Parasitic resistances are detrimental to solar cell performance because they reduce the device power output. In this paper the effects that series- and shunt resistance have on photovoltaic module performance parameters are discussed. Techniques used to measure these resistances are presented and the results show how current-voltage characteristics and module performance are affected. An analysis of the performance degradation of an a-Si module revealed that, apart from the Staebler-Wronski effect, an increase in series resistance contributed to a power loss of 50% after outdoor exposure of 130 kWh/m(2). A 29% decrease in shunt resistance of a CuInSe2 module, over the same exposure, translated to a power loss of 6%. This study shows that the effects of shunt and series resistances on module performance are significant and cannot be ignored. These effects must be understood and taken into consideration when employing photovoltaic modules in systems and when analysing performance degradation. (C) 2003 Elsevier Ltd. All rights reserved.