In solar thermal power plants, at least the solar part undergoes a daily start-up process. Since duration and energy consumption of the start-up depend on irradiance and temperature boundary conditions, differences occur between the individual days and especially between the seasons. For a good representation in annual electricity yield calculations, the start-up process should be modeled on a daily basis. This paper presents a closed approach for the calculation of start-up and cool-down in solar thermal power plants especially designed for annual calculations. It is demonstrated by one example how the required parameters can be obtained and how the methodology is applied. A sensitivity analysis reveals the large impact of start-up consumption on the annual yield.