# Efficiency

The of an electrical device depends on:

• how much 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:

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

### Example

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

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 by . 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 bulbs
• using insulation to prevent thermal energy losses to the surroundings