kWh Calculator
A kilowatt-hour (kWh) is the standard unit of electrical energy on every electricity bill in the United States. Knowing how many kWh an appliance consumes helps you estimate running costs, compare energy-efficient alternatives, and identify which devices are driving up your bill. This calculator takes an appliance's power rating in watts, your daily usage in hours, and the number of days per month, then returns daily kWh, monthly kWh, and projected annual kWh. The underlying formula is straightforward: divide the wattage by 1,000 to convert to kilowatts, then multiply by the number of hours used. For example, a 100-watt light bulb running for 8 hours a day uses 0.80 kWh per day, 24.00 kWh per month, and 288.00 kWh over a full year. Power ratings appear on the label affixed to most appliances or in the product manual. If the label shows only amps and volts, multiply the two to get approximate watts. Use this calculator alongside the electricity cost calculator to see the dollar value of that energy at your local rate.
How kWh is calculated
The formula converts watts to kilowatts then multiplies by time:
Daily kWh = Power (W) × Hours per day / 1,000
Monthly kWh = Daily kWh × Days per month
Annual kWh = Monthly kWh × 12
Worked example: 100W light bulb, 8 hours/day, 30 days/month
- Daily kWh = 100 × 8 / 1,000 = 0.80 kWh
- Monthly kWh = 0.80 × 30 = 24.00 kWh
- Annual kWh = 24.00 × 12 = 288.00 kWh
Frequently asked questions
What is a kilowatt-hour (kWh)?
A kilowatt-hour is a unit of energy equal to consuming 1,000 watts for one hour. It is the standard unit on electricity bills. For example, a 100-watt light bulb running for 10 hours uses 1 kWh of energy. The kWh figure on your utility bill represents the total energy consumed in the billing period, and your rate (in dollars per kWh) determines your charge.
How do I read the power rating on an appliance label?
Appliance labels show power in watts (W) or kilowatts (kW). Look for a label on the back or bottom of the device, or check the user manual. If only amperage (A) and voltage (V) are shown, multiply them to get approximate watts (W = A x V). Some devices list a range (e.g., 800-1200W); use the higher figure for a conservative estimate. Energy Star-rated products also list annual kWh consumption for a standard usage pattern.
What is the difference between kW and kWh?
Kilowatts (kW) measure power, which is the rate of energy use at a given moment. Kilowatt-hours (kWh) measure energy, which is the total amount of electricity consumed over time. A 2 kW heater running for 3 hours uses 6 kWh of energy. The distinction matters because your electricity bill charges you for kWh (energy used), not kW (power rating) alone.
Why does kWh matter for my electricity bill?
Your electricity provider charges per kWh consumed. Knowing how many kWh an appliance uses lets you estimate its monthly running cost by multiplying kWh by your rate. Reducing kWh consumption, whether by switching to more efficient appliances or reducing usage hours, directly cuts your bill. The U.S. Energy Information Administration (EIA) publishes average residential rates by state so you can compare your own rate.
How much electricity do common appliances use?
Typical power ratings vary widely. A standard LED light bulb uses around 8-10W. A desktop computer uses 100-400W. An electric kettle uses 1,000-1,500W. A central air conditioner uses 3,000-5,000W. A clothes dryer uses 4,000-6,000W. The U.S. Department of Energy provides detailed appliance energy use data to help you understand where household consumption is concentrated.
Sources
- U.S. Department of Energy, Estimating Appliance and Home Electronic Energy Use: energy.gov.
- U.S. Energy Information Administration, Electricity Explained: eia.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology. General information only.