Watts to kWh Calculator: Convert Watts and Hours to kWh

A watt measures how fast a device uses energy at one moment. A kilowatt-hour measures how much energy it used over a stretch of time. To go from one to the other you need the time.

1. Convert watts to kilowatts by dividing by 1,000.
2. Convert the time to hours. Divide minutes by 60.
3. Multiply kilowatts by hours. The result is kWh.

Worked example one: a 1,500 watt heater is 1.5 kW. Two hours of use is 1.5 times 2, which is 3.0 kWh. At the 2025 US average price of 17.23 cents that costs $0.52.

Worked example two: a 60 watt bulb is 0.06 kW. Ninety minutes is 1.5 hours. That is 0.06 times 1.5, or 0.09 kWh, which costs $0.02.

Small loads add up when they run all day. This table shows the energy and cost of a steady draw over a 30 day month and over a year.

Source: Price: 17.23 cents per kWh, the 2025 US residential average from EIA Form EIA-861.

• Watt (W). A rate of power. A 100 watt bulb draws 100 watts whenever it is on.
• Kilowatt (kW). 1,000 watts. Large appliances and chargers are rated in kW.
• Watt-hour (Wh). One watt used for one hour. Batteries and small devices use it.
• Kilowatt-hour (kWh). 1,000 watt-hours. Your electric bill is built on it.

Your bill gives kWh, and sometimes you want the average power. Divide the kWh by the hours in the period and multiply by 1,000.

A bill that shows 720 kWh for a 30 day month covers 720 hours. That is 720 divided by 720, times 1,000, or 1,000 watts on average. For comparison, the average US home used about 874 kWh a month in 2025, an average draw of about 1,214 watts.

• Saying a device uses watts per hour. Watts are already a rate, so use watts, then multiply by hours.
• Using the label wattage for something that cycles. A refrigerator or air conditioner runs part of the time, so the average is lower than the label.
• Forgetting minutes. Ten minutes is one sixth of an hour, not 10.
• Mixing units. Enter kW as kW and watts as watts.