The power production and efficiency of PV systems risks to be overestimated within the planning stage, if the influence of shading effect on PV modules is neglected. Several experiments consisting of measuring the current-voltage and power-voltage curves of photovoltaic modules within a real PV plant, both unshaded and applying shading profiles, have been carried out in clear sunny days. The measurements were performed on poly-crystalline and mono-crystalline modules, determining the possible PV mismatches and investigating the influence of nonuniform irradiation conditions on PV power losses.