The power or a device is measured in watts (W). A kilowatt means 1000 watts.
Energy is normally measured in joules (J) but when calculating the cost of using electricity energy is measure in kilowatt-hours (kWh).
Energy transfered = power x time
(kilowatt-hours, kWh) (kilowatts, kW) (hours, h)
total cost = number of kilowatt-hours x cost of one kilowatt-hour
Example; If a 3 kW heater is used for 2 hours what was the total cost if every kW h of electricity costs 7 pence ?