The power generation of photovoltaic systems is significantly affected by partial or complete shading of its cells and it depends of the PV array configuration, shading characteristics, and presence of bypass diode. The paper deals with the investigation of the impact of partial shading on poly-crystalline and mono-crystalline PV modules operation. Several experiments consisting of measuring the current-voltage and power-voltage curves of photovoltaic modules of a real PV plant, both unshaded and applying shading profiles, have been carried out in clear sunny days. The experimental analysis can be applied to derive mathematical models for evaluating the power losses under shading conditions influencing PV modules operation. (C) 2013 Elsevier Ltd. All rights reserved.