Having a photovoltaic (PV) system raises the question of whether it runs as expected.Measuring its energy yield takes a long time and the result still contains uncertainties from varyingweather conditions and possible shading of the modules. Here, a free software PVcheck to measurethe peak power of the system is announced, using the power data of a single sunny day. The softwareloads a data file of the generated power as a function of time from this day. This data file is providedby typical inverters. The software then simulates this power curve using known parameters like angleand location of the PV system. The assumed peak power of the simulation can then be adjusted sothat the simulated curve matches the measured one. The software runs under Microsoft Windows™and makes use of the free library pvlib python. The simulation can be refined by importing weatherdata like temperature, wind speed, and insolation. Furthermore, curves describing the nominalmodule efficiency as a function of the illumination intensity as well as the power-dependent inverterefficiency can be included in the simulation. First results reveal a good agreement of the simulationwith experimental data. The software can be used to detect strong problems in PV systems afterinstallation and to monitor their long-time operation.