A kilowatt hour is calculated by adding the watts together and deviding by 1000, this will give you kilowatts (kW). Then work out how many hours you are using this load for. multiply the kW by the hours and you get kWh. eg 15 lamps at 100w = 1500w/1000 = 1.5 kW. If these were left on for 6 hours then 6 x 1.5 = 9 kWh.