How is energy efficiency expressed in a percentage?

Energy efficiency is expressed as the ratio of useful output work to the total input work, represented mathematically as Output Work divided by Input Work. This ratio is typically multiplied by 100 to express the efficiency as a percentage. The concept of efficiency in this context highlights how much of the energy supplied to a system is converted into useful work, as opposed to being lost or wasted in the process.

In this scenario, choosing the correct formulation underscores the importance of understanding energy conversion processes. An efficient machine or system maximizes output while minimizing losses, making this calculation critical in evaluating performance. The other options either misrepresent the relationships involved or suggest different metrics that do not convey the overall energy efficiency accurately.