Compositionally graded InxGa1-xN layers of high structural and optical quality grown on Si have the potential for cheap and efficient solar harvesting systems1. The theoretical maximum efficiency for tandem photoelectrochemical water-splitting systems could reach over 22.5% with Si as bottom cell and InxGa1-xN as top cell with a bandgap between 1.6-1.8eV, an indium content of 0.37-0.44, respectively1,2. However, the use of InxGa1-xN as a water-splitting photoelectrode (PE) for solar hydrogen production has yet to show promising performance as initial demonstrations of InxGa1-xN on a GaN substrate have exhibited photocurrents below 0.1mA/cm2 at AM1.5G irradiation, i.e. a current even below what has been reported for pure n-GaN3. We developed a numerical model of InxGa1-xN water-splitting PEs which aims at identifying and quantifying the losses in the system. The model included electromagnetic wave propagation calculations for the determination of the locally resolved light absorption and charge generation terms, which built the generation terms in the charge transport and conservation equations solved in the semiconductor. The charge transfer at the semiconductor-electrolyte interface, a boundary condition to our model, accounted for Fermi level pinning and a potential drop in the Helmholtz layer due to surface states. The complete numerical model was validated using linear sweep voltammograms of InxGa1-xN PEs grown by plasma-assisted molecular beam epitaxy with varying indium content (0.095, 16.5, 23.5, 33.3 and 41.4) and under varying light irradiance (1%, 10%, 50% and 100% of AM1.5G). Parametric analyses were performed on optical and electronic properties to identify key performance parameters and evaluate their impact on the performance of compositionally graded InxGa1-xN PEs.