Final answer:
Energy efficiency measures the amount of useful work derived from each unit of energy used. It reflects the proportion of input energy that is transformed into useful output, rather than being lost as waste, typically as heat.
Step-by-step explanation:
Energy efficiency refers to the measure of the effectiveness of an energy conversion process, which is how much useful work can be obtained from each unit of energy used. In the simplest terms, it's the fraction of energy that is converted into useful energy or work, rather than being wasted, for example, as thermal energy. This concept is critical when considering the operation of machines and appliances and their impact on the environment and resource conservation.
The calculation for efficiency (Eff) can be expressed as Eff = Wout/Ein, where Wout is the useful work output and Ein is the energy consumed. Therefore, when considering the multiple-choice options given, the correct answer for what energy efficiency means is A. How much useful work we can get from each unit of energy used.