Numerous proposals have been and are being made to define rational efficiencies for variousHVAC processes and systems, total-energy systems, and energy-conversion processes and systemsof all types. There is much debate, and as the systems get more complicated the arguments becomeless coherent. The problem is this: In the majority of instances, the definitions arebased on energy. The solution is this: Employ the available energy. Basically, availableenergy is easier to comprehend than energy. Once accepted as the basis for defining efficiencies,the complications and arguments disappear; the definition of an appropriate efficiencybecomes simple.