Energy efficiency

Devices are designed to waste as little energy as possible. This means that as much of the input energy as possible should be transferred into useful energy stores.

How good a device is at transferring energy input to useful energy output is called efficiency.

A very efficient device will waste very little of its input energy. A very inefficient device will waste most of its input energy.

The efficiency of a device is the proportion of the energy supplied that is transferred in useful ways. The efficiency can be calculated as a decimal or a percentage, using the equations:

\text{efficiency}   = \frac{\text{useful energy transferred}}{\text{total energy supplied}}

\text{percentage efficiency} = \text{efficiency} \times 100

\text{percentage efficiency} = \frac{\text{useful energy transferred}}{\text{total energy supplied}} \times 100

This is when both useful energy transferred and total energy supplied is measured in joules (J).


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?

\text{efficiency}   = \frac{\text{useful energy transferred}}{\text{total energy supplied}}

\text{efficiency} = \frac{28}{200}

\text{efficiency} = 0.14

\text{percentage efficiency} = \text{efficiency} \times 100

= 0.14 × 100

percentage efficiency = 14%

The light bulb is not very efficient since most of the energy supplied is not transferred usefully. Most of the energy is dissipated as infrared radiation and only 14% is transferred usefully as light radiation.

Increasing efficiency – Higher

Understanding how energy moves through a system allows scientists and engineers to modify the system to improve the efficiency.

A normal filament light bulb works by heating a thin wire until it glows. However, most of the energy that goes into a light bulb is actually used to heat the room!

A transfer diagram breaking down the energy pathways of a bulb filament

Light emitting diodes emit light without getting hot, so it is much more efficient to use these to provide light as much less input energy is wasted.

There are other, simpler, ways of improving the efficiency of devices. For example:

  • putting the right amount of water into a kettle means that energy will not be wasted heating water that won't be used
  • not putting fridges and freezers next to cookers and ovens in the kitchen - the air that is being cooled by the fridge/freezer will not be too warm
  • allow room around fridges/freezers for the air to circulate otherwise the device may overheat
  • lower the temperature setting that the washing machine uses to wash clothes - to avoid energy being wasted in heating the water
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