How is energy efficiency defined?

Energy efficiency is defined as the ratio of useful energy output to total energy input. This definition highlights the effectiveness of a system in converting the energy it consumes into useful work or energy that can be utilized for a specific purpose. A higher efficiency indicates that a larger proportion of the input energy is being used for its intended purpose rather than being wasted as unusable energy, such as heat or sound.

Understanding energy efficiency is crucial in various fields, including engineering, environmental science, and economics, as it directly impacts resource conservation and sustainability. By focusing on how much of the energy input is converted into useful output, it becomes easier to assess the performance of different systems and make informed decisions about energy use.

The other options do not capture the concept of efficiency properly, as they either refer to energy inputs or total energy consumed without distinguishing the useful output that efficiency measurements focus upon.