Although the concepts of “efficiency” or “coefficient of performance” are useful in relating the output to input energies of a system, they do not give any indication of the manner in which this input energy varies within the system.

This shortcoming may be overcome by using “energy flow diagrams” which have the great advantage of providing a pictorial, yet mathematically accurate, representation of the energy balance of the system, from which an overall appreciation of the energy variations can be obtained at a glance.