Our Methodology

Cost = (Wattage ÷ 1,000) × Hours of Use × Electricity Rate An appliance's wattage divided by 1,000 gives kilowatts. Multiplying by hours of use gives kilowatt-hours (kWh), and multiplying by your electricity rate per kWh gives the cost. Example: a 1,500-watt space heater running 4 hours a day at the U.S. average rate of $0.18 per kWh costs about $1.08 per day. Monthly figures assume 30 days of use; yearly figures assume 365 days.

- United States: average residential electricity rates for all 50 states and DC from the U.S. Energy Information Administration (EIA) Electric Power Monthly (eia.gov). - United Kingdom: the electricity unit rate under the Ofgem energy price cap (ofgem.gov.uk). - Appliance wattages: manufacturer specifications and U.S. Department of Energy reference data. Defaults are typical values — your model may differ, which is why every calculator lets you adjust the wattage.

We review and update electricity rates quarterly, when the EIA publishes new state-level data and Ofgem announces price cap changes. The current dataset is Q3 2026, last verified July 2026. Every calculator and state page reads from the same central data file, so no page can fall out of sync.

All results are estimates. Your actual costs depend on your exact tariff, appliance efficiency, thermostat cycling, and usage patterns. For precise measurements, use a plug-in energy monitor; for billing questions, check your utility statement.

Spotted a number that looks wrong? Email hello@wattitcost.com and we will verify it against the source data.