Watts, kilowatts, and kilowatt-hours
Watts (W) describe the rate at which a device uses power. One kilowatt (kW) equals 1,000 watts. A kilowatt-hour (kWh) measures energy used over time: running a 1 kW device for one hour uses 1 kWh.
The U.S. Energy Information Administration explains that a 40-watt bulb used for five hours consumes 0.2 kWh. The same unit is used on most residential electricity bills.
The basic cost formula
For a simple planning estimate, convert watts to kilowatts, multiply by hours of use, multiply by the number of active days, and then multiply by the price per kWh.
- Power in kW = watts ÷ 1,000.
- Energy in kWh = power in kW × hours used.
- Estimated cost = energy in kWh × your local price per kWh.
For irregular schedules, estimate a representative day and enter the number of days that the device actually runs. This keeps the result tied to your routine instead of an unrealistic full-time assumption.
Why your bill may differ
The rate printed on a bill may not be the same as a headline average. The EIA notes that delivered electricity prices can include generation, transmission, distribution, taxes, and fees, while a utility tariff may apply charges differently.
Standby power, thermostatic cycling, changing schedules, seasonal rates, demand charges, and other household loads can also change the final bill. For a closer estimate, use a measured average or the appliance’s energy label and enter the all-in rate from your bill.
Continue with a calculator
Use the Electricity Cost Calculator for a recurring usage estimate, or review the Appliance Running Cost Calculator for a single appliance schedule. Keep the input values and assumptions with the result so you can revisit them when your rate or routine changes.