We present a method for modelling the degradation of solar cells in space, induced by electron or proton irradiations. It applies to modern cells, single, double or triple junctions made of GaInP. GaAs and Ge materials. It is based on classical semiconductor equations, after the values of all the various material parameters involved, as well as the electronic characteristics of irradiation induced defects, which act as recombination centers, have been experimentally determined. Because the nature of the irradiation induced defects in GaAs and GaInP is independent of the concentration and nature of the native defects and doping impurities, the method does not introduce empirical parameters and is valid for all types of cells. Modelling the degradation of 3J cells, as well as the associated top and middle subcells, of two origins (Emcore and Azurspace), has been performed and the results are confronted successfully with experimental data for the case of 1 MeV electron irradiation. Modelling has also been performed for several types of GaAs and GaInP, n/p and p/n, single cells, using the same electronic characteristics for the irradiation induced defects. The fits with experimental data are correct in all cases, thus illustrating the generic character of the method. (C) 2010 Elsevier B.V. All rights reserved.