2. The electricity consumption of an appliance (which may be expressed in kWh per 24 hours) is a function of the category of appliance to which it belongs (e.g. 1-star refrigerator, chest freezer, etc.), its volume and the energy efficiency of its construction (thickness of insulation, compressor efficiency, etc.) and the difference between ambient temperature and the temperature inside the appliance. In setting energy efficiency standards, therefore, allowance must be made for the main endogenous factors which influence energy consumption (i.e., the category of the appliance and its volume). For this reason the maximum allowable electricity consumption of an appliance is defined by a linear equation which is a function of the volume of the appliance, with different equations laid down for each category of appliance.