How do you calculate efficiency?

• A. Efficiency = total energy input / useful energy output × 100%
• B. Efficiency = energy input / energy output × 100%
• C. Efficiency = useful energy output / total energy input × 100%
• D. Efficiency = useful energy output × total energy input

Answer: (C)

Efficiency is a measure of how much useful energy you get from the energy you put in. To express it as a percentage, you compare the useful energy output to the total energy input and multiply by 100. So the formula is useful energy output divided by total energy input, times 100%.

For example, if a motor uses 200 J of electrical energy and delivers 120 J of mechanical work, efficiency = 120/200 × 100% = 60%.

The other ways would misplace the quantities or give a number that doesn’t represent the fraction of input energy that becomes useful. Swapping the numerator and denominator reverses the ratio, and multiplying instead of forming a ratio would not yield a percentage of useful energy.