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

## 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 useful **energy output**, divided by the **total energy input**, and can be given as a decimal always less than 1 or a percentage. No machine is 100% efficient.

## Efficiency equation

To calculate** Efficiency** we use this equation.

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

## Efficiency demo

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

## Chilled practice question

Calculate the **efficiency** of a light bulb with a **total input energ**y 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 the **total input energy** in **Joules**.

## Science in context

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

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