After installation of a PV system, the question arises if it works correctly. Comparing the yield with some prediction gives a hint if the system is OK, but takes a long time and depends on the weather during that period. Also the effect of shading by trees or buildings is hard to estimate and often not included in the prediction. We present a solution based on the evaluation of the power curve of the PV system from a sunny day. The peak power of the PV system is obtained by simulating this power curve using the peak power as a fit parameter. The simulation uses the public library pvlib python including the clear sky model of Ineichen and Perez, and a database for the air turbidity. The temperature of the PV modules is calculated from air temperature and wind speed. The software with its graphical user interface for Microsoft Windows is freely available. The simulation of irradiance and module temperature is compared to experimental data. As an example, the peak power of some newly installed PV systems was evaluated. Furthermore, a degradation analysis of the peak power over a 10-year period of a test system is performed.