The electric power system of a modern vehicle has to supply enough electrical energy to numerous electrical and electronic systems. The electric power system of a vehicle consists of two major components: a generator and a battery. A detailed understanding of the characteristics of the electric power system, electrical load demands, and the driving environment such as road, season, and vehicle weight are required when the capacities of the generator and the battery are to be determined for a vehicle. In order to avoid the over/under design problem of the electric power system, an easy-to-use and inexpensive simulation program may be needed.In this study, a vehicle electric power simulator is developed. The simulator can be utilized to determine the optimized capacities of generators and batteries appropriately. To improve the flexibility and easy usage of the simulation program, the program is organized in modular structures, and is run on a PC. Empirical electrical models of various generators and batteries, and the structure of the simulation program are presented. For the simulation, data of engine speed profile and electric loads of a vehicle are given, and these data are acquired from real driving conditions. Numerous driving data of a vehicle are logged and analyzed to improve the accuracy of the simulator.Compared with the real data from the vehicle, the energy generation and distribution data from the simulator are comparable. Beyond that, more information from the simulator are obtained.