Which of the following best describes the efficiency of a device that uses 120 J of input energy to produce 30 J of useful output energy?

Confusing Efficiency Equations

Students often confuse the two efficiency equations, using useful energy output for total power input instead of total energy input.

Remember that efficiency can be calculated using either useful energy output divided by total energy input or useful power output divided by total power input. Always check which type of input is required.