Low-emission hydrogen production needs to be economically viable and competitive compared to fossil-fuel-based production methods. Several techno-economic studies are proposed to develop a methodology for optimizing the design of the elements that contain these systems. In this study, a direct connection of a photovoltaic (PV) source to an electrolyzer through a DC-DC converter is analyzed. The sizing optimization is based on the Levelized Cost of Hydrogen (LCOH) and the installation oversizing ratio, which considers the installed nominal PV power and the nominal electrolyzer power. Compared to previous studies, this analysis is based on a per-minute environmental data and includes a precise model of the electrolyzer and DC-DC converter, resulting in higher precision for the optimization process. The results confirm the dependence on the electrolyzer CAPEX and OPEX values and the installation size. Finally, robustness of the proposed optimal system sizing is validated through sensitivity analysis, and location dependency of the results are discussed.