What Is a kWh, and How Is Electricity Usage Calculated?
A kilowatt-hour (kWh) is the energy used by a 1,000-watt device running for one hour — it’s the standard unit your utility measures and bills you on, calculated as watts × hours used ÷ 1,000.
The basic formula
To estimate how many kWh a device uses, multiply its wattage by the number of hours it runs, then divide by 1,000. A 100-watt lightbulb left on for 10 hours uses 1 kWh (100 × 10 ÷ 1,000). A 1,500-watt space heater running for 4 hours uses 6 kWh — which shows why space heaters and other high-wattage devices move the needle on a bill so much faster than lighting does.
What a kWh actually costs
Multiply your usage in kWh by your supply rate (shown as $/kWh, or sometimes as cents per kWh, on your bill) to get the supply portion of your cost for that device or period. National average residential electricity rates are typically in the range of $0.12–$0.20 per kWh, though this varies significantly by state and by whether you’re on utility default supply or a competitive contract — your own bill is the only reliable source for your actual rate.
Which appliances use the most
- Central air conditioning and electric heating — typically the single largest usage source in a home.
- Electric water heaters, especially older tank-style units running continuously.
- Clothes dryers, which use far more energy than washers.
- EV chargers, which can add hundreds of kWh a month depending on driving distance.
- Older refrigerators and freezers, which can use 2–3x the electricity of a modern efficient model.
Frequently asked questions
How many kWh does an average household use per month?
U.S. residential usage commonly falls somewhere around 800–1,200 kWh per month, though this varies widely by home size, climate, and heating/cooling type — your own bill history is the most accurate benchmark for your household.
Is a kWh the same as a kW?
No — a kW (kilowatt) is a rate of power, like the speed of a car; a kWh (kilowatt-hour) is an amount of energy used over time, like the distance traveled. A device’s kW rating times the hours it runs gives you kWh.
What is a therm, for gas bills?
A therm is the unit gas utilities typically bill in, roughly equal to the heat produced by burning 100 cubic feet of natural gas — the gas-bill equivalent of a kWh on an electric bill.
See this on your own bill
Upload your electric or gas bill and Certo reads it for you — a free bill health score, a plain-English breakdown, and whether your rate is competitive.
Want it tracked automatically every month?
Plus and Pro track bill history, flag rate hikes and unusual usage, and (on Pro) build a custom efficiency checklist and savings tracker — so you're not re-checking this by hand.