A new model for determining solar-cell degradation has been developed in electron and proton space radiation environments. This technique is based on results derived from displacement damage dose modeling software that predicts solar-cell damage caused by high-energy particles trapped in the Van Allen radiation belts. Although damage-dose-based solar-cell models have been shown to provide acceptable accuracy, they require an unwieldy two-step computational process: 1) determining the trapped particles’ energy spectra for a given orbit, and 2) determining the displacement damage dose for the integral energy spectra and a given shielding thickness. For orbit transfers, this is especially cumbersome because the process must be performed at each step along the evolving trajectory. Hence, it is impossible to integrate high-fidelity solar-cell degradation models into a trajectory optimization program that involves iterating among multiple orbital paths. The new solar-cell degradation modeling method is computationally efficient and is therefore well suited for use in low-thrust trajectory optimization programs. Numerical comparisons with the high-fidelity model demonstrate that this new method can accurately determine solar-cell degradation for a wide array of Earth orbits and low-thrust orbit transfers.