Enter a power in watts and a number of hours to calculate the total energy used in kilowatt-hours.
Left a space heater running overnight and wondering what that actually cost in energy? This is the calculator for that question — power in watts, plus a duration, converted straight into kilowatt-hours.
How to Use the Watts to kWh Calculator
Enter the device's wattage and how many hours it ran; the energy figure in kWh updates immediately below.
It's the mirror image of the kWh to Watts Calculator, run the other direction, and the formula is just as short:
Energy (kWh) = (Power (W) × Time (hours)) ÷ 1,000Plug in the calculator's own defaults and the arithmetic is easy to check by hand: a 100 W device left running for 24 hours uses exactly 2.4 kWh — 100 × 24 = 2,400 watt-hours, divided by 1,000. That's the everyday version of the question this tool exists to answer: not an abstract power-vs-energy distinction, but "if I leave this thing on all day, or all night, how much does that actually add up to." A 60 W fan running for 8 hours comes out to 0.48 kWh; a 2,000 W space heater for the same 8 hours is 16 kWh — thirty-three times more energy for thirty-three times the wattage, since the hours are held constant.