Based on a comprehensive literature review, the requirements and capabilities of fossil fueled thermal power plants are investigated with regard to the current framework in the energy sector. Therefore, the foreseeable consequences of an increasing supply of renewable energies (on the example of wind power and photovoltaics) are demonstrated by calculating the operating hours of fossil fueled thermal power plants by the fundamental relationships of the spot market (merit order). The analysis shows that technologies are so far established or in development, which enables the fossil fueled thermal power plant fleet to cover the fluctuating residual load. Optimised load ranges, load change rates, part load efficiencies as well as start-up procedures are necessary for the economic operation of these plants in future. Furthermore, these adaptations are essential with regard to the security of power supply.