This paper originated from an existing cooperation between BMW Group and TU Munich about energy management. It deals with the integrated system design of an energetically optimized dual voltage power net for premium class cars, which offers future functionalities like start-stop, recuperation of braking energy and limited electrical driving. Based on legal driving cycles as well as "real" driving cycles criteria for dimensioning the two most important components, the short time storage device and the electric motor, are derived. By using the object orientated simulation tool Dymola/Modelica simulations have been carried out, that present the attainable reduction in fuel consumption corresponding to different driving strategies.