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.

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

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

Calculating efficiency

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

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.

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

 percentage \ of \ efficiency = efficiency \times 100

 (percentage \ of \ efficiency = \frac{useful \ energy \ transferred}{total \ energy \ supplied} \times 100)

Both useful energy transferred and total energy supplied are measured in joules (J).


The energy supplied to a LED bulb is 140 J. Out of this total energy supplied, 108 J is usefully transferred. How efficient is the light bulb?

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

 efficiency= \frac{108}{140}

 efficiency= 0.77

 percentage\  efficiency= efficiency \times 100

 percentage\  efficiency= 0.77 \times 100

 percentage\  efficiency= 77 \ %

The LED bulb is quite efficient since most of the energy supplied is transferred usefully. Most of the energy dissipated goes to raising the temperature of the surroundings.

Increasing efficiency - Higher

It is not possible to have an efficiency of greater than 1 or an efficiency percentage greater than 100%. This would mean that more energy is being transferred than is being supplied, which would mean that energy is being created. This would break the law of conservation of energy.

Devices waste energy for various reasons, including friction between their moving parts, electrical resistance, and unwanted sound energy. In general, it is useful to increase the efficiency of any process or device. This can be achieved by reducing wasted energy transfers so more of the input energy is usefully transferred.

Mechanical devices can be made more efficient through lubrication to reduce the friction between moving parts of a machine and increase the amount of useful energy transferred.

For systems that are designed to transfer thermal energy, the efficiency can be improved by reducing the wasteful dissipation of thermal energy to the surroundings, for example by using insulation.