The paper focuses on the determination of technical conditions and degradation of photovoltaic modules installed in grid-on operated photovoltaic systems in Ukraine and the Czech Republic. Detected malfunctions of PV modules are classified and Fault Tree Analyses (FTA) is created for particular PV modules. For assessment of the technical condition of the PV modules it is proposed to use residual resource index (RRI), that is being successfully used for determination of technical condition of transformers, breakers and synchronous generators. The main contribution is the method for the detection of photovoltaic modules condition with the use of a neuro-fuzzy network based on the experience of operational (maintenance) staff of particular plants.