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:

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

percentage~efficiency = efficiency \times 100

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

This is when both useful energy transferred and total energy supplied are 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?

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

efficiency = \frac{28}{200}

efficiency = 0.14

percentage~efficiency=efficiency \times 100

percentage~efficiency=0.14 \times 100


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.

Efficiency and power

As power is equal to useful energy transferred per second, another way to calculate efficiency is to use the formula:

efficiency = \frac{useful~power~transferred}{total~power~supplied}

percentage~efficiency = efficiency \times 100

This is when both useful power transferred and total power supplied are measured in watts (W).


A transformer usefully transfers 190,000 W of the 200,000 W of energy supplied to it. How efficient is the transformer?

efficiency = \frac{useful~power~transferred}{total~power~supplied}

efficiency = \frac{190,000}{200,000}

efficiency = 0.95

percentage~efficiency = efficiency \times 100

percentage~efficiency = 0.95 \times 100

percentage~efficiency = 95\%

The transformer is very efficient, since 95% of the energy supplied to it is usefully transferred. This means that the transformer only wastes 5% of the energy supplied to it.

It is not possible to have an efficiency of greater than 1 or efficiency percentage of 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.

Wasted energy

Devices waste energy for various reasons including friction between their moving parts, electrical resistance, and unwanted sound energy.

Devices can be made more efficient by reducing the energy that they waste or dissipate to the surroundings. One example is lubrication being used to reduce the friction between moving parts of a machine.