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Watt-Hour Energy from Charge or Power & Time

Calculate energy in two independent ways: constant voltage times charge, or constant power times duration.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

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Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

At a constant 12 V, 2 Ah represents 24 Wh. Separately, a constant 6 W load operating for 4 hours uses 24 Wh.

Energy = constant voltage × charge; energy = constant power × duration

Worked example

Enter these known values and leave the other values blank.

Constant or charge-weighted average voltage
12 V
Transferred charge
2 Ah
Constant or time-averaged power
6 W
Duration
4 hrs
Energy from voltage and charge
24 Wh
Energy from power and duration
24 Wh

Assumptions and limitations

  • The two input groups are independent; their energies are not automatically forced to match. All input and output quantities must be positive.
  • Voltage multiplied by ampere-hours gives watt-hours when the voltage is constant or is the appropriate charge-weighted average. A changing battery voltage requires integration or a suitable average.
  • Power multiplied by time gives energy for constant power or a suitable time average. This does not model conversion losses, usable battery capacity or discharge limits.
  • The month and year options are fixed durations of 30.4375 and 365.25 days. They are not calendar arithmetic. Mechanical and electric horsepower use different conventions.

Common questions

Is a watt-hour a unit of power?

No. A watt measures power and a watt-hour measures energy. One watt-hour is exactly 3,600 joules.

Does the first result predict actual battery runtime?

No. It is a voltage–charge energy model. Actual usable capacity, varying voltage, load and losses require additional information.

References

The power unit uses the International Table BTU per hour consistently with the reviewed energy convention. Mechanical horsepower uses exactly 550 foot-pounds-force per second instead of a rounded factor.

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