This paper presents a new algorithm for determination of the series resistance of crystalline-Si PV modules from individual illuminated I V curves. The ideality factor and the reverse saturation current are then extracted in the classic way. The approach is applied to in-situ measured data from modules based on two types of mc-Si feedstock. The results indicate that the method yields physically meaningful parameters. An improved definition of local ideality factor is suggested, resulting in m-V plots unaffected by the series resistance. In addition, m-I plots are introduced for the first time. The novel differential techniques reveal an unexpected rise of the ideality factor at open circuit by about 5%. (C) 2012 Elsevier Ltd. All rights reserved.