A novel equivalen diagram of a solar cell is developed. A solar cell array is represented by a voltage source E and two internal resistances rsc and ro c which are determined from short-circuit and open-circuit tests. E is constant and does not depend on irradiation; rsc is constant for given irradiation; ro c is constant for given irradiation under the open-circuit condition, but changes its value with varying load. An example is presented for calculation of the parameters in the equivalent diagram, using the experimental output characteristic of a cell. An analysis is made of the trend in parameters of modern solar cells for the developed diagram. The diagram allows engineers and researchers to use ordinary calculation and design techniques for the analysis of circuits with solar cells.