Enter a device's wattage and a number of hours to find the total energy it used in kilowatt-hours.
How to Use the Watts to Kilowatt-Hours Calculator
Enter a device's wattage and the hours it ran. The energy used over that stretch appears in kWh, the unit an electricity bill actually charges for.
A Small Aquarium Heater Adds Up More Than It Looks
A 300 W aquarium heater cycling on for an estimated 8 hours a day doesn't look like much next to a refrigerator or an air conditioner, but run the multiplication and it comes to 2.4 kWh a day — 300 × 8 ÷ 1,000. Over a 30-day month that's 72 kWh, a bigger contribution to a bill than its modest wattage would suggest, precisely because it's a duration multiplied against a wattage, not the wattage alone.
Energy (kWh) = (Power (W) × Time (hours)) ÷ 1,000Nameplate wattage is a rate, not a cost — nothing on a bill gets charged in watts. This is the multiplication that turns a device's rating into the number that actually shows up on a utility statement, once a rate per kWh gets applied on top of it.