The growth of renewable energy and especially photovoltaic energy in Greece is remarkable. There exist many grid-connected systems in Greek territory which have completed ten years operation. A challenging task for these systems is performance analysis, investigation of possible degradation in systems efficiency and formulation of a detailed solar potential record for each region. This paper focuses in the performance of a grid-connected PV system in central Greece which has completed six years' operation. The analysis methodology is based on three axes. The first is a calculation of the daily PR and yearly PR and their comparison for the 6 years of operation. The daily PR is further correlated with the averaged clearness index in order to assess the atmospheric effect on the PV performance. The second axis involves the application of a mathematic model, which describes PV power, to the available data. Computed values act as the reference values and deviation of the measured values thereof hint to probable changes in PV system performance. The third axis involves the computation of normalized efficiency to STC conditions. This includes (i) the computation of DC power from available AC data and inverters' efficiency, (ii) temperature normalization according to the temperature coefficients of manufacturer and (iii) comparison of efficiency at various weather conditions and computation of yearly average values, respectively, with Airmass. The analysis results hint to a small performance deterioration over the years, with degradation rates ranging from 1 to 4%. (C) 2019 Elsevier Ltd. All rights reserved.