Energy and power
‘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.
For example, when dealing with food energy we’ll typically find the consumption of food energy per year; when dealing with commercial energy we’ll find the consumption of TOE per year. These data are expressed as rates of energy over time, for example GJ/year, and therefore have the same dimension (energy per unit of time) as power levels. Such confusion should be carefully avoided.
Therefore, it is critical that a meaningful energy analysis has both types of information: the consumption of energy input and the power level at which the energy conversion is expected to take place. For example, one gallon (3.87 litres) of gasoline a given amount of energy can be used to fuel a small motorbike for more than 200km, a minivan for about 50km, and a Formula 1 car for no more than a few kilometres. If we do not specify the purpose of the vehicle which will be linked to different requirements of power level at the outset of our analysis, it is impossible to perform any meaningful energetic assessment. Indeed, it is essential to first define the power level required by the given task.
