A sustainable energy supply confronts urban planners and engineers with new challenges: building power plants, minimizing heating demand and balancing irregular revenues from renewable energy sources by storage techniques and cogeneration. The city of Freiburg manages energy supply through the creation of decentralized power supply units at correct scales. The technology of cogeneration saves energy and allows to generate it at the moments and at the places where it is needed. Some smart grid networks have been already installed in order to experiment on the better harmonization of production and consumption.