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Battery Runtime Calculator

Estimate hours of operation from battery energy and a steady device load. Enter nominal capacity with explicit allowances, or use energy already known to be available at the output.

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Your inputs

Starting values are examples. Replace them with the details for your setup.

Wh
%

The portion of nominal capacity available for this run.

%

Conversion losses between usable battery energy and the load.

W

Add the watts used by the devices you plan to run. Include relevant standby loads.

Estimates hours on a charge, not battery lifespan. Load, temperature and battery condition can change the result.

Calculations stay in your browser.

EXAMPLE RESULT

Estimated runtime per charge

7 h 12 min

Nominal battery energy
600 Wh
Usable output energy
432 Wh
Constant output-side load
60 W

What this assumes

  • Usable capacity 80%; discharge/conversion efficiency 90%, applied once each.
  • Constant-load estimate, not battery lifespan. Standby draw, temperature, age, chemistry, and changing loads can alter runtime; lead-acid capacity may vary with discharge rate.

A share link includes the inputs used for this result.

How the calculation works

Nominal Wh = Ah × nominal volts, when entering Ah. Usable output Wh = nominal Wh × usable capacity fraction × discharge efficiency. Runtime hours = usable output Wh ÷ output load watts. For already-usable output Wh, divide directly by output load watts.

WORKED EXAMPLE

Example: capacity with explicit allowances

An illustrative 600 Wh battery with 80% usable capacity and 90% discharge efficiency provides 600 × 0.8 × 0.9 = 432 usable output Wh. At a constant 60 W output load, estimated runtime is 432 ÷ 60 = 7.2 hours, or 7 hours 12 minutes.

WORKED EXAMPLE

Example: already-usable energy

If an example system has 300 Wh already available at its output and the load is 50 W, runtime is 300 ÷ 50 = 6 hours. Do not apply another capacity or efficiency reduction if those losses are already included in the 300 Wh.

Define the load at the output

Use the average power consumed by the device or devices being supplied. Conversion losses belong in the separate discharge-efficiency assumption. Account for system standby consumption without counting it again if it is already represented in a measured delivered-energy figure.

Usable capacity and efficiency are separate

The usable-capacity fraction describes the portion of nominal battery energy available for this run, including any starting-charge or reserve allowance. Discharge efficiency describes the share of that energy reaching the output. Neither is an automatic setting for a particular battery chemistry.

Runtime is not battery lifespan

This estimates time per charge, not years before replacement. Load changes, temperature, age, and battery condition affect actual runtime. Victron notes that available capacity also changes with discharge rate, particularly for lead-acid batteries; this simple energy model does not calculate a Peukert correction.

Sources & methodology

Technical guidance checked September 27, 2026. Example inputs are illustrative, not specifications for a particular vehicle or battery.