We present a methodology for the analysis and the evaluation of the electrical effects of partial shadings on a PV array. This methodology is namely developed in a pedagogic tool of the software PVsyst, which aims to explain how a shaded cell or group of cells behaves within a PV array. We use this tool for understanding the functions of the by-pass diodes, and quantify the array electrical losses in some typical situations, when one or several cells are shaded. We show that the electrical shading effect is dependent either on the length of the strings, and on the number of strings in parallel. This method should be applied for accurate shading calculations within the simulation process of PVsyst in a near future.