This paper describes computer simulation and modeling as applied to spacecraft power systems. Included is work done on the Modular Electrical Power System (MEPS). MEPS was an Independent Research and Development project (D30-D) developed at Martin Marietta. The primary simulation tool used was Saber from Analogy Inc., which runs on a UNIX based workstation. This paper includes the philosophy used in modeling and simulating MEPS utilizing the latest simulation tools available. It discusses when it is more advantageous to do behavioral vs. piece part modeling, different types of behavioral and functional modeling possible, validation of models, using behavioral, functional, and piece part models as building blocks, and simulations as a tool for doing system level trade studies and analyses.