Solar-driven processes inspire hopes and dreams of a sustainable future on planet earth. However, the design and optimization of photoreactors for specific reaction systems is a complex task at the interface of material science and engineering. The objective of the design process is, thereby, to derive a photoreactor that provides both optimal operating conditions and a high efficiency of radiation transport. The published data set accompanies the publication ‘Low-cost photoreactors for highly photon/energy-efficient solar-driven synthesis’, DOI: XYZ. The referenced manuscript introduces reflective multi-pass photoreactors, an optimization strategy for such photoreactors, underlines the validity of the chosen approach by the experimental demonstration of a potassium iron(III) oxalate photoreduction, and lastly derives a concise design guideline that supports the adaptation to other reaction systems. The published dataset comprises the optical simulation environment, the optimization algorithm code, the simulation results, and experimental data presented in the manuscript.