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Home Battery Backup Time Calculator

Calculates how long a battery can power a load before running out.

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Result

Backup time

5.1 hours

Usable energy

1,020 Wh

Notes

  • · Inverter and battery efficiency losses reduce usable capacity — 85% is a typical real-world estimate. Actual runtime also depends on battery age and depth of discharge.

Home Battery Backup Time Calculator — How It Works

Home Battery Backup Time Calculator — How It Works

Calculates how long a battery can power a load before running out.

How it works

  1. Enter the load (in watts) that the battery needs to power.
  2. Enter the battery’s voltage and capacity (in amp-hours).
  3. Enter the system’s efficiency — typically 80-90% due to inverter and conversion losses.
  4. The tool calculates the usable energy in the battery and divides it by the load to estimate backup time in hours.

Formula

Usable energy (Wh) = Battery voltage × Battery capacity (Ah) × Efficiency
Backup time (hours) = Usable energy ÷ Load (W)

Example

A 200W load powered by a 12V, 100Ah battery at 85% efficiency provides about 1,020 Wh of usable energy — roughly 5.1 hours of backup.

Notes

Real-world runtime is also affected by battery age, temperature, and how deeply the battery is discharged — deep-cycle batteries are typically not discharged below 50% to preserve lifespan.

Try the Home Battery Backup Time Calculator — instant results in your browser, no sign-up.

FAQ

Is this free?

Yes — the Home Battery Backup Time Calculator runs entirely in your browser, no sign-up required.

Are these figures exact?

The calculator performs the math precisely from the numbers you enter, but real-world rates, emissions factors, and efficiency figures vary by location and equipment — treat results as planning estimates.

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