Electrical appliances

Appliances, power and energy

All electrical appliances transfer energy from one store to another, for example chemical energy in the fuel in power stations. This is transferred into kinetic energy in a fan or heat energy in a cooker.

The amount of energy transferred depends on the power (the energy transferred each second) and the amount of time the appliance is switched on for. The energy transferred by an appliance can be calculated using the equation:

energy = power × time

E = P \times t

This is when:

  • energy (E) is measured in joules (J)
  • power (P) is measured in watts (W)
  • time (t) is measured in seconds (s)

One watt is the power when one joule of energy is transferred in one second.

Time should be converted from minutes into seconds - this is done by multiplying the number of minutes by 60.


How much energy is transferred by a 1,500 W hair dryer in 15 minutes?

E = P \times t

E = 1,500 \times (15 \times 60)

E = 1,500 \times (900)

E = 1,350,000~J~or~1.35~mega~joules~(MJ)

Power can also be calculated using the equation:

power = potential difference × current

P = V \times I

This is when:

  • Power (P) is measured in watts (W)
  • potential difference (V) is measured in volts (V)
  • current (I) is measured in amps (A)
When working with mains electricty and appliances, the potential difference is 230 V.

Example 1

What is the power of an electric heater that draws a current of 8 amps (A) when plugged into the mains?

P = V \times I

P = 230 \times 8

P = 1,840~W

Example 2

If the electric heater is used for 20 minutes, how much energy is transferred in that time?

E = P \times t

E = 1,840 \times (20 \times 60)

E = 1,840 \times (1,200)

E = 2,208,000~J~or~2.21~MJ