The first law of thermodynamics states that for any given system and time interval, the change in total energy is equal to the difference between the heat transferred to the system and the work done by the system on its surroundings. In hydraulic terms, changes in the total energy of a fluid do not consider changes in its internal (molecular) forms of energy, such as electrical and chemical energy, because these are usually relatively small.

In hydraulic terms, energy is often represented as energy per unit weight, resulting in units of length. At any point in a hydraulic system, the total energy of a fluid consists of three components that can be expressed as an equivalent elevation, or head:

Converting the total energy to an equivalent head allows it to be plotted on the same scale as elevation for any point in the system, either on pipeline profiles or maps, allowing engineers to visualize changes as slopes or contour lines, respectively. This gives a better feel for the resulting behavior of the system, especially when reviewing the results of an EPS or transient analysis. Further, the difference between this energy level and the pipeline elevation is equal to the total gauge pressure.