The proper adjustment of the sizing of flat-plate solar thermal collectors is a crucial factor in their optimization, and it must be determined based on specific heating requirements. An undersized dimension can lead to a decrease in efficiency, while an oversized dimension can increase manufacturing and installation costs, as well as result in a degradation of its thermal performance. In this context, a numerical approach has been employed to enhance the outlet temperature by adjusting the length of the conduit. This adjustment depends on several other factors, such as fluid flow rate, thermal properties of the conduit material, and environmental conditions. The results highlight a direct correlation between pressure losses and fluid flow rate, which increase proportionally with a higher flow rate. As the fluid passes through the conduit and encounters these pressure losses, energy is dissipated as work to overcome the resistance to flow, potentially causing a decrease in pressure and, ultimately, a reduction in outlet temperature.