The short answer
A kilowatt (kW) measures power: how fast energy is used or delivered at a given moment. A kilowatt-hour (kWh) measures energy: how much was used over a stretch of time. Power is like the speed of a car in miles per hour, energy is like the miles you drove. A 1 kW heater that runs for 3 hours uses 3 kWh, just as a car going 60 mph for 3 hours covers 180 miles.
Your power company bills energy, so the number that costs you money is kWh. The kW rating on an appliance only tells you how fast it adds kWh while it runs.
What the units mean in SI terms
The National Institute of Standards and Technology (NIST) lists the joule (J) as the SI unit of energy, work and heat, and the watt (W) as the SI unit of power, defined as one joule per second (J/s). The prefix kilo (k) means 10³, so a kilowatt is 1,000 watts. The EIA puts it the same way: one kilowatt is equal to 1,000 watts.
The hour is not an SI unit, but NIST lists it among the units accepted for use with the SI, with 1 h = 3,600 s. Put the pieces together and a kilowatt-hour is 1,000 joules per second for 3,600 seconds, which is 3,600,000 joules. NIST's conversion table gives exactly that: 1 kWh = 3.6 × 10⁶ J, or 3.6 megajoules.
The EIA's own example: a 40 W light bulb (0.04 kW) that is on for five hours uses 200 Wh, or 0.2 kWh. The same 0.2 kWh could also be a 200 W computer for one hour, or a 2,000 W kettle for six minutes. Very different power, same energy, same cost.
How your electric bill counts kWh
The EIA notes that utilities use meters to measure how much electricity their customers use, and that utilities and power plants generally measure electricity in kilowatt-hours. So the usage your bill charges for is energy in kWh, not power in kW. The Virginia Cooperative Extension's method for the cost of an appliance follows the same logic: kWh per year times your utility's rate per kWh.
Two numbers from the EIA give a sense of scale. The average residential price in the US was 18.19 cents per kWh from January to July 2026, according to table 5.6.B of its Electric Power Monthly. By state it ran from 12.36 cents in North Dakota to 46.28 cents in Hawaii, with 33.25 cents in California and 16.06 cents in Texas. And the average US residential customer bought 10,791 kWh in 2022, about 899 kWh a month, the latest year in the EIA's answer to how much electricity an American home uses.
Combine the two, keeping in mind that they come from different years: 899 kWh × $0.1819 = $163.53 for a month of average use at the 2026 average price. Your own bill will differ, because your use, your rate and any fixed monthly fees are your own. To find the price you really pay per kWh, divide the charges that grow with your use by the kWh on the bill.
Appliance examples, worked through
To get kWh from a label, turn the watts into kilowatts and multiply by the hours. The ratings below are example label values for the arithmetic, not measurements of any particular product. Read the rating on your own device, or better, measure it with a plug-in meter. Cost is at the 18.19 cent average.
| Device | Power | Hours | Energy | Cost |
|---|---|---|---|---|
| Space heater | 1.5 kW | 4 | 6 kWh | $1.09 |
| LED grow light | 0.3 kW | 18 | 5.4 kWh | $0.98 |
| Desktop computer | 0.2 kW | 8 | 1.6 kWh | $0.29 |
| Light bulb (EIA example) | 0.04 kW | 5 | 0.2 kWh | $0.04 |
| EV on a Level 2 charger | 7.2 kW | 2 | 14.4 kWh | $2.62 |
The space heater line in detail: 1,500 W is 1.5 kW. Four hours at 1.5 kW is 6 kWh. At $0.1819 per kWh that is $1.09 a day, or $32.74 over 30 days. The electricity cost calculator does this for any appliance and rate, per day, month and year.
The grow light shows why hours matter as much as watts. At 0.3 kW it draws a fifth of the heater's power, but at 18 hours a day it uses 5.4 kWh, almost as much as the heater's 6 kWh. Over 30 days that is 162 kWh, or $29.47. The grow light cost calculator handles several lights, on/off schedules and extras like fans.
A label rating does not tell you how long a device actually draws that power. The Virginia Cooperative Extension points out that many appliances have several settings, so the power drawn depends on the setting in use. A refrigerator cycles on and off to hold its temperature, and the extension suggests dividing the time it is plugged in by three to estimate how long it runs at full wattage. For devices like these, a plug-in meter or the yearly kWh on an energy label beats watts × hours.
Electric cars: kW for the charger, kWh for the trip
An electric car puts both units in front of you at once, and it is where they get mixed up most.
- Charger power in kW. The Department of Energy's Alternative Fuels Data Center lists Level 1 charging from a 120 V plug at about 1.9 kW, which adds about 5 miles of range per hour. Most residential Level 2 chargers run at up to 30 amps and deliver 7.2 kW, about 25 miles of range per hour, and Level 2 units range from 2.9 to 19.2 kW. DC fast chargers go up to 500 kW.
- Consumption in kWh per distance. fueleconomy.gov rates the 2026 Chevrolet Equinox EV with front-wheel drive at 31 kWh per 100 miles, which is 0.31 kWh per mile.
- Motor power in kW. The same listing gives the motor as 180 kW. That is how much power the car can put to the wheels at full throttle, not how much energy it uses on an average mile.
Put them together. One hour on a 7.2 kW Level 2 charger delivers 7.2 kWh. Divided by 0.31 kWh per mile that is about 23 miles of range, close to the 25 miles the AFDC quotes as typical. fueleconomy.gov lists 9.5 hours to charge this car's empty battery at 240 V. A 100-mile day uses 31 kWh, which costs $5.64 at the national average price. The EV charging cost calculator works out the cost per charge, per mile and per month with your own rate and efficiency, and the EV vs gas car guide compares that with a gas car.
A bigger charger does not make the trip cheaper. It delivers the same kWh faster. The price per kWh, and how many kWh the car needs per mile, decide the cost.
Converting kWh to other energy units
Energy has other units too. If you have a gas bill in therms or a heater rated in Btu, NIST's conversion table gives the factors in joules, so every unit can go through joules:
- 1 kWh = 3,600,000 J = 3.6 MJ
- 1 Btu (IT) = 1,055.056 J, so 1 kWh = about 3,412 Btu
- 1 therm (US) = 105,480,400 J, so 1 therm = about 29.3 kWh
That last line is handy if you heat with gas and want to compare a gas bill in therms with an electric bill in kWh. Keep in mind that this compares the energy content only, not what a furnace or a heat pump makes of it. The energy converter converts between kWh, joules, Btu, calories and more.
Common mix-ups
- "kW per hour". Usage is kW times hours, not kW per hour. If a contractor or a forum post says "kilowatts per hour" about a bill, they mean kWh.
- Forgetting the factor of 1,000. A 1,500 W heater is 1.5 kW. Multiply 1,500 by the hours and the price and you get a bill 1,000 times too high.
- Multiplying the label by every hour. A fridge plugged in for 24 hours does not run for 24 hours. By the extension's rule of thumb it runs about a third of that time, and a device on a low setting draws less than on high.
- Comparing charger kW with battery kWh. A 7.2 kW charger and a battery measured in kWh answer different questions: how fast, and how much. Time to charge is roughly the kWh you add divided by the charger kW.
- Using cents as dollars. 18.19 cents per kWh is $0.1819. Enter the dollar figure in calculators, or the result will be off by 100.
Frequently asked questions
Is a kWh the same as a kW per hour?
No. A kWh is kilowatts multiplied by hours, an amount of energy. A "kW per hour" would be kilowatts divided by hours, a rate at which power changes, which almost never comes up for a household. When someone says "kilowatts per hour" about a bill, they mean kWh.
How many kWh does a 1,000 W appliance use?
One kWh for every hour it runs at full power. 1,000 W is 1 kW, and 1 kW × 1 h = 1 kWh. Half an hour is 0.5 kWh, ten hours is 10 kWh. Devices with a thermostat or a compressor cycle on and off, so they usually use less than the label times the hours.
What does 1 kWh cost?
Whatever your bill says per kWh. The US Energy Information Administration put the average residential price at 18.19 cents per kWh for January to July 2026, with large differences between states, from 12.36 cents in North Dakota to 46.28 cents in Hawaii.
Why does my bill show kWh but my charger shows kW?
The meter counts energy, which is what you pay for, so the bill is in kWh. A charger is limited by how fast it can deliver energy, which is power, so it is rated in kW. Multiply the charger kW by the charging hours to get the kWh that will show up on the bill.
How do I convert kWh to BTU or therms?
NIST gives 1 kWh = 3.6 million joules, 1 Btu (IT) = 1,055.056 joules and 1 US therm = 105,480,400 joules. So 1 kWh is about 3,412 Btu, and 1 therm is about 29.3 kWh. The energy converter does these and other conversions.
Sources
- NIST Guide to the SI, Chapter 4: SI derived units (watt = J/s, joule) and SI prefixes (kilo = 10³)
- NIST Guide to the SI, Chapter 5: Units outside the SI (hour = 3,600 s)
- NIST Guide to the SI, Appendix B.8: conversion factors (kilowatt hour, Btu, therm)
- US Energy Information Administration, Electric Power Monthly, table 5.6.B: average price of electricity by state, year to date through July 2026
- US Energy Information Administration, How much electricity does an American home use?
- US Energy Information Administration, Measuring electricity
- Grisso, Walker and Ignosh, Estimating Appliance and Home Electronic Energy Use, Virginia Cooperative Extension publication 2901-9014, 2020
- US Department of Energy, Alternative Fuels Data Center, Charging electric vehicles at home and in public
- US EPA and DOE, fueleconomy.gov, 2026 Chevrolet Equinox EV FWD