The decarbonization of the energy sector drives the implementation of building-integrated and building-applied photovoltaic (BIPV–BAPV) systems. However, these systems face space and design limitations in urban environments. This study proposes an innovative methodology for the design and sizing of urban photovoltaic systems, considering diverse distributions and introducing metrics that link performance to occupied area. The methodology was applied to a university building in southern Spain, comparing the performance of rooftop photovoltaic (RTPV) and facade-applied photovoltaic (FAPV) systems. FAPV showed a larger useful area, resulting in similar self-sufficiency indices (RTPV: 22%, FAPV: 21%) and a 5% higher total emission reduction compared to the RTPV system. The proposed metrics demonstrate that FAPV outperforms RTPV both in final yield (49 vs. 21 kWh/kWp·m2) and total emission reduction (3.1 vs. 1.3 kgCO2eq/kWp·m2) normalized by installed power and occupied area. These complementary metrics are crucial for evaluating and selecting optimal photovoltaic configurations with varying generation densities and efficiencies, driving urban decarbonization and the creation of Zero Energy Buildings (ZEBs).