GCSE & IGCSE physics practice

Efficiency questions and answers

Efficiency compares the useful output with the total input: efficiency = useful output energy ÷ total input energy (or the same with power). It's a decimal between 0 and 1, or a percentage when multiplied by 100. No real device is 100% efficient: the rest is wasted, usually as heat.

15 questions · Foundation tier · Every answer is checked as you type it, with a full worked solution. Updated 10 Nov 2026.

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Where efficiency comes up

GCSE (AQA)	Physics and Combined Science, Foundation and Higher. Recall both efficiency equations, for energy and for power (on AQA's list of equations to learn).
Cambridge IGCSE (0625)	Core: understand efficiency qualitatively. Supplement (Extended): define efficiency and recall and use (%) efficiency = useful energy (or power) output ÷ total input (× 100%).

The method

Efficiency = useful output ÷ total input.
× 100 for a percentage.
Useful output = efficiency × total input (use the decimal).
Wasted = total input − useful output.

Efficiency questions

Try each one before looking at the hint. If you're stuck, the AI tutor can talk you through it step by step.

Warm-up

Question 1Warm-upFoundation

A lamp uses 200 J of energy and gives out 40 J as light. What is its efficiency as a decimal?

Question 2Warm-upFoundation

A motor is supplied with 500 J and does 350 J of useful work. What is its efficiency as a percentage?

Question 9Warm-upFoundation

A motor is supplied with 1000 J and gives 750 J of useful energy. Calculate its efficiency as a percentage.

Question 10Warm-upFoundation

A 60 W filament bulb gives out 6 W of light. What is its efficiency as a decimal?

Exam-style

Question 3Exam-styleFoundation

A kettle has an input power of 2400 W and a useful output power of 2160 W. Calculate its efficiency as a percentage.

Question 4Exam-styleFoundation

A power station is 40% efficient. It is supplied with 5000 MJ of energy. How much useful energy does it output?

Question 5Exam-styleFoundation

A TV uses 150 J every second and wastes 45 J of it as heat. Calculate its efficiency as a percentage.

Question 6Exam-styleFoundation

An electric motor is 80% efficient and gives a useful output of 240 W. What is its input power?

Question 11Exam-styleFoundation

A car engine is 25% efficient. It is supplied with 8000 J of energy from fuel. How much useful energy does it output?

Question 12Exam-styleFoundation

A device wastes 30 J of every 120 J it is supplied with. Calculate its efficiency as a percentage.

Question 13Exam-styleFoundation

A loudspeaker is 85% efficient and has an input power of 20 W. How much power is wasted?

Challenge

Question 7ChallengeFoundation

A motor lifts a 20 kg mass through 3 m using 1200 J of electrical energy. Calculate its efficiency as a percentage. ( $g = 9.8$ N/kg)

Question 8ChallengeFoundation

A 60% efficient heater transfers 4.5 kJ of useful energy. How much energy is wasted?

Question 14ChallengeFoundation

A motor lifts a 15 kg load through 4 m in 5 s. Its input power is 200 W. Calculate its efficiency as a percentage. ( $g = 9.8$ N/kg)

Question 15ChallengeFoundation

A kettle uses 400 000 J of electrical energy to heat 1 kg of water by 80 °C. Calculate its efficiency as a percentage. ( $c = 4200$ J/kg °C)

Common mistakes

Dividing input by output (giving more than 1).
Using a percentage as a decimal in a calculation (75% is 0.75).
Adding wasted energy to the useful output.

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Sources

The questions are our own. The “where it comes up” notes are from each board's published specification, checked on 27 October 2026:

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