Watts to kWh Calculator
Convert power in watts (W) to energy in kilowatt-hours (kWh). Enter the power in watts and the consumption time period in hours, and press Calculate to get the energy in kWh. Everything runs locally in your browser.
Enter the power in watts
Type the average power draw of your device in W — that is the unit printed on most appliance nameplates and LED bulbs, so no unit juggling is needed here. Scientific notation like 1.5e3 works too.
Enter the time in hours
Type how long the device runs, in hours. Fractional hours are fine: 30 minutes is 0.5 hr, 15 minutes is 0.25 hr, and a full day is 24 hr.
Press Calculate, read kWh
The energy in kilowatt-hours appears at once (at most 4 decimal places). Copy the result to your clipboard, or press Reset to start over.
How watts become kilowatt-hours
- What is the watts to kWh formula?
- The energy E in kilowatt-hours (kWh) is equal to the power P in watts (W), times the time period t in hours (hr), divided by 1000: E(kWh) = P(W) × t(hr) / 1000. The division just converts watts to kilowatts.
- How much energy does a 1500 W heater use in 8 hours?
- 1500 W × 8 hr = 12000 W·hr, and 12000 / 1000 = 12 kWh. That is the same energy a 1.5 kW load would use in 8 hours — the /1000 in this calculator is exactly that kW conversion step.
- How many kWh does a 60 W bulb use per day?
- Left on around the clock: 60 W × 24 hr / 1000 = 1.44 kWh per day. Used only 3 hours a day, it drops to 60 × 3 / 1000 = 0.18 kWh per day.
- What is the difference between watts and kilowatt-hours?
- A watt (W) measures power — how fast energy is used at an instant. A kilowatt-hour (kWh) measures energy — power sustained over time. It is the speed-vs-distance analogy: kWh is the distance driven, W is the speed.
- How do I turn the kWh result into a cost?
- Multiply the kWh figure by your electricity tariff. For example, a 100 W load running 24 hours uses 2.4 kWh; at $0.15 per kWh that costs 2.4 × 0.15 = $0.36. Utilities bill in kWh, so this calculator gives you the quantity they charge for.
Privacy: every calculation runs in your browser tab — no value you type is sent anywhere. The copy button uses your clipboard only. Real devices vary their draw over time; use average power for realistic energy estimates.