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    Efficiency calculations: 6 common traps

    By Engr. Mehmood Kakar · Updated

    Quick answer

    Efficiency = useful output ÷ total input, using energy for both or power for both, in the same units. The answer is a decimal less than 1; multiply by 100 for a percentage. To find the input, rearrange: total input = useful output ÷ efficiency, using the decimal.

    The efficiency equations

    There are two versions on the equation sheet:

    • efficiency = useful output energy transfer ÷ total input energy transfer
    • efficiency = useful power output ÷ total power input

    An electric motor takes in 500 J and transfers 400 J usefully as kinetic energy. Calculate its efficiency. (2 marks)

    1. efficiency = 400 ÷ 500
    2. efficiency = 0.80 (80%)

    Trap 1: mixing energy and power

    Use energy for both, or power for both. If a question gives the input power in watts and the useful energy in joules, convert one using E = P t before you divide.

    An LED bulb has an input power of 60 W. It gives out 9 W as light. Calculate its efficiency. (2 marks)

    1. Both values are powers, so use the power version
    2. efficiency = 9 ÷ 60
    3. efficiency = 0.15 (15%)

    Trap 2: different units

    2.4 kJ ÷ 3000 J is not 0.0008. Convert first: 2.4 kJ = 2400 J, so the efficiency is 2400 ÷ 3000 = 0.80.

    Trap 3: putting a percentage in the equation

    If a question says a kettle is 90% efficient, use 0.90 in the equation, not 90. Using 90 gives an answer 100 times too small (or too big).

    Trap 4: using wasted energy as useful

    Read carefully. "A lamp wastes 48 J of every 60 J" gives a useful output of 60 − 48 = 12 J, so the efficiency is 12 ÷ 60 = 0.20, not 48 ÷ 60.

    A machine is 75% efficient. It takes in 2000 J. How much energy is wasted? (3 marks)

    1. useful output = efficiency × total input = 0.75 × 2000 = 1500 J
    2. wasted = total input − useful output = 2000 − 1500
    3. wasted energy = 500 J

    Trap 5: rearranging the wrong way

    To find the total input, divide the useful output by the efficiency. Multiplying gives a smaller number, which can’t be right because the input is always bigger than the useful output.

    A kettle is 90% efficient. It transfers 270,000 J usefully to the water. Calculate the total energy supplied. (3 marks)

    1. total input = useful output ÷ efficiency
    2. total input = 270,000 ÷ 0.90
    3. total input = 300,000 J

    Trap 6: an efficiency above 100%

    If you get 1.25 or 125%, you have divided the total by the useful output. Flip it. No device can give out more energy than it takes in, so the answer must be less than 1.

    Increasing efficiency

    Efficiency goes up when less energy is wasted. Common examples are lubricating moving parts to reduce friction, insulating a kettle or house to reduce heat loss, and using LED bulbs, which waste less energy heating their surroundings than filament bulbs.

    Practice

    1. A motor is 35% efficient. Its input power is 1200 W. Calculate its useful power output.

    useful = 0.35 × 1200 → 420 W

    2. A light bulb gives out 500 J of light and is 25% efficient. Calculate the energy it takes in.

    total = 500 ÷ 0.25 → 2000 J

    3. A crane uses 48 kJ of electrical energy to lift a load, giving it 36 kJ of gravitational potential energy. Calculate the efficiency as a percentage.

    36 ÷ 48 = 0.75 → 75%

    Frequently asked questions

    What is the equation for efficiency?

    Efficiency = useful output energy transfer ÷ total input energy transfer, or useful power output ÷ total power input. Both are on the AQA equation sheet.

    Does efficiency have a unit?

    No. It is a ratio of two energies or two powers, so the units cancel. You can give it as a decimal (0.75) or a percentage (75%).

    Can efficiency be more than 100%?

    No. A device can never give out more useful energy than it takes in. An answer above 1 (or 100%) means the numbers have been put in the wrong way round.

    How do you work out wasted energy?

    Wasted energy = total input − useful output. For example, if a motor takes in 500 J and gives 400 J of useful energy, 100 J is wasted.

    Engr. Mehmood Kakar

    Written by Engr. Mehmood Kakar

    Physics educator and engineer (MSc Physics, B.Ed, M.Ed) with 8 years of hands-on teaching, from college students to teacher training. About the author · LinkedIn (opens in a new tab)

    Checked against the AQA GCSE Physics (8463) specification on 14 November 2026.