The liberalization of energy markets and the aims of the climate protection promote distributed systems. These systems permit inclusion of all practicable energy resources in the energy mix in connection with storage facilities, controllable loads as well as combined heating and power generation. This article describes an energy management system for distributed energy supply systems and it's application as a planning tool to prepare economy studies. The function of the energy management system is short-term optimization in co-generation systems, incorporating generating units using renewable primary energy. It is based essentially on the function packages forecast (weather, generation and load), operations planning and online modules. The operations planning takes into account different generating units, energy storage mechanisms, demand-side management, contracts and co-generation. The online modules perform redistribution of generation and load schedules, generation control and load control. The studies reported here deal with contract management, evaluation of emissions and renewable energy combined with battery systems.