Averaging-based and detailed dynamic models of hybrid electric power systems are presented, with emphasis on electric machines and drives. The objective is to compare technical and practical aspects associated with hybridization of refrigeration units in delivery trucks. Challenges unique to this hybrid application, including the thermal system interface and drive-cycle response, are introduced. The system topology is presented, and modeling approaches for each major subsystem, including an ac machine, the battery set, and converters, are discussed. An average modeling technique is fast enough to allow system-level power and efficiency to be evaluated over a long time interval. Compared to the average model, a detailed model including transient response and harmonics gives more accurate power loss estimates at the cost of slower simulation speed. The two models, interfaced with the thermal system, are examined with simulation studies in MATLAB/Simulink, showing the features of each model. Experimental setup and results are presented to validate the models.