E l e c t r i c e n e r g y Power is usually defined as energy divided by time P = E / t , so that energy is power times the time, E=Pxt E=energy (joules) , P = power (watts), t = time (seconds) C a l c u l a t i o n s 1a) An electric shower has 1500 W. If it is used during 200 seconds, how much energy has been used? b) How much energy is used if you use a 2 kW electric shower during half an hour? Please give the answer in Joules (J). c) Considering that a kWh is charged at $ 0.10, how much does it cost to run a 1000 W kettle for 30 min? 2. An electric iron uses 4050 joules of energy for 15 seconds. What’s the power rating on the iron? 5. A computer that is 87% efficient consumes 375 000 J of energy. How much useful energy does it provide? 6. A television that is 83% efficient provides 4 600 J of useful energy. How much energy does it consume?