Energy efficiency is a guiding principle widely used in EU policies concerning environment and sustainability, as its application may well guarantee lower primary energy consumption, less energy distribution losses, reduced operational costs, and decarbonization.
First, it is worth underlining what energy efficiency means. According to the EN ISO 50001:2018, energy efficiency may be defined as the ratio between an output (of performance or energy, etc.) and an input of energy. Then, the efficiency can be increased by reducing the input, increasing the output, or applying a combination of the two.