A straightforward non-invasive method is proposed to accurately evaluate the shunt resistance of an elementary cell of a photovoltaic module connected in an installed string without requiring prior knowledge of the parameters of the intrinsic diodes. The approach relies on the measurement of the current-voltage characteristic of the whole string after intentionally shading the selected cell. Calibrated PSPICE simulations are employed to illustrate the method and test its reliability. As a case study, the shunt resistances of several cells belonging to a series array of 10 commercial panels are determined. (C) 2011 Elsevier Ltd. All rights reserved.