Energy. Power x Time. Example 1: How much energy does a 40 Watt bulb use in 2 minutes? ... How long does it need a 1500 Watt vacuum cleaner to produce 3000 Joules of energy? ... How many units of electricity are used when you run a 2kW cooker for 4 hours ... How much would it cost to run a 3kW heater for 10 hours.