Efficiency

This lesson covers efficiency and how to calculate it, including the use of Sankey diagrams, as required by the Edexcel GCSE Combined Science specification (1SC0). Efficiency is a key concept that links energy stores, transfers and dissipation.

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What Is Efficiency?

Efficiency is a measure of how much of the input energy (or power) is usefully transferred. It is expressed as a decimal or a percentage.

$\text{Efficiency} = \frac{\text{useful output energy transfer}}{\text{total input energy transfer}}$Efficiency=total input energy transferuseful output energy transfer​

$\text{Efficiency (\%)} = \frac{\text{useful output energy transfer}}{\text{total input energy transfer}} \times 100\%$Efficiency (%)=total input energy transferuseful output energy transfer​×100%

The same equations work with power:

$\text{Efficiency} = \frac{\text{useful output power}}{\text{total input power}}$Efficiency=total input poweruseful output power​

Quantity	Unit
Efficiency (decimal)	No unit (between 0 and 1)
Efficiency (percentage)	% (between 0% and 100%)

Exam Tip: Efficiency can never be greater than 1 (or 100%). If your calculation gives a value above 100%, you have made an error — check which is the input and which is the output.

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Worked Examples

Worked Example 1

A light bulb is supplied with 100 J of energy. It transfers 10 J as light and 90 J as heat. Calculate the efficiency.