The basic relationship
One kilowatt-hour equals 1,000 watts used for one hour. A 100 W device running 10 hours also uses 1 kWh.
Why runtime matters
A high-power appliance used briefly can consume less than a low-power appliance left on continuously. That is why both watts and hours belong in a cost estimate.
From energy to cost
Multiply kWh by the applicable electricity rate. At $0.18 per kWh, 10 kWh of energy costs $1.80 before charges not represented by that rate.
Use the numbers
Apply this guide with one of WattMeasure’s free calculators. Results are planning estimates and should be verified for consequential decisions.
Last reviewed August 6, 2026