In photovoltaics the actual curve of the current-voltage characteristic of a solar generator is often needed, for example if the maximum power point is to be determined. It is convenient to use a model for the description of the relationship between current and voltage, which is able to describe the solar cell in the generation region as well as the breakdowns at positive and negative voltages. As the mathematical expression cannot be given in an explicit form, a set of suitable numerical algorithms to compute the currents for a given voltage and vice versa is given. For more complex solar generators, it can neither be assumed that the solar cells have identical electrical characteristics, nor that they are evenly illuminated. A general model for the description of solar generators is proposed that gives all voltages and currents as well as the voltage and current at the outputs of the solar generator itself. This new model is also implemented in a numerical algorithm. The characteristics of partially shaded solar generators were calculated and discussed using the proposed models.