Considerations on selecting the appropriate modeling level when modeling energy systems are presented in this paper. A general three-level model leveling consisting of: "Overall efficiency", "Process" and "System" is suggested. As thermodynamic cycle models are central for modeling of the process of many energy systems, identification of so-called cycle specification variables, their physical meaning and their numerical implications are discussed specifically. The example chosen is a Rankine power or cooling cycle. Similarities between modeling of power cycles and cooling cycles are finally summed up.