What does overall efficiency compare?

Overall efficiency in the context of heating appliances refers to the ratio of useful energy output to the total energy input. This concept is critical in understanding how effectively a heating system converts energy into useful heat versus what is lost in the process.

The correct answer addresses this by comparing the total energy put into the system (input heat) with the amount of energy that successfully generates heat for use (output heat). Input heat includes all energy sources the system utilizes, while output heat pertains solely to the heat that can be used effectively for heating a space or water.

Choosing this answer accurately reflects the fundamental principle of efficiency measurement, which aims to determine how much of the supplied energy is converted into useful work or heat, and thus provides a clear understanding of a system’s performance.