How to calculate electricity cost
Convert watts and hours into kilowatt-hours, then multiply by your rate:
kWh per day = Watts × Hours ÷ 1,000
Cost = kWh × Rate per kWh
A 100-watt device run 5 hours a day uses 0.5 kWh daily. At $0.15 per kWh that's about 7.5¢ a day — roughly $2.25 a month and $27 a year.
Finding the numbers
Wattage is on the appliance label (or multiply volts × amps). Your rate per kWh is on your utility bill. This calculator uses 30 days per month and 365 per year; actual bills also include fixed fees and tiered or time-of-use rates.
Frequently asked questions
How do I calculate the cost of running an appliance?
Multiply the wattage by the hours used and divide by 1,000 to get kilowatt-hours (kWh), then multiply by your electricity rate. A 100-watt device run 5 hours a day uses 0.5 kWh daily; at $0.15 per kWh that is about 7.5 cents a day, or roughly $2.25 a month.
What is a kilowatt-hour (kWh)?
A kilowatt-hour is the energy used by a 1,000-watt appliance running for one hour. It is the unit your utility bills you in. Watts measure power at an instant; kWh measures energy used over time, which is what you pay for.
How do I find an appliance's wattage?
Check the label on the device or its power supply, usually on the back or bottom, listed in watts (W). If only volts and amps are shown, multiply them: 120 V × 2 A = 240 W. A plug-in power meter gives the most accurate real-world figure.
What is a typical electricity rate?
In the U.S. the average residential rate is roughly $0.15–$0.18 per kWh, but it varies widely by state and plan. Check your utility bill for your exact rate — it is usually listed per kWh — and enter it for an accurate estimate.
Does standby power add up?
Yes. Many devices draw a few watts even when "off" — so-called phantom or vampire load. Individually small, but across a whole home it can add several percent to your bill. Estimate it by entering the standby wattage and 24 hours a day.
Disclaimer: Estimates use a flat per-kWh rate and ignore fixed charges, taxes, and tiered pricing. Provided for educational purposes.