What is efficiency, and how do you calculate it for a device?
Efficiency: the meaning of efficiency, the efficiency equation as a ratio of useful to total energy (or power), and why no device is perfectly efficient.
A focused answer to Edexcel GCSE Physics 3.11, covering the meaning of efficiency, the efficiency equation in terms of useful and total energy transferred (and as a percentage), the power form of the equation, Sankey diagrams, and why no real device is 100% efficient, with worked calculations.
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What this dot point is asking
Edexcel statement 3.11 wants you to recall and use the efficiency equation, expressed as the ratio of the useful energy transferred to the total energy supplied (and as a percentage), and to understand why mechanical and electrical devices are never perfectly efficient.
What efficiency means
A device takes in energy and transfers some of it usefully and the rest wastefully, almost always as heating of the surroundings. Efficiency compares the useful output with the total input, so it tells you how good the device is at doing its intended job without wasting energy. Because energy is conserved, the useful plus the wasted energy always equals the total input.
The efficiency equation
Efficiency has no unit because it is a ratio of two energies (or two powers). It is given on the Edexcel equation sheet, but you must know how to use it, including rearranging it to find the useful energy or the total energy when efficiency is known. An efficiency above or is impossible and signals an arithmetic error.
Sankey diagrams and wasted energy
A Sankey diagram is a visual way to see efficiency: the wider the useful-output arrow compared with the total input, the more efficient the device. The wasted branches (usually thermal energy to the surroundings) show where the energy goes. No real device transfers all its input usefully, so there is always at least one wasted branch.
How Edexcel examines this
Efficiency is examined on both tiers, most often as a two to four mark calculation, sometimes followed by a request to explain what happens to the wasted energy. The mark scheme rewards the correct ratio (useful over total), the value, and, where asked, the conversion to a percentage, so set the calculation out clearly. A frequent style provides the total input and the useful output and asks for the percentage efficiency; another gives the efficiency and one energy and asks you to find the other by rearranging. Higher-tier papers may present a Sankey diagram and ask you to read off the useful and wasted energies and calculate the efficiency from them. When a question asks about the wasted energy, the full-mark answer states that it is dissipated to the thermal store of the surroundings and that it is still conserved overall, linking back to the conservation and dissipation statements. Watch for the power form of the equation: if the data are in watts, use useful power over total power, which gives the same efficiency.
Try this
Q1. A device transfers usefully from a total input of . Calculate the efficiency as a percentage. [2 marks]
- Cue. efficiency .
Q2. State why a real device can never be efficient. [1 mark]
- Cue. Some energy is always dissipated (wasted), usually as heating of the surroundings.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20203 marksA motor is supplied with of energy and transfers usefully to a kinetic store. Calculate the efficiency of the motor as a percentage.Show worked answer →
Use efficiency with useful and total (1 mark). Substitute: efficiency (1 mark), and as a percentage (1 mark). Markers reward the correct ratio, the value and the conversion to a percentage. Putting the total on top, or the wasted energy in the numerator, is the usual error.
Edexcel 20224 marksA lamp is supplied with of energy and transfers usefully as light. Calculate the efficiency, and explain what happens to the rest of the energy.Show worked answer →
Efficiency , or (2 marks for the calculation and percentage). The remaining is dissipated to the thermal store of the surroundings, heating the lamp and the air around it (1 mark), and this energy is wasted because it is no longer useful, though it is still conserved (1 mark). Markers reward the efficiency value, identifying the wasted , and stating it is dissipated as thermal energy while still being conserved overall.
Related dot points
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A focused answer to Edexcel GCSE Physics 3.4 and 3.6 to 3.8, covering the principle of conservation of energy, why the total energy in a closed system does not change, how energy is dissipated to less useful stores, and why mechanical processes waste energy by heating the surroundings.
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A focused answer to Edexcel GCSE Physics 3.9 and 3.10, covering ways of reducing unwanted energy transfer including lubrication and thermal insulation, and how the thickness and thermal conductivity of the walls of a building affect its rate of cooling.
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A focused answer to Edexcel GCSE Physics 3.1 and 3.2, covering the change in gravitational potential energy equation, the kinetic energy equation, the units and what each symbol means, and how energy transfers between the gravitational and kinetic stores, with worked calculations.
- Power: power as the rate of energy transfer or work done, the power equation, the watt as a joule per second, and the core practical measuring personal power.
A focused answer to Edexcel GCSE Physics 8.12 to 8.14, covering the definition of power as the rate of energy transfer or work done, the power equation, the watt as a joule per second, comparing devices by power, and the core practical measuring personal power, with worked calculations.
Sources & how we know this
- Pearson Edexcel GCSE (9-1) Physics (1PH0) specification — Pearson (2016)
- Edexcel GCSE Physics and Combined Science equation list (1PH0/1SC0) — Pearson (2025)