Partial shading of photovoltaic (PV) power generators causes typically multiple maximum power points (MPPs) on the electrical characteristics of the generators. In this paper, the effect of partial shading on the number of MPPs has been studied by using MATLAB Simulink simulation models of a real PV generator composed of 18 series-connected PV modules. In most cases, partial shading conditions lead to the occurrence of multiple MPPs, but also only one MPP can be present despite of the partial shading. Conditions, which lead to one or multiple MPPs in electrical characteristics of PV generators, have been studied systematically. It was found out that reasons for having one or more MPPs originated directly from physical properties of PV modules: the ratio of MPP current to short-circuit current, power losses in bypass diodes, and the value of the parasitic shunt resistance of PV cells. These phenomena are thoroughly explained in this paper.