Standby power cost: watts to annual kWh
Estimate the yearly electricity cost of an always-on device, and learn when a nameplate wattage is the wrong input.
Convert watts and hours to energy
Energy in kWh = watts × hours ÷ 1,000. A device drawing a measured 5 W in standby for 24 hours a day over 365 days uses 5 × 24 × 365 ÷ 1,000 = 43.8 kWh. At a hypothetical tariff of 0.20 per kWh, the energy charge is 8.76 per year before fixed charges or taxes. Use the electricity cost calculator.
Measure standby rather than guessing
A device can use electricity while appearing off, but its active nameplate wattage may be much higher than actual standby draw. A suitable plug-in meter can measure average standby power over a representative interval. The U.S. Department of Energy describes standby measurement. Follow the meter and appliance instructions.
Add devices without double-counting
For multiple devices, calculate each measured wattage and its real hours, then add kWh before multiplying by the tariff. A device used in standby only 12 hours daily would use half the 24-hour estimate at the same power. The watts, kWh and electricity cost guide explains the distinction between power and energy.