We describe a way to obtain the degradation, induced by proton and electron irradiations, of solar cell parameters (short-circuit current, open-circuit voltage and maximum power) versus fluence, directly from the calculation of the characteristics of the cell and of the irradiation-induced recombination centres. The calculation can be performed for any energy of the irradiating particle and for any specific thicknesses and doping levels of the base and emitters. The validity of this approach is illustrated in the case of GaAs cells of different origins and extended to GaInP cells. It will allow us to deduce the degradation of multijunction cells.