To assure the maximal energy yield of grid connected PV systems, system faults have to be identified as quickly as possible. For this task several procedures that offer an almost continuous performance check are in development or application. One of them is the EU-funded project PVSat2. In the framework of this project, a model, able to reflect the efficiency characteristics η(G,T) at MPP of both the classic crystalline silicon and the various thin film technologies is analysed. The applicability of this model for the application to grid connected PV systems using cSi, aSi and CIS modules is demonstrated.