Energy lesson
Calculating Efficiency: Learn how to calculate efficiency using energy and power equations.
Calculating Efficiency is part of Conservation and dissipation of energy in GCSE AQA Higher Physics.
3 min
Reading time
14
Lessons in this topic
GCSE AQA Higher
Pathway
Expert reviewed
Content review
Calculating Efficiency: Learn how to calculate efficiency using energy and power equations.
Use this lesson to stay inside Conservation and dissipation of energy while connecting the detail back to the wider Energy topic.
Read the core explanations from Calculating Efficiency before testing yourself from memory.
Your phone battery dies. Your car needs petrol. Your electricity bill arrives. All these problems come down to one thing: energy isn't perfectly transferred from one form to another.
When you charge your phone, not all the electrical energy becomes stored chemical energy in the battery. Some becomes heat in the charger. When your car burns petrol, not all the chemical energy becomes kinetic energy. Most becomes waste heat through the exhaust.
This is where efficiency comes in. It tells us exactly how much of our input energy actually does what we want it to do.
Think of efficiency like this: if you put 100 J of energy into a system, how much useful energy comes out? A perfect system would give you all 100 J back in the form you want. Real systems give you less.
The missing energy hasn't disappeared - that would violate energy conservation. It's been transferred to unwanted forms, usually heat and sound. Your car engine is about 25% efficient. That means 75% of the petrol's energy becomes waste heat, not motion.
There are two ways to calculate efficiency, depending on whether you're given energy values or power values. Both give the same answer because power is just energy per second.
Notice that efficiency has no units - it's just a number. You can express it as a decimal (0.25) or percentage (25%). Both equations give identical results because power = energy ÷ time, so the time cancels out.
Let's work through some typical exam calculations. The key is identifying what energy is useful versus what's wasted.
Different energy transfer processes have very different efficiencies. Understanding these helps explain why we use certain technologies and avoid others.
LED lights are about 80% efficient - most electrical energy becomes light. Old filament bulbs are only 5% efficient - most energy becomes unwanted heat. That's why LEDs are replacing filament bulbs everywhere.
Electric cars are about 90% efficient at converting stored electrical energy to kinetic energy. Petrol cars are only 25% efficient. This huge difference explains why electric cars need much less energy per mile.
Efficiency questions appear frequently in GCSE exams. Here's how to maximise your marks and avoid the traps that catch most students.
The biggest mistake students make is confusing useful output with wasted energy. If a question says '400 J is wasted as heat' and the input is 2000 J, the useful output is 2000 - 400 = 1600 J, not 400 J.
Another common error is using the wrong equation. If you're given 'The motor has an input power of 500 W', use the power equation. If you're given 'The motor uses 500 J of energy', use the energy equation.
Efficiency calculations are most useful when you compare what you want from a system with what is wasted. If a device is 80% efficient, that means 80% of the input becomes useful output and 20% is transferred in unwanted ways, often as heating. This helps explain why improving insulation, reducing friction and using better materials can reduce waste without breaking energy conservation.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Learn how to calculate efficiency using energy and power equations.
By the end of this lesson, you should be able to Calculate efficiency using the energy transfer equation; Calculate efficiency using the power equation; Apply efficiency calculations to real-world energy systems; Evaluate the efficiency of different energy transfer processes.
Read each section, then close the notes and explain the idea from memory. Check your explanation against the page and finish with an exam question from Energy.