Fiducia TogetherEfficiency and power
How much of the energy in does something useful, and how fast energy is transferred.
Please note: This page explains a word or a process in education. It is general information about the system, the law and the guidance in England — not advice about your child, your school or your exam. If a decision is going against you, get advice from one of the free services we link to before you agree to anything or miss a deadline. If you are revising, check your own exam board's specification: boards word things differently and yours is the one that marks you.
What it is
Efficiency is useful output energy or power divided by total input, as a decimal or percentage. Power is energy transferred per second, in watts.
Where it applies
GCSE physics and combined science.
How it works
Efficiency can never exceed 100%, because energy is conserved. The remainder is dissipated, usually as thermal energy to the surroundings.
Power and energy are different quantities: a 2 kW heater transfers energy twice as fast as a 1 kW one, not twice as much.
How to use it in your work
Check your answer is at or below 100%. Above means an arithmetic error.
Use consistent units — watts and joules and seconds, or kilowatts and kilojoules.
Say where the wasted energy goes rather than that it is lost.
Where to get help
Your teacher and your board's specification and mark scheme.
Revise this with a song
These are ours. We write revision songs because a lot of people remember a tune when they cannot remember a page, and because revising out loud in the kitchen beats staring at a highlighter.
- The Physics Song The GCSE physics specification, set to music.
Where to read more
Last reviewed 2026-08-28 by Fiducia Together · Next review due 2027-08-28
Please note: This page explains a word or a process in education. It is general information about the system, the law and the guidance in England — not advice about your child, your school or your exam. If a decision is going against you, get advice from one of the free services we link to before you agree to anything or miss a deadline. If you are revising, check your own exam board's specification: boards word things differently and yours is the one that marks you.