‘Energy’ indicates (in general terms) a given amount of energy, with no reference to time, and it is measured in joules. In energy analysis it can refer to a given amount of a primary energy source, or to a given amount of an energy carrier. ‘Power’, on the other hand, indicates the given pace of an energy conversion in time the rate at which useful work is performed and its unit of measure is watts (joules per second). Power is intrinsically linked to the characteristics of the energy converter (generating power) and the useful work performed with such a power.

Unfortunately, in many applications of energy analysis the distinction between energy and power often becomes blurred because of the way data on energy flows are presented. In fact, when dealing with the analysis of the metabolism of human beings (endosomatic metabolism) or socio-economic systems (exosomatic metabolism), one gets easily confused because data on energy inputs are usually expressed on a time basis.
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The rapidly growing literature on biofuels and many initiatives looking for alternative energy sources shows an amazing variety of options proposed by experts and visionaries. All these options are being proposed as feasible and/or highly desirable solutions to replace our use of oil.

In order to ascertain whether or not a certain source of energy would be an appropriate input for a system, one must first carefully observe the characteristics of the energy system. We cannot feed gasoline to humans, or power a refrigerator with pizzas. When refuelling a vehicle in a modern gas station, the driver must first select the appropriate type of fuel: gas or diesel, regular or premium octane rating.
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