# What does the energy efficiency ratings on products mean?

Mar 18, 2016

$\text{Efficency(%)" = "useful energy"/"total energy} \times 100$

#### Explanation:

On a product there will be an efficiency number, this tells you how much of the energy produced is useful.

For example, a light bulb may be 20% efficient. If the bulb has a power of $\text{100 W}$, only $\text{20 W}$ are useful, light energy, while the other $\text{80 W}$ are waste such as sound or heat.