kWh Usage Calculator
A kilowatt hour usage calculator converts an appliance's power draw and run time into the energy it consumes, measured in kilowatt hours. A kilowatt hour is the standard unit utilities use to bill electricity, equal to the energy of a 1,000 watt device running for one hour. This tool takes the wattage of a device, the hours it runs each day, and the number of days, then divides the watts by 1,000 to get kilowatts and multiplies by hours and days to return total kilowatt hours. Every input is editable so you can measure a single appliance over a day or a whole circuit over a year. Knowing kilowatt hours is the first step in understanding an electricity bill, comparing two appliances, or sizing a battery system. The wattage is usually on the appliance label or in its manual, sometimes shown as amps and volts that you multiply. Unlike a cost calculator, this tool reports energy only, so it stays valid no matter what you pay per kilowatt hour. The US Department of Energy publishes guidance on estimating appliance energy use. Every figure is computed deterministically from the formula shown below, with a worked example that reconciles exactly to the calculator so you can follow each step.
Energy use converts watts to kilowatts, then scales by time: kWh = watts / 1,000 x hours x days. A 1,500 W appliance running 4 hours a day for 30 days uses 180 kWh.
kWh usage formula
Energy (kWh) = W / 1,000 x H x D
W = power draw (watts)
H = hours run per day
D = number of days
Watts divided by 1,000 gives kilowatts; multiplied by the total hours of operation gives kilowatt hours, the energy your meter records.
Worked example
A 1,500 watt space heater runs 4 hours a day for 30 days.
- Power = 1,500 / 1,000 = 1.5 kW
- Total hours = 4 x 30 = 120 hours
- Energy = 1.5 x 120 = 180 kWh
The appliance uses 180 kilowatt hours over the month. These are the calculator's default inputs, so the result above matches the widget exactly.
kWh Usage Calculator: frequently asked questions
What is a kilowatt hour?
A kilowatt hour (kWh) is the amount of energy a 1,000 watt device uses in one hour. It is the unit electric utilities bill in. A 100 watt bulb left on for 10 hours uses 1 kWh, and a 1,500 watt heater running one hour uses 1.5 kWh.
How is kWh different from watts?
Watts measure power, the rate of energy use at an instant. Kilowatt hours measure energy, the total amount used over time. A high-wattage device used briefly can use fewer kilowatt hours than a low-wattage device left on for a long time.
How do I find my appliance's wattage?
Look for a wattage rating on the appliance label or in its manual. If only amps and volts are listed, multiply them to get watts. For appliances that cycle, such as fridges, use the annual kilowatt hour figure from the energy guide label instead of nameplate watts.
How do I turn kWh into a cost?
Multiply the kilowatt hours by your electricity rate per kilowatt hour, which appears on your utility bill. For example, 180 kWh at $0.15 per kWh costs $27.00. Our appliance energy cost calculator does this step for you.
Does this work for a whole house?
Yes, if you know the average power draw. For a whole house it is easier to read total kilowatt hours directly from your electric meter or utility bill, since many appliances run at once and cycle on and off.
Official sources
- Estimating appliance energy use in kilowatt hours: US Department of Energy (DOE). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.