Power rating of an electrical appliance       Power rating of an electrical appliance is the rate of transferring electrical energy.      Power rating is measured in Watts (W).      Example: A 100 W bulb transfers 100 joules of electrical energy into light and heat every second.       Electrical energy transferred (Joules) = Power rating (Watts) x time (seconds)        Cost of electricity      The electricity meter shows the number of kilowatt hours used since it was last read.      Kilowatt hour (kWh) is another unit of electrical energy.       Electricity bill uses the kilowatt hour.       Electrical energy transferred (kWh) = Power rating (kW) x time (hours)       The electricity bill refers to kilowatt hour as a unit.     The electricity company has a set price for one kilowatt hour.      Cost of electricity  =  Total number of kilowatt hours used x Cost per kilowatt hour