Watts to kWh: calculate appliance electricity cost
Convert power and running time into energy, then estimate cost using actual tariffs and realistic operating hours.
Power is not energy
Watts (W) measure the rate of energy use. Kilowatt-hours (kWh) measure energy over time. A 1,000 W appliance draws 1 kW while operating at that power. It consumes 1 kWh if it maintains that power for one hour. A watt rating alone is not a monthly electricity bill.
Use the formula
Energy in kWh = watts ÷ 1,000 × hours operated. Energy charge = kWh × price per kWh. For a 1,500 W heater running at full power for two hours daily over 30 days: 1.5 × 2 × 30 = 90 kWh. At an illustrative tariff of 0.20 currency units per kWh, the energy charge is 18. Use the electricity cost calculator with your own tariff.
Estimate actual operation
A refrigerator or thermostat-controlled heater does not necessarily draw its rated power continuously. Use measured average consumption or a reasonable estimate of time actively running. For example, a 200 W device active 40% of a 24-hour day uses 0.2 × 24 × 0.4 = 1.92 kWh daily, excluding any separately relevant standby use. This 40% is an example, not a recommended duty cycle for your appliance.
Compare a replacement fairly
Replacing a 60 W lamp with a 9 W lamp used four hours daily saves (60 − 9) ÷ 1,000 × 4 × 365 = 74.46 kWh annually. At 0.20 per kWh, that is 14.89 per year before rounding effects and other costs. Compare equivalent light output, not just wattage. The lighting savings calculator can test purchase-cost and use assumptions.
Why the bill can differ
Fixed charges, tiered rates, time-of-use prices and taxes can change the bill. At variable tariffs, calculate each time period separately and add the charges. Do not multiply the whole bill by an appliance’s energy share if fixed charges are included. For an EV, see charging cost and losses; add the result to your monthly budget.
Common questions
Does a device marked 2,000 W always use that much? No. The label may describe a rated or maximum condition rather than average consumption.
Should I use a plug-in energy meter? It can help for compatible plug-in appliances within the meter’s ratings. Follow the manufacturer’s instructions; do not modify wiring or use a meter beyond its limits. This is an energy estimate, not electrical installation advice.
Reference
Virginia Cooperative Extension: estimating appliance energy use