Define power and its SI unit. Show that 1 kW h = 3.6 × 10⁶ J. A 2 kW heater runs for 3 hours; find the energy used in kW h and in joules.

1. 1 · Define power. Power is the rate of doing work: P = W/t.
2. 2 · Give the unit. The watt: 1 W = 1 J s⁻¹. Larger units are the kW and the MW; 1 hp = 746 W.
3. 3 · Convert the kilowatt hour. 1 kW h = 1000 J s⁻¹ × 3600 s = 3.6 × 10⁶ J.
4. 4 · Solve the numerical. Energy = 2 kW × 3 h = 6 kW h = 6 × 3.6 × 10⁶ J = 2.16 × 10⁷ J.
5. 5 · Real-life link. These 6 kW h show up as 6 units on the electricity bill.
6. 6 · Close in one line. So the kilowatt hour is a unit of energy, not of power.

Scoring tip: Show the hour as 3600 s in your working. Leaving out that step is the usual way to lose the mark.