The individual PV outputs to the MPP trackers for all inverters are compared with each other. In order to do so, the MPP trackers are subdivided into one or more groups, depending on pitch and orientation. An average output is calculated per group and output deviations which are unexpectedly low and under the specified tolerances are reported. For very small systems, the comparison is made with other systems from the same region. The tolerances can be entered separately for the summer and winter seasons, in order to be better able to take weather fluctuations into consideration.