PRACTICAL METHOD
How to estimate appliance electricity use and cost
A short reference for reading the estimate and checking its assumptions.
Estimated energy is watts ÷ 1,000 × hours of use. Multiply kilowatt-hours by your electricity price per kWh to estimate cost. For a 100-watt lamp used 5 hours a day, the estimate is 0.5 kWh per day. At an entered rate of $0.18 per kWh, that is $0.09 per day before fixed charges or other bill adjustments.
Use the energy rate from your bill if available. Nameplate watts and operating hours are assumptions: refrigerators, air conditioners, computers, and other devices cycle or change load. For a closer measurement, compare a suitable plug-in electricity meter or utility interval data over a representative period. Monthly estimates here use 30 days; yearly estimates use 365 days.