A method is proposed for calculating the diffusion length damage coefficient for minority carriers ([Formula: see text]) in GaAs and InGaP solar cells under electron irradiation using the Shockley–Read–Hall (SRH) model for defect-assisted recombination. In the SRH model, the damage coefficient [Formula: see text] is proportional to the product [Formula: see text], where [Formula: see text] is the defect introduction rate under particle radiation and [Formula: see text] is the minority carrier capture cross section of the said defects. The introduction rate [Formula: see text] is evaluated using the atomic theory for displacement under electron radiation, and the calculation for [Formula: see text] is adapted from Henry and Lang’s high-temperature multiphonon emission formulation. A linear scaling relationship is observed between [Formula: see text] and nonionizing energy loss—validated by bibliographic data in the radiation energy range [Formula: see text]–12 MeV. Our model reproduces the increasing trend in [Formula: see text] with doping, as observed in the literature, while also capturing the anisotropy between the [Formula: see text]-type and [Formula: see text]-type materials, with the [Formula: see text]-type exhibiting greater radiation resistance than its [Formula: see text]-type counterpart. The calculated [Formula: see text] is fed into the physical model for solar cell operation to obtain the post-irradiated [Formula: see text] at a given fluence [Formula: see text]. The degradation of the electrical quantities is consistent with the measurements recorded in the literature. The findings show that InGaP is more radiation resistant than GaAs. It is demonstrated that calculating [Formula: see text] not only aids in determining the degradation of solar cell parameters from first principles, but also in obtaining the empirical function for degradation: [Formula: see text], used in fitting the experimental measurements. The limitations and potential scope of improvements in the model are also discussed.