On this page
Formula
kWh = Watts × Hours ÷ 1,000, Watts = Amps × Volts × Power factor
- Watts
- the power the appliance draws while running
- Hours
- total running time (hours per day × number of days)
- Amps × Volts
- current and voltage, used when the wattage isn't known
- Power factor
- 1 for heaters and other resistive loads; often 0.8–0.9 for motors
What a kilowatt-hour is
A kilowatt-hour is the unit your meter counts and your utility bills you for. It’s an amount of energy: one kilowatt of power flowing for one hour. The EIA’s example makes the scale clear: a 40-watt bulb left on for five hours uses 200 watt-hours, which is 0.2 kWh.
The kWh calculator answers one question: how much energy does this thing use? Give it the appliance’s watts and running time, or the amps and volts if that’s what the nameplate shows, and it returns kilowatt-hours per day, per month, per year and over any number of days you choose.
How to use the kWh calculator
- Pick From watts if you know the power rating, or From amps and volts if the label lists current instead.
- Enter the power (W or kW), or the amps, the volts and, for motors, the power factor.
- Enter hours used per day and the number of days you want the total for.
- Optionally add a rate in dollars per kWh to see a cost beside the energy.
The steps under the result show each conversion, so you can check how the watts became kWh.
The kWh formula
kWh = W × h ÷ 1,000
Divide by 1,000 because a kilowatt is 1,000 watts. If you prefer, convert first: 750 W is 0.75 kW, and 0.75 kW × 4 h = 3 kWh.
Worked example from watts
A 1,200 W microwave runs 15 minutes a day. Fifteen minutes is 0.25 hour, so:
- Per day: 1,200 × 0.25 ÷ 1,000 = 0.3 kWh
- Over 30 days: 0.3 × 30 = 9 kWh
- Per year: 0.3 × 365 = 109.5 kWh
Worked example from amps and volts
A clothes dryer on a 240 V circuit draws 12 A. Its power is 12 × 240 = 2,880 W, or 2.88 kW. Run for 3 hours a week, it uses 2.88 × 3 = 8.64 kWh a week.
Adding the power factor
Motors, compressors and some electronics don’t turn all of their volt-amps into real work. Real power = amps × volts × power factor. A pump drawing 5 A at 120 V with a power factor of 0.85 uses 5 × 120 × 0.85 = 510 W, not 600 W. Heaters, toasters and incandescent bulbs have a power factor of 1, so you can leave it blank for them.
kWh by wattage and running time
Read across to see how running time multiplies energy. Monthly figures assume 8 hours a day for an average 30.4-day month.
| Power | 1 hour | 8 hours | 24 hours | Month at 8 h/day |
|---|---|---|---|---|
| 10 W | 0.01 kWh | 0.08 kWh | 0.24 kWh | 2.4 kWh |
| 60 W | 0.06 kWh | 0.48 kWh | 1.44 kWh | 14.6 kWh |
| 100 W | 0.1 kWh | 0.8 kWh | 2.4 kWh | 24.3 kWh |
| 300 W | 0.3 kWh | 2.4 kWh | 7.2 kWh | 73 kWh |
| 1,000 W | 1 kWh | 8 kWh | 24 kWh | 243.3 kWh |
| 1,500 W | 1.5 kWh | 12 kWh | 36 kWh | 365 kWh |
| 5,000 W | 5 kWh | 40 kWh | 120 kWh | 1,216.7 kWh |
A small device left on around the clock can rival a big one used briefly. A 10 W device running 24 hours uses as much as a 1,000 W toaster running 15 minutes, about 0.24 to 0.25 kWh.
Units you’ll meet
- Watt (W): a rate of energy use, one joule per second.
- Kilowatt (kW): 1,000 watts. Appliance power, EV chargers and solar arrays are rated in kW.
- Watt-hour (Wh): energy of 1 W for an hour. Phone and laptop batteries are rated in Wh.
- Kilowatt-hour (kWh): 1,000 Wh, equal to 3.6 megajoules. The unit on your electric bill.
- Power factor: the share of the apparent power (volts × amps) that does real work, from 0 to 1.
Where kWh figures are useful
Reading and checking your bill
Your bill shows total kWh for the billing period. Adding up your largest appliances with this calculator shows where those kilowatt-hours go. Once you know the energy, the electricity cost calculator turns it into dollars per day, month and year at your rate.
Sizing backup power and solar
Generators, home batteries and solar arrays are all sized in energy terms. A list of daily kWh for the essential loads is the starting point for any of them, and your monthly kWh total feeds straight into the solar panel calculator.
Circuit planning
The amps mode doubles as a load check. For the relationship between volts, amps, ohms and watts on a circuit, use the Ohm’s law calculator.
Common mistakes
- Forgetting to divide by 1,000. 500 W for 2 hours is 1,000 Wh, which is 1 kWh, not 1,000 kWh.
- Entering minutes as hours. Twenty minutes is 0.33 hour, not 0.2.
- Confusing kW with kWh. A 7 kW EV charger running 3 hours delivers 21 kWh.
- Using amps × volts for a motor. Without the power factor, the result overstates real power.
- Counting the nameplate for cycling appliances. Their real daily kWh is usually far below the label figure × 24.
Worked examples
A space heater for a month
A 1,500 W space heater running 6 hours a day for 30 days: 1.5 kW × 6 h × 30.
Inputs (From watts): Power rating = 1500; Power rating unit = Watts; Hours used per day = 6 h/day; Number of days = 30 days
Daily energy = 1,500 W × 6 h ÷ 1,000 = 9 kWh/day Total = 9 kWh/day × 30 days = 270 kWh
Answer: 1,500 W for 6 h a day uses 9 kWh a day, 270 kWh over 30 days and 273.75 kWh in an average month.
A gaming PC with a price
A 450 W gaming PC used 4 hours a day for a week, at 21¢ per kWh.
Inputs (From watts): Power rating = 0.45; Power rating unit = Kilowatts; Hours used per day = 4 h/day; Number of days = 7 days; Electricity rate (optional) = $0.21 /kWh
Daily energy = 450 W × 4 h ÷ 1,000 = 1.8 kWh/day Total = 1.8 kWh/day × 7 days = 12.6 kWh Cost = 12.6 kWh × $0.21/kWh = $2.65
Answer: 450 W for 4 h a day uses 1.8 kWh a day, 12.6 kWh over 7 days and 54.75 kWh in an average month, costing $2.65 at $0.21/kWh.
A window air conditioner from its amps
A unit drawing 7.5 A at 120 V (900 W) for 8 hours a day over a 31-day month.
Inputs (From amps and volts): Current = 7.5 A; Voltage = 120 V; Hours used per day = 8 h/day; Number of days = 31 days
Power = 7.5 A × 120 V = 900 W Daily energy = 900 W × 8 h ÷ 1,000 = 7.2 kWh/day Total = 7.2 kWh/day × 31 days = 223.2 kWh
Answer: 900 W for 8 h a day uses 7.2 kWh a day, 223.2 kWh over 31 days and 219 kWh in an average month.
A LED bulb for one day (edge case)
A 9 W LED bulb left on for half an hour uses only 4.5 Wh (0.0045 kWh).
Inputs (From watts): Power rating = 9; Power rating unit = Watts; Hours used per day = 0.5 h/day; Number of days = 1 days
Daily energy = 9 W × 0.5 h ÷ 1,000 = 0.0045 kWh/day Total = 0.0045 kWh/day × 1 day = 0.0045 kWh
Answer: 9 W for 0.5 h a day uses 0.0045 kWh a day, 0.0045 kWh over 1 day and 0.14 kWh in an average month.
Assumptions and limitations
- Energy = power × time. The power is treated as steady while the appliance is on, so the result is exact for heaters, kettles and lamps and an upper estimate for appliances that cycle or throttle.
- In amps mode, power = amps × volts × power factor. Leaving the power factor blank uses 1. Single-phase power is assumed; three-phase loads are not covered.
- Hours per day must be between 0 and 24. The total covers the number of days you enter; the monthly figure uses an average month of 365 ÷ 12 = 30.4 days.
- The optional rate is in dollars per kWh (18¢ = 0.18) and is applied as a flat price; tiered and time-of-use tariffs are not modeled.
- Results are rounded for display only (up to 4 decimals for small loads), after the full calculation.
Frequently asked questions
How do you calculate kWh from watts?
Multiply the watts by the hours of use and divide by 1,000. A 2,000 W kettle that runs 6 minutes (0.1 hour) a day uses 2,000 × 0.1 ÷ 1,000 = 0.2 kWh a day.
How do I calculate kWh from amps?
First find the watts, amps × volts, then use the kWh formula. A tool drawing 6 A on a 120 V outlet uses 720 W; running 2.5 hours, that's 720 × 2.5 ÷ 1,000 = 1.8 kWh. For motors, multiply by the power factor too.
What is the difference between kW and kWh?
A kilowatt (kW) is a rate of power, how fast energy is used right now. A kilowatt-hour (kWh) is an amount of energy, the power multiplied by the time it runs. A 1 kW heater uses 1 kWh every hour it's on.
How many kWh does a house use per month?
The EIA's figure for the average US residential customer is about 899 kWh a month (10,791 kWh in 2022). Homes with electric heat, pools or EVs use more; apartments usually use less.
How many watts is 1 kWh?
The two measure different things, so there's no single answer. 1 kWh is 1,000 watts for one hour, 100 watts for 10 hours, or 10 watts for 100 hours.
Why does my meter show fewer kWh than the label suggests?
The label lists the maximum draw. Refrigerators, air conditioners and dehumidifiers cycle on and off, and computers draw far less at idle than at full load. A plug-in energy meter gives the real figure.
Sources
- Electricity explained: Measuring electricity — U.S. Energy Information Administration. Checked October 10, 2026.
- How much electricity does an American home use? — U.S. Energy Information Administration. Checked October 10, 2026.
- NIST Guide to the SI, Appendix B.9: Factors for Units Listed by Kind of Quantity or Field of Science — National Institute of Standards and Technology. Checked October 10, 2026.