Efficiency
Elementary School
Definition
The proportion of input energy that a system converts into useful output, usually expressed as a percentage. No machine is perfectly efficient because some energy is always lost, most often as heat due to friction.
Worked examples
\(\)Efficiency\( = \frac{\)useful energy out\(}{\)total energy in\(} \times 100\% = \frac{450\,\)J\(}{600\,\)J\(} \times 100\% = 75\%\)
A motor converts 450 J of its 600 J input into useful work; the remaining 150 J is lost as heat.
A light bulb receives 100 J of electrical energy but emits only 5 J as visible light; the rest becomes heat.
This bulb is only 5% efficient because most input energy is wasted rather than producing useful light.
Common mistakes
- \(\)Efficiency\( = \frac{\)total energy in\(}{\)useful energy out\(} \times 100\%\) → \(\)Efficiency\( = \frac{\)useful energy out\(}{\)total energy in\(} \times 100\%\) Useful output goes in the numerator; total input goes in the denominator.
- A machine can be 100% efficient if it's well-designed. → No real machine is 100% efficient; some energy is always lost to heat and friction. The second law of thermodynamics guarantees energy is always lost in real systems.
- Efficiency can exceed 100% if the machine is very powerful. → Efficiency is always less than or equal to 100%; output cannot exceed input. Power measures how fast energy is converted, not the proportion converted efficiently.
Where you'll use it next
You'll apply efficiency when studying energy transformations, comparing machines and power plants, analyzing heat engines in thermodynamics, and evaluating renewable energy systems in environmental science.
See also
Reviewed by Pat Cheng, M.Ed. — StudyPug Curriculum Lead · Last updated June 6, 2026