Energy from batteries and the mains

When work is done on an object, energy is transferred. The rate at which this energy is transferred is called power. So the more powerful a device is, the more energy it will transfer each second. This is important when considering the total cost of electricity used as shown on an electricity bill.

Work done is also known as energy transferred.

Calculating energy transferred

Joules and seconds

The energy transferred can be calculated using:

energy transferred = power × time

This is when:

  • energy transferred is measured in joules, J
  • power is measured in watts, W
  • time is measured in seconds, s


A 250 W computer is used for 10 minutes. Calculate the energy transferred.

10 minutes = 10 × 60 = 600 s

energy transferred = 250 × 600

energy transferred = 150,000 J (150 kJ)

Calculating electrical power

Power can be calculated using:

power = \frac{energy~transferred~(work~done)}{time}

This is when:

  • power is measured in watts (W)
  • energy transferred (work done) is measured in joules (J)
  • time is measured in seconds (s)

The power output of a device increases if:

  • more energy is transferred in a given time
  • the same amount of energy is transferred in a shorter time

The energy transferred by an electrical device can be investigated using a joulemeter. The power output of a device can be calculated if the length of time the device is switched on is also measured.


An electric lamp transfers 1,200 J in 2 minutes. Calculate its power.

2 minutes = 2 × 60 = 120 s

power = 1,200 ÷ 120

power = 10 W

Kilowatt-hours and hours

The electricity bills from an energy supplier show the energy used in kWh, rather than in J or kJ.

The energy transferred is still calculated using:

energy transferred = power × time


  • energy transferred is measured in kilowatt-hours, kWh
  • power is measured in kilowatts, kW
  • time is measured in hours, h


A 10 kW shower is used for 12 minutes. Calculate the energy transferred in kWh.

12 minutes = 12 ÷ 60 = 0.2 h

energy transferred = power × time

energy transferred = 10 × 0.2

energy transferred = 2 kWh