Physics

Instant, private, and free

Watt-hours Calculator.

Calculate energy from power and elapsed time.

On-device calculationNo signup
01

Set your values

Results update as you type.

Energy: 200 Wh

Energy

200.000000Wh
Energy: 720,000 J

Energy

720,000.000000J

Results update automatically as you type.

Use Cases

Estimate appliance energy consumption

Determine how much energy an appliance uses over a period to understand its impact on your electricity bill or battery capacity.

Example: A 1500 W space heater used for 2 hours consumes 3000 Wh.

Size a solar or battery system

Calculate total daily energy needs by summing watt-hours of devices to help size solar panels or battery storage.

Example: Summing 50 W fan for 5 hours and 10 W LED for 6 hours gives 250 Wh + 60 Wh = 310 Wh.

Frequently Asked Questions

How do I calculate watt-hours?
Multiply the power in watts by the time in hours. For example, a 100 W device running for 3 hours consumes 300 watt-hours (Wh).
What is the difference between watts and watt-hours?
Watts measure power (rate of energy use), while watt-hours measure total energy consumed over time. One watt-hour is one watt of power used for one hour.
Can I use this calculator for any device?
Yes, as long as you know the power in watts and the time in hours. It works for appliances, electronics, lights, and other electrical devices.

Tips & Common Mistakes

Tips

  • Ensure power is in watts and time in hours. Convert kilowatts to watts (1 kW = 1000 W) and minutes to hours (divide by 60) before calculating.
  • For devices with variable power, use the average power consumption over the time period.
  • Use this calculator to compare energy usage of different devices by calculating their watt-hours for the same duration.
  • Remember that watt-hours measure energy, not power. To get kilowatt-hours (kWh), divide the result by 1000.

Common Mistakes to Avoid

  • Forgetting to convert time to hours: using minutes directly gives incorrect results. For example, 30 minutes should be 0.5 hours.
  • Mixing up power and energy: watts are not the same as watt-hours. Always multiply power by time to get energy.
  • Using peak power instead of average power for devices that cycle on and off, leading to overestimation.

Last updated: August 13, 2026