**3. The concept of energy efficiency**

Several definitions for energy efficiency have emerged over the years. Energy efficiency may be considered as investing in more energy-efficient technologies or appliances which results in more energy savings. Butler et al. [24] defines energy efficiency as "using energy cautiously and economically to sustain everyday life, live comfortably and support wellbeing". According to [25], energy efficiency is achieving the same service and performance while using technology with less energy use and therefore enhancing the security of the energy supply. For [26], energy efficiency is an effective tool for reducing electricity or energy consumption which limits greenhouse gas (GHG) emission and thereby reducing global warming.
