Power of an electrical appliance  Power of an electrical appliance is the rate at which it transforms electrical energy. . Power is measured in Watts (W).     Cost of electricity The electricity meter shows the number of kilowatt-hours used since it was last read. . The number of Kilowatt-hours is the amount of energy transferred from the mains.       Energy transferred    =    Power      x     time             kilowatt-hour             kilowatt          hour               kWh                       kW               h The electricity bill refers to kilowatt-hour as a unit. . The amount of electrical energy a device transforms depends on how long the appliance is switched on and the rate at which the device transforms energy. . The cost of energy transferred from the mains is given by the following formula:        Total cost  =  Number of kilowatt-hours  x  Cost per kilowatt-hour