Manufacturers of photovoltaic cells and modules usually provide temperature coefficients referring to the short-circuit current, the open-circuit voltage and the maximum power. Few studies analyse the variation of the series resistance with respect to the cell or module temperature. In this paper, this dependency is studied by suitably processing a set of current-voltage curves acquired on several modules working under outdoor conditions. The curves are measured at an increasing module temperature. The temperature coefficient of the series resistance is estimated by using the single diode model and the double diode one. Some hundreds of current vs voltage curves referring to degraded photovoltaic modules are used in this paper to analyse the effects of the degradation on the series resistance and on the temperature coefficient thereof. (C) 2020 Elsevier Ltd. All rights reserved.