Motivated by the radiation damage of solar panels in space, firstly, the results of Monte Carlo particle transport simulations are presented for proton impact on triple-junction Ga$_{0.5}$In$_{0.5}$P/GaAs/Ge solar cells, showing the proton projectile penetration in the cells as a function of energy. It is followed by a systematic {\it ab initio} investigation of the electronic stopping power for protons in different layers of the cell at the relevant velocities via real-time time-dependent density functional theory (RT-TDDFT) calculations. The electronic stopping power is found to depend significantly on different channeling conditions, which should affect the low velocity damage predictions, and which are understood in terms of impact parameter and electron density along the path. Additionally, we explore the effect of the interface between the layers of the multilayer structure on the energy loss of a proton, along with the effect of strain in the lattice-matched solar cell. Both effects are found to be small compared with the main bulk effect. The interface energy loss has been found to increase with decreasing proton velocity, and in one case, there is an effective interface energy gain.