In the last years, the requirements for electrical energy systems in motor vehicles have increased considerably. In the past, many studies were focused on single components of the electrical system. However, to shorten the development process, reduce costs, improve reliability and also to optimize the fuel consumption due to the electrical system, the electrical system must be regarded as a whole.The Robert Bosch GmbH has developed a simulation environment, which is intended to improve the development process of new vehicle electrical systems by means of computer simulation.On the basis of a freely selectable driving cycle and various driver models, it is possible to simulate the behavior of electrical energy supply structures. The model of the electrical system is coupled to a dynamic model of the drivetrain. The characteristics of this drivetrain can also be modified and various vehicle models can be selected for simulation. The influence of different types of alternators, batteries, starters, electrical consumers and electrical supply structures on the performance of the electrical system can be investigated. The voltage and current levels of the electrical components, the battery condition, the starting capability of the vehicle, the fractional fuel consumption and all characteristic quantities of the drivetrain can be calculated.Simulation control, evaluation of the results, data base management of the models and the automatic generation of models from sub models, is easily achieved via user-friendly graphical interface.The simulation environment is designed to reduce the time required for the conception and development phase of a product and to decrease the number of prototypes that need to be built. This simulation system reflects the state of development during the whole development process from the early conception phase right up to series production.