The efficiency of an electrical device depends on:

  • how much energy is wasted - transferred to unwanted stores
  • how much energy is transferred to useful stores

The more energy a device wastes, the less efficient it is.

To calculate efficiency, use the equation:

efficiency = \frac{useful~output~energy~transfer}{input~energy~transfer}

This is when:

  • energy transfer is measured in joules (J)
  • useful output energy refers to the useful energy that is transferred by the device (eg thermal energy by a heater)
  • input energy refers to the total energy supplied to a device


The energy supplied to a light bulb is 200 J. A total of 28 J of this is usefully transferred. How efficient is the light bulb?

efficiency = \frac{useful~output~energy~transfer}{input~energy~transfer}

efficiency = \frac{28}{200}

= 0.14

Efficiency can be calculated as a percentage. Multiply by 100 to convert this answer into a percentage:

percentage~efficiency = 0.14 \times 100

= 14\%

In this example, the light bulb is not very efficient since most of the energy supplied is not transferred usefully. Most of the energy will have been dissipated by infrared radiation. This is because light bulbs become very hot when they are switched on, which increases the thermal store of the surroundings.

Improving efficiency - Higher

Efficient devices use less fuel than less efficient device. They can operate at a lower power because less of the supplied energy store is wasted.

Efficiency can be improved by:

  • making devices and machines from materials which reduce unwanted energy transfer
  • improving the technology used, for example modern LEDs are much more efficient than traditional filament bulbs
  • using insulation to prevent thermal energy losses to the surroundings