Energy & Workshop Math

Battery Runtime Calculator

Estimate how long a battery will run a load, with explicit usable-energy and efficiency assumptions.

Units

Your measurements

Results update as you type.

Example shown — use your own measurements.

Enter a fraction from 0 to 1 that respects the battery maker's operating limits and any required reserve. This calculator does not choose a safe depth of discharge.

Extra options & cost (optional)
Fractions and number formats

Use decimals (12.5), fractions (12 1/2 or 12-1/2), or explicit lengths (12ft 6in, 12′ 6″, 25 mm). A comma followed by one or two digits is a decimal comma: 12,5 = 12.5. Grouped commas mean thousands: 1,200 = 1200. Use a decimal point for ambiguous values. Scientific notation is not accepted.

Enter your measurements

Results update as you type.

About this calculator

Enter nominal battery energy in Wh, or voltage and amp-hours, a usable fraction consistent with the battery maker's operating limits, and the load. Choose ideal efficiency only for a mathematical best case; otherwise enter converter efficiency. Battery-side standby draw is added separately. Zero total draw is reported as not applicable; no emergency-backup duration is guaranteed.

How to measure

  • Enter nominal watt-hours, or nominal volts and amp-hours.
  • Enter your chosen usable fraction and either an explicit ideal model or the converter efficiency.
  • Include average output load and any battery-side standby draw; runtime is an estimate under those assumptions.

How to use this calculator

  1. Enter the battery's nominal energy, or its voltage and amp-hours.
  2. Choose how much of the capacity you are willing to use.
  3. Choose ideal efficiency or enter a converter efficiency.
  4. Enter the average load and standby draw.
  5. Calculate for the estimated runtime.

Formula

Usable energy = nominal energy × usable fraction

Battery-side draw = average load ÷ efficiency + standby draw

Runtime = usable energy ÷ battery-side draw

If battery-side draw is zero, runtime is reported as not applicable

Assumptions

  • The load is steady at the average you enter.
  • Nominal energy is taken at the battery, before conversion losses.
  • Standby draw is added on top of the load-converted draw.

Common mistakes

  • Using full nameplate energy as guaranteed usable capacity.
  • Leaving conversion efficiency and idle draw out of the budget.
  • Ignoring discharge rate, age, cutoff and temperature effects.

Worked example

Try it: press “Load example” above the form.

100 Wh at 80% usable is 80 Wh; a 10 W load at 90% efficiency draws 11.11 W, so runtime is 7.2 hours.

Limitations

Nominal model only — no Peukert, temperature, voltage sag or aging correction. Not a guaranteed backup duration or battery sizing design.

Frequently asked questions

Why is runtime shorter than watt-hours divided by watts?

Usable capacity can be below the nameplate energy, and conversion losses plus idle draw consume power. This model uses the usable fraction and efficiency you provide.

Can this predict every battery’s discharge behaviour?

No. Temperature, age, rate capability, cutoff voltage and battery chemistry affect real runtime. The result is an energy-budget estimate rather than a battery discharge-curve model.

What does the worked example show?

100 Wh at 80% usable is 80 Wh; a 10 W load at 90% efficiency draws 11.11 W, so runtime is 7.2 hours.

Which assumptions should I check before using the result?

Nominal model only — no Peukert, temperature, voltage sag or aging correction. Not a guaranteed backup duration or battery sizing design.

Reviewed by Codex AI editorial agent (Editorial, 2026-11-12) · Codex independent specialist-review AI agent (educational arithmetic/source review; not licensed human approval) (Electrical, 2026-11-12).

Sources

Sources explain the methods and any named product presets. Check your product label where coverage, yield or density differs.

Calculation documentation · Updated 2026-11-11 · Version 1.6.4. Estimates are planning aids. Confirm quantities, coverage and prices with your supplier before purchasing. Found an error? Report it.

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