Annual electricity cost from rated power, hours of use and your own tariff.
Formula
annual cost = power (W) / 1000 x hours per day x days per year x price per kWh, plus standby power (W) / 1000 x standby hours x days x price per kWh
Assumptions
Rated power is the maximum draw on the label. Appliances with heating elements or motors cycle on and off, so real consumption is usually lower than rated power multiplied by time.
A single flat tariff is applied. Time-of-use and tiered tariffs are not modelled.
Standby hours are taken as 24 minus hours of use.
Limits
Use a plug-in energy meter or the appliance's annual consumption label for a figure you can rely on.