Today, in most countries, electricity is produced either on hydropower or on large steam turbines on the basis of fossil fuels or nuclear power. Electricity from distributed generation constitutes only small amounts. Until now, the task of balancing supply and demand and the task of securing frequency and voltage on the grid has been left solely to large production units. Meanwhile, the implementation of cleaner technologies, such as renewable energy, combined heat and power production and energy conservation, is necessary for future sustainable energy systems. Consequently, such distributed production units sooner or later need to contribute to the task of securing a balance between electricity production and consumer demands. This paper presents technical designs of potential future flexible energy systems, which will be able both to balance production and demand and to secure voltage and frequency requirements on the grid.