The performance of a photoelectrochemical device can be improved significantly by using porous fibrous photoelectrode substrates since they enhance the electrochemically active surface area, decrease resistivity, and reduce diffusion length for charge carriers. However, the fibers may also substantially scatter the incident radiation, reducing sunlight utilization as well as transmission to the second photoelectrode in a tandem configuration. In this study, a Monte Carlo ray-tracing model was developed to quantify the normal-hemispherical reflectance, transmittance, and absorptance associated with fibrous substrates (generated in-silico) to optimize their design. The effect of a wide range of fiber diameters, substrate thicknesses, and porosities were evaluated for both absorbing and non-absorbing fibers. The effect of anisotropy of fibers on radiation transfer was also studied. Overall, it was found that the fibers should be large and isotropic, and the ensembles should be thin and highly porous to minimize optical losses.