Photovoltaic (PV) systems are utilized all over the world for clean energy production. Photovoltaic simulation software is used to predict the energy produced by photovoltaic array structures. Due to Pakistan's geographical location in the equatorial region, the prospect of harnessing photovoltaic energy is too high. In the context of this fact, this work conducted extensive research to optimize photovoltaic energy output by using different PV modules sizes. For fixed areas where photovoltaic modules are installed, the output energy would remain more or less the same for any size of PV modules with insignificant differences in PV modules efficiency or quality. Moreover, in this research, it is found that by using different PV module sizes (i.e. 250 watt to 540 watt) at a time, while keeping all other parameters and conditions constant, a large variation in the output energy of the system can be observed. This difference in output energy with the change of PV modules sizes raises fundamental concerns about how to choose the right PV modules size to generate maximum output energy at any given location. This study intends to emphasize the fact that when designing an On-Grid photovoltaic system, relatively little consideration is given to selecting the appropriate type and size of PV modules, which can result in significant energy loss of the system. In this research, different PV modules of various sizes and power ratings with nearly identical efficiencies were analyzed in four selected locations. HelioScope simulation software is used to simulate all PV systems having PV modules power rating to analyze their monthly, annual energy generation, and system losses. The simulation results show that the appropriate PV modules size must be determined in order to generate the maximum output energy from the proposed PV system.