Science · Grades 7–11
Energy efficiency
Useful output cannot exceed total input in this simple energy accounting model.
The key idea
Useful output cannot exceed total input in this simple energy accounting model.
efficiency % = useful output / input × 100
Worked examples
A device receives 4100 J and delivers 615 J of useful energy. Find its efficiency as a percentage.
615/4100 × 100 = 15%
15 %
A device receives 6700 J and delivers 5360 J of useful energy. Find its efficiency as a percentage.
5360/6700 × 100 = 80%
80 %
Try it yourself
A device receives 300 J and delivers 135 J of useful energy. Find its efficiency as a percentage.
A device receives 1600 J and delivers 1360 J of useful energy. Find its efficiency as a percentage.
Check your understanding
Answers and marking notes
- 45 %
135/300 × 100 = 45%
- 85 %
1360/1600 × 100 = 85%
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Learning preview · teacher review required. Grade bands are approximate, not official curriculum alignment.