

Measure or improve the real-world result

These tools are relevant to the numbers above; you do not need them to use the calculator.

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Portable power stationBackup power for outages and a way to test battery runtime in practice.See optionsBattery monitorTrack state of charge and runtime accurately.See options
Written & maintained by the HowMuchWatts team · Last updated June 30, 2026 · How we verify our numbers

How it works

Usable energy = capacity × usable % ÷ 100 × inverter efficiency ÷ 100. Runtime = usable energy ÷ load. Real runtime is usually a bit shorter because of startup surges, temperature and ageing.

Frequently asked questions

Why can’t I use the full battery capacity?

Discharging a battery completely shortens its life. Lithium batteries safely use ~90–100% of capacity; lead-acid only ~50%. The “usable capacity” field accounts for this.

What is inverter efficiency?

Converting battery DC power to household AC power loses a little energy as heat — typically 5–15%. That loss is included here.

How do I find my total load?

Add up the wattage of every device you plan to run at once. Our appliance running cost calculator can help you find each device’s wattage.

These calculators provide general estimates for educational purposes only and are not financial advice. Real-world results depend on factors not captured here. Verify figures independently before making any purchase or financial decision.

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