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Battery Runtime Calculator

Estimate runtime from battery capacity, nominal voltage, continuous load and usable-energy assumptions.

01

INPUTS

Basic inputs

Ah

Rated capacity shown on the battery.

V

Nominal direct-current voltage.

W

Continuous power draw of the device or system.

Assumptions

%

Portion of rated capacity available for use.

%

Accounts for inverter, wiring and conversion losses.

02

RESULT

Basic estimate

Estimated runtime

10h 12m

At a continuous 100 W load, the battery is estimated to run for approximately 10h 12m.

Calculation summary

Nominal energy
1,200 Wh
Continuous load
100 W
Usable energy
1,020 Wh
Usable capacity
85%

Calculation trace

  1. 100 Ah × 12 V
    = 1,200 Wh
    Nominal energy
  2. 1,200 Wh × 85%
    = 1,020 Wh
    Usable energy
  3. 1,020 Wh ÷ 100 W
    = 10.2 hours
    Runtime (decimal)
  4. 10.2 hours
    = 10h 12m
    Hours and minutes
03

HOW IT WORKS

About this calculationUnderstand the basic energy model used.

Stored energy equals capacity (Ah) times nominal voltage (V). Multiplying by usable capacity and conversion efficiency gives the energy delivered to the load; dividing by the continuous load power (W) gives the runtime in hours.

Assumptions & limitationsConstant load; no chemistry, ageing or temperature model.
  • Ignores the Peukert effect — high loads reduce usable capacity.
  • Ignores temperature, battery age and state of health.
  • Assumes a constant load and a flat discharge voltage.
Example scenariosSee how input changes affect runtime.
  • 100 Ah · 12 V · 50 W load · 90% efficiency: 21h 36m
  • 200 Ah · 24 V · 300 W load · 85% usable · 90% efficiency: 12h 14m
04

SOURCES & REVIEW

Source records
Method version battery-runtime-energy@1.0.0. Energy relation E = Q·V and runtime t = E/P; SI units per the BIPM SI Brochure (bipm.org).
Review status
Published with a cited primary source. This is an ideal-energy model and has not yet been independently peer-reviewed; real batteries deliver less (see the warnings above).
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05

RELATED TASKS

Choose a battery size
Battery Capacity Calculator — coming soon.
Estimate stored energy
Battery Energy Calculator — coming soon.
Model inverter losses
Inverter Runtime Calculator — coming soon.