# Efficiency

## What is Efficiency?

The efficiency of a device is the proportion of input energy that is converted to useful energy. Efficiency is a measure of how much work or energy is conserved in an energy transfer, work or energy can be lost, for example as wasted heat energy. The efficiency is the energy output, divided by the energy input, and can be given as a decimal always less than 1 or a percentage. No machine has an efficiency of 100%.

## Efficiency equation

To calculate Efficiency we write the equation like this.

$efficiency = {\text{useful energy output} \over\text{total energy input}}$

## Efficiency demo

In this tutorial you will learn how to calculate how efficient a device is at changing energy from one form to another.

## Chilled practice question

Calculate the efficiency of a light bulb with an input energy of 500 J. The bulb emits 200 J of light energy and 300 J of heat energy.

## Frozen practice question

A tumble drier is 80 % efficient. Its useful energy is 45 KJ what is its input energy in Joules.

## Science in context

The efficiency of a device is the proportion of input energy that is converted to useful energy

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