Solar and wind energy pose a challenge to direct power grid acceptance due to their randomness, intermittency and fluctuation, whereas hydropower with rapid response, good adjustability and stable output has become the most preferred option for complementary energy. In this study, a novel approach to optimal sizing of large-scale hydro-photovoltaic (PV) hybrid systems considering electricity delivery demand and reservoir characteristics has been proposed. We establish a multi-objective mathematical model that minimizes the difference between the daily output process of the hydro-PV hybrid system and the setting-up characteristic load process while maximizing the daily power generation of the hydro-PV hybrid system. The proposed method was applied to a multi energy complementary base in the upper reaches of the Yellow River in Qinghai Province, China. The results show that the optimal PV capacity for a hydro-PV hybrid system and the complementary guarantee rate are closely related to electricity delivery demand and reservoir characteristics. It was observed that at the complementary guarantee rate of 0.7, the optimal PV capacities of the Yangqu-PV hybrid system were 402 and 425 MW under three- and five-segment line modes, respectively, with the corresponding optimal PV capacities of the Banduo-PV hybrid system at 129 and 154 MW under three- and five-segment line modes, respectively. Additionally, the Yangqu-PV hybrid system meets bimodal and unimodal load shaving well; however, the Banduo-PV hybrid system only satisfies unimodal load shaving well. Finally, simulation results indicate that upstream reservoir regulation, hydropower expansion and regulated storage capacity expansion benefit the complementarity between hydro and PV. The highlighted results verify the feasibility of the proposed method. Therefore, the proposed method provides technical support and reference for planning and constructing PV power plants and can be used to implement other hybrid systems in the future.