Energy efficiency is understood to mean the utilization of energy in the most cost effective manner to carry out a manufacturing process or provide a service, whereby energy waste is minimized and the overall consumption of primary energy resources is reduced. In other words, energy efficient practices or systems will seek to use less energy while conducting any energy-dependent activity; at the same time, the corresponding (negative) environmental impacts of energy consumption are minimized. Energy efficiency tends to be associated—often wrongly—with “deprivation” of some sort, such as lower levels of comfort in buildings, lower industrial production levels. Energy efficiency generally means a lower consumption of energy and this may not be accompanied by changes in the quality or quantity of an output or activity.
Energy efficiency is a term that is used in different ways, depending on the context. The strict technological usage relates an energy output to an energy input, and is used typically by engineers for machines and equipment. Thus, the energy efficiency of an electric motor is the ratio of mechanical output (that is, the work done using the motor) to the electrical input. This approach is used extensively in industrial plants and buildings for a wide range of equipment including motors, pumps, compressors, furnaces, boilers and etc.
At national level, the term “energy efficiency” is not often used. Rather, “energy intensity” is typically adopted. Energy intensity is the ratio of energy consumption to some measure of the demand for energy-related activities, and it can be applied to an entire sector of the economy. An example is the energy intensity of say the industrial sector of a country, expressed as X joules per unit of GDP generated by that sector. Sometimes the energy intensity is expressed entirely in monetary terms, e.g. energy expenditure of Z dollars per dollar of GDP. Thus energy intensity (similar in many respects to the energy efficiency concept illustrated for processes) will typically include structural and behavioural components.