Key Stage 4 science · Physics – Energy

GCSE efficiency and energy resources: a guide for parents

This GCSE physics topic has two linked parts. Students calculate how efficiently a device or process turns the energy supplied into useful energy, and they look at the energy resources the world relies on, sorting them into renewable and non-renewable and weighing up their costs, reliability and environmental effects. They also look at how the mix of resources used has changed over time.

Efficiency and energy resources, explained

Efficiency measures how much of the energy put into a device ends up doing the job it was meant to do. It is found by dividing the useful energy transferred by the total energy supplied, or the useful power output by the total power input. The answer is a decimal between 0 and 1, or a percentage if multiplied by 100. Because some energy is almost always dissipated to the surroundings as unwanted thermal energy, real devices are less than 100% efficient, and an answer above 100% means that something is wrong with the working.

Students are taught to identify the useful and the wasted energy for a device, and to draw or read a diagram showing how the input splits. A filament lamp, for example, transfers electrical energy to light and a much larger amount to thermal energy, so it is inefficient as a lamp. Raising efficiency means cutting unwanted transfers: lubrication reduces friction in moving parts, and thermal insulation such as loft insulation, cavity wall insulation and double glazing slows heat escaping from a building. Students may be asked to compare two devices and decide which is better value, which means looking at both efficiency and cost.

Energy resources are the second half. Non-renewable resources will run out because they are used much faster than they form: the fossil fuels coal, oil and natural gas, and nuclear fuel such as uranium. Renewable resources are replaced naturally as we use them: wind, solar, hydroelectricity, tidal and wave power, geothermal energy and biofuels. Students learn the main uses, which are transport, heating and generating electricity, and for each resource, the advantages and disadvantages. Fossil fuels are reliable and relatively cheap but release carbon dioxide; wind and solar produce no carbon dioxide while operating but depend on the weather; nuclear produces very little carbon dioxide but leaves radioactive waste that has to be stored for a long time.

Students also look at how patterns of use have changed. Britain once generated most of its electricity from coal, and its last coal-fired power station closed in 2024; wind and solar now supply a much larger share. Reasons for change include cost, concern about climate change, new technology and government policy. Examiners do not expect students to take sides but to give balanced, evidence-based reasons for an energy choice.

Words your child will hear

Efficiency: Useful energy (or power) out divided by total energy (or power) in. Never more than 1, or 100%.
Useful energy: The energy transferred to the store you wanted, such as light from a lamp.
Wasted energy: Energy transferred to stores you didn’t want, usually thermal energy that spreads into the surroundings.
Non-renewable: A resource that will run out because it cannot be replaced as quickly as it is used.
Renewable: A resource that is naturally replaced, such as wind, solar or tidal power.
Fossil fuel: Coal, oil or natural gas, formed from ancient living things over millions of years.
Insulation: A material that slows the transfer of thermal energy, such as loft insulation.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about calculating energy efficiency for any energy transfers
Students should be taught about renewable and non-renewable energy sources used on Earth, and changes in how these are used

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

An electric motor lifts a load. For every 400 J of electrical energy supplied, 300 J is transferred to the gravitational store of the load and the rest is wasted as thermal and sound energy. Calculate the efficiency, say how much energy is wasted, and find the efficiency if oiling the bearings cuts the wasted energy to 60 J for the same input.

Write the equation and identify the numbers. Total energy in is 400 J. Useful energy out is the 300 J that lifts the load.
```
efficiency = useful energy out
             ----------------------
             total energy in
```

Substitute and divide.

efficiency = 300 ÷ 400
           = 0.75

As a percentage, multiply by 100: the motor is 75% efficient. The wasted energy is the total in minus the useful out.
```
wasted = 400 - 300 = 100 J
```
After oiling, only 60 J is wasted, so the useful energy is now 400 - 60 = 340 J. Divide again.
```
efficiency = 340 ÷ 400
           = 0.85 (85%)
```
Explain the change. Oil reduces friction between moving parts, so less energy is dissipated as thermal energy. The efficiency rises, but it is still below 100%, because some energy is always wasted.

The motor starts at 0.75 (75%) efficient, wasting 100 J. After oiling it is 0.85 (85%) efficient, because friction wastes less energy.

Common mistakes and how to help

Turning the efficiency fraction upside down

Students recall that it involves dividing useful and total energy and sometimes divide the wrong way, giving an answer above 1.

What helps: Remember that the useful part can never be bigger than the total, so it goes on top. A quick check is that an efficiency can never exceed 100%. If it does, swap the numbers.

Thinking wasted energy disappears

The word “wasted” suggests it has gone, and Sankey diagrams show the arrow for wasted energy ending at the edge of the page.

What helps: Stress that the wasted energy is still there, spread into the thermal store of the surroundings. Energy is conserved, which is why the useful and wasted amounts must add up to the total input.

Treating all renewable resources as reliable and all non-renewable ones as bad

Debates in the media simplify the choice, and the word “renewable” sounds like “always available”.

What helps: Ask for one advantage and one disadvantage of each resource. Wind turbines produce nothing on a calm day, and a gas power station can be switched on when needed. A balanced answer names both.

Calling nuclear fuel renewable, or saying nuclear power stations burn fuel

Nuclear power gives very little carbon dioxide, and “low carbon” gets mixed up with “renewable”.

What helps: Uranium is a finite resource, so nuclear fuel is non-renewable. Energy comes from nuclear fission in the fuel rods, which heats water to make steam, rather than from burning.

How to help at home

Read the energy labels. Look at the efficiency labels on a fridge, washing machine or light bulb. Your child can explain what the rating tells you and why a more efficient model may cost more to buy but less to run.
Do a draught and insulation walk. Walk round the home looking for ways thermal energy escapes: single glazing, gaps under doors, an uninsulated loft. Ask which fix would give the biggest gain for the lowest cost, and why.
Compare two bulbs by their labels. Compare the power rating printed on an older-style filament bulb, or its box, with an LED bulb that gives similar brightness, for example 60 W and about 8 W. If both give out about the same light each second, most of the older bulb’s extra power must be wasted, mainly as thermal energy. Ask your child which is more efficient and how they know. Do not touch a bulb that has been on.
Run a family energy debate. Imagine an island that needs new electricity. Each person argues for one resource, such as wind, nuclear, gas or tidal, and everyone must give one weakness of their own choice. Good scientific answers admit trade-offs.
Draw a Sankey-style sketch. Choose a device such as a kettle, a bicycle or a hairdryer. Your child draws an arrow whose width shows the energy in, splitting into a thick useful arrow and a thinner wasted one, and says where the wasted energy goes.

Builds on

Key Stage 3: Fuel uses and energy costs at home
Key Stage 3: Energy changes and transfers
Key Stage 3: Energy in systems and conservation