The Energy efficiency of a system is the ability of the same system to use energy provided in order to obtain a desired result.
The lower the fuel consumption is, in relation to fulfillment of a particular need, the better the energy efficiency of the system in question is.
Energy efficiency is a ratio. It's expressed by a a number between 1 and 10 (or, multiplying it per cent, by the percentage from 0% to 100%), where the 0% represents the total waste of a system consuming energy without producing results, and the 100% is the optimal efficiency, where every part of the input energy is transformed in a result.
Considering that any physical process is able to transform energy without waste, both 0% and 100% are purely theoretical conditions.
Energy efficiency, in general terms, means the ability to use energy in the most optimal way, with the goal to save energy in the “end-use of energy” stage, to be specific, in industry, transport, agriculture, infrastructure, as well as in the houses where we live and their lighting, and, in general, all the energy consumption.
"Energy efficiency" refers to the series of measures used for programming, planning, designing and constructing, allowing to obtain the same services through the consumption of less energy. If the term "Energy efficiency" refers to a whole energetic system, it means the capacity to guarantee a specific productive process or a service supply (for example, heating), through the use of the least possible amount of energy. Sistems related to energy efficiency can be many: from performances of engines to those industrial, from those belonging to a building to those related to appliances and lighting, from those associated with the integrated water service to those belonging to waste management.
So, to understand the possible applicative solutions and technologies, it's necessary to know the charaterization of the different areas of investigation for each branch and their relative energy requirements.
Through the production "in loco", it's possible to reduce dependence on energy or fuel supply, utilized for different uses. Among more affordable technologies, there are systems of energy management and of builiding automation, lighting, compressed air, electric engines and inverters, UPS and co-generation.
To represent a precise picture of the possible technological solutions, it's necessary to refer to their scope and their destination of use. In particular, in the industrial field, instead of thermal generation by means of traditional boiler and of taking electric energy from the network, it's useful to think at co-generation plants. The technology of the ORC (Organic Rankine Cycle) plants and the district heating are, on the other hand, among the most interesting alternatives to produce electric energy and to provide heat. In housing, about satisfaction of the thermal need, it's appropriate to install a condensing boiler or a solar thermal plant; in the service industry, it's more convenient to use heat pumps. Other solutions regard building envelopes, such as closing glass doors and matt surfices, and general services, such as refrigeration.
Energy efficiency should not be confused with energy conservation. Energy savings regard the actions reducing the economic cost of the electricity bill but it's not necessarily equal to the perceived common good. In other words, energy efficiency always saves money, but the opposite is not necessarily true. Being efficient means “do more with less.”