Watt Hours Calculator

Calculating Watt-Hours

Watt-hours measure electrical energy — how much power a device uses over time. This is essential for battery sizing, solar system design, and estimating electricity costs.

Formulas

$$Wh = P \times t$$ $$kWh = \frac{Wh}{1000}$$ $$J = Wh \times 3600$$ $$Ah = \frac{Wh}{V}$$

$P$ is power in watts, $t$ is time in hours, $V$ is voltage, and $Ah$ is amp-hours useful for battery capacity calculations.

Example

A 100 W load running for 2 hours consumes 200 Wh (0.2 kWh or 720,000 J). At 12 V, that equals about 16.67 Ah — useful for sizing a 12 V battery bank.

Related tools: Watt Converter and Generator Power Calculator.

Frequently Asked Questions

What is the difference between Wh and kWh?

Watt-hours (Wh) and kilowatt-hours (kWh) measure the same thing. 1 kWh = 1000 Wh. Utility bills use kWh.

How do I convert Wh to amp-hours?

Divide watt-hours by voltage: Ah = Wh / V. This tells you how many amp-hours a battery must supply at that voltage.

Why multiply by 3600 for joules?

One watt equals one joule per second. One hour has 3600 seconds, so Wh × 3600 = joules.

Can I use this for battery runtime?

Yes. Divide battery capacity (in Wh or Ah × V) by load power to estimate runtime. Account for efficiency losses in real systems.

What if my device power varies?

Use average power over the time period. For variable loads, measure or estimate the average wattage during operation.

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Watt Hours Calculator

Calculating Watt-Hours

Formulas

Example

Frequently Asked Questions

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