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Efficiency calculator
Energy and power
Efficiency is the proportion of the input energy that is transferred usefully. It can be given as a decimal or a percentage.
efficiency = Euseful / Etotal
This free calculator is for GCSE and A-level physics. Choose the quantity you want to find, enter the other values with their units, and it shows the working in the layout used in exams, with the answer to the number of significant figures you choose. The values shown are an example. Change them to your own.
Quantity to find
Working
- Write the equationefficiency = Euseful / Etotal
- Substitute the valuesefficiency = 180 ÷ 750
- Calculateefficiency = 0.24
- Give the answer to 3 significant figuresefficiency = 24.0 %
As a decimal: 0.240
Worked example
Find efficiency, the efficiency. Given: useful energy output Euseful = 180 J; total energy input Etotal = 750 J.
- Write the equationefficiency = Euseful / Etotal
- Substitute the valuesefficiency = 180 ÷ 750
- Calculateefficiency = 0.24
- Give the answer to 3 significant figuresefficiency = 24.0 %
As a decimal: 0.240
Symbols and units
| Symbol | Quantity | SI unit |
|---|---|---|
| efficiency | Efficiency | no unit |
| Euseful | Useful energy output | J |
| Etotal | Total energy input | J |
Efficiency: rearranged
- To find efficiency: efficiency = Euseful / Etotal
- To find useful energy output: Euseful = efficiency Etotal
- To find total energy input: Etotal = Euseful / efficiency
Exam tip
Efficiency cannot be more than 1 (100%). You can use power instead of energy if both are for the same time.
Common questions
What is the formula for efficiency?
The equation is efficiency = Euseful / Etotal, where efficiency is the efficiency; Euseful is the useful energy output (J); Etotal is the total energy input (J).
What units should I use for efficiency?
In SI units: efficiency in no unit (a ratio), useful energy output in J, total energy input in J. The calculator also accepts common alternatives such as kilometres, grams or minutes, and converts them for you.
How do I rearrange efficiency = Euseful / Etotal?
Rearranging gives these forms: to find efficiency: efficiency = Euseful / Etotal. to find useful energy output: Euseful = efficiency Etotal. to find total energy input: Etotal = Euseful / efficiency.
Practise efficiency questions
Press the button for a question with random numbers. Work it out on paper first, then type your answer.
Related
Read the full explanation on the Efficiency formula page. More energy and power equations: Work done, Work done by an angled force, Elastic potential energy, Power from force and speed, Kinetic energy, Gravitational potential energy, Power, Energy transferred. See the full list on the physics equation calculator page.