Electrical power and energy

Electrical power tells you how quickly an appliance transfers energy. Energy companies charge for energy in kilowatt-hours (kWh).

Key facts

  • P = IV; also P = I²R.
  • Energy E = Pt (J if P in W and t in s).
  • 1 kWh = 1 kW for 1 hour = 3.6 MJ.
  • Cost = kWh × price per kWh.
  • UK mains: 230 V, 50 Hz a.c.

Worked examples

Example 1

A 2.5 kW kettle is used for 6 minutes a day for 30 days. At 28p per kWh, how much does it cost?

  1. Time: 6 × 30 = 180 min = 3 h.
  2. Energy: 2.5 × 3 = 7.5 kWh.
  3. Cost: 7.5 × 28 = 210p = £2.10.

Answer: £2.10

Example 2

A lamp draws 0.25 A from a 12 V supply. Find its power.

  1. 0.25 × 12 = 3 W

Answer: 3 W