Which statement best defines energy efficiency?

Which statement best defines energy efficiency?

• A. the total energy input required to produce a unit of work
• B. the useful work produced divided by the energy used
• C. energy efficiency is the energy saved by efficient devices
• D. the rate of energy consumption per unit time

Answer: (B)

Energy efficiency is about how well a device converts input energy into useful work. It’s measured as the ratio of useful work (or useful energy output) to the total energy put in, often expressed as a percentage. This captures how much energy is not wasted as heat, friction, or other losses. Why this definition fits best: it directly quantifies how effectively energy is used in producing the desired outcome. If a motor uses 100 joules of energy and does 80 joules of useful work, its efficiency is 80%, illustrating the idea that only a portion of input energy becomes useful output. The other ways to think about it don’t describe efficiency itself. Describing energy input per unit of work points to energy intensity or specific energy, which is essentially the inverse of efficiency. Saying energy saved by efficient devices refers to potential savings rather than the immediate conversion efficiency of a device. And describing the rate of energy use per unit time is power, not efficiency.

Energy efficiency is about how well a device converts input energy into useful work. It’s measured as the ratio of useful work (or useful energy output) to the total energy put in, often expressed as a percentage. This captures how much energy is not wasted as heat, friction, or other losses.

Why this definition fits best: it directly quantifies how effectively energy is used in producing the desired outcome. If a motor uses 100 joules of energy and does 80 joules of useful work, its efficiency is 80%, illustrating the idea that only a portion of input energy becomes useful output.

The other ways to think about it don’t describe efficiency itself. Describing energy input per unit of work points to energy intensity or specific energy, which is essentially the inverse of efficiency. Saying energy saved by efficient devices refers to potential savings rather than the immediate conversion efficiency of a device. And describing the rate of energy use per unit time is power, not efficiency.