Efficiency
Ratio of useful output to input power.
Formula
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Example
80 W out of 100 W = 80%
How it works
Efficiency tells you how much of the power supplied to a device is converted into useful power instead of being wasted as heat, sound, friction, or other losses. It works as a ratio because both output and input are measured in the same unit of power, so comparing them shows the fraction of the input that is actually useful. Multiplying by 100 turns that fraction into a percentage.
When to use it
Use this formula when evaluating machines, motors, engines, lamps, or electrical devices to see how well they convert supplied power into useful work. It is also useful when comparing different designs or checking whether a system is wasting too much energy.
Worked example, step by step
A motor takes in 250 W of electrical power and delivers 200 W of useful mechanical power. Efficiency = (P_out / P_in) × 100 Efficiency = (200 / 250) × 100 Efficiency = 0.8 × 100 Efficiency = 80% So, the motor is 80% efficient.
Common mistakes to avoid
- Using energy values without making sure they refer to the same time period as the power values
- Swapping the input and output power in the ratio
- Forgetting to multiply by 100 when the answer is meant to be a percentage
Did You Know?
No real machine is 100% efficient, because friction and heat always skim off part of the input energy.
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