The values of absolute exergy and exergetic efficiency depend on the assumed values of the “reference states,” which are essentially a model of the environment of the processes and components considered. The environmental model plays an important role in any optimization process that uses any form of exergy-based analysis. This paper examines the influence of eleven previously proposed models for the environment on the absolute values of the exergy and on the exergetic efficiencies, associated with the operation of three different power plants: a methane cogeneration plant, a coal-fired plant, and a dual-flash geothermal power plant. It is observed that although there are some common characteristics in most of the values obtained from all the models, the absolute exergies and exergetic efficiencies calculated using some of these models deviate substantially from those obtained with the others. The reasons for these deviations are also examined here.