In this paper an application of the System Dynamics approach in the field of energy supply is presented. Decentralized energy supply is introduced as a new energy system with a high potential for meeting ecological requirements and sustainability targets. System Dynamics models of one power supply unit (PSU) and of the decentralized energy supply (New Energy System Model) are described. The electrical and thermal power flow and the power conversion processes represent the basis for the stock-and-flow diagrams. At any time and for every sub process of the overall system the precise power and energy state can become certain and comprehended for the user. With examples some results of simulation are represented. Investigations of technical and ecological potential of decentralized energy supply systems are the primary objective of the models. The models are also used for education purposes for the elucidation of energetic supply processes with different power station technologies.