# Efficiency

The fraction of input energy that becomes useful output. Efficiency = useful output / total input.

Physics (school) · Part 03: Energy, work and power · Card 7 of 8 · Last updated 2026-10-19

## Analogy

A leaky bucket: efficiency is how much water arrives compared with how much you poured in.

## Example

**Motor**

Input = 500 J
Useful output = 400 J
efficiency = 400 / 500
= 0.8 or 80%

## In practice

An LED bulb is far more efficient than a filament bulb, which wastes most energy as heat.

## Practice

### Flashcards

- **How do you calculate efficiency?** Useful output energy divided by total input energy.

## Questions people ask

### Can a machine be 100% efficient?

No real machine can. Some energy always escapes as heat, sound or friction, so the useful output is always less than the input. The wasted energy is not destroyed, it simply spreads into the surroundings. Efficiency is always less than 1, or less than 100% as a percentage.

### How can you make a machine more efficient?

Reduce the energy wasted. Oil moving parts to cut friction, use insulation to stop heat escaping, and choose a shape that cuts air resistance. A better-insulated house, for example, wastes less heat, so the heating gives more useful warmth for the same energy input.

## Related concepts

- [Energy stores and transfers](https://studyvisually.com/learn/science/physics-school/physics-school-part-3/physics-school-part-3-concept-1)
- [Conservation of energy](https://studyvisually.com/learn/science/physics-school/physics-school-part-3/physics-school-part-3-concept-2)
- [Kinetic energy](https://studyvisually.com/learn/science/physics-school/physics-school-part-3/physics-school-part-3-concept-3)
- [Gravitational potential energy](https://studyvisually.com/learn/science/physics-school/physics-school-part-3/physics-school-part-3-concept-4)

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