This study presents an experimental evaluation aimed at enhancing the efficiency and sustainability of renewable energy systems using photovoltaic thermal (PVT) collectors employing nanofluids. The experimental setup features a flat-plate PVT collector with a single-pass water flow, utilizing H2O and graphene sheets dispersed in water at a concentration with 0.04 wt%. Various operational conditions, including different flow rates and nanoparticle concentrations, were examined to assess the PVT collector’s performance. Outdoor assessments, using water as a reference fluid, were conducted at flow of 0.7 L per minute and 1.3 L per minute for the considered nanofluids. The results, rigorously analyzed and confirmed, revealed that the graphene water nano-colloid outperformed water, despite water exhibiting superior thermal performance during peak solar radiation and elevated ambient temperatures. The introduction of water to the PVT collector led to an 8.1% and 8.9% increase in average daily electrical efficiency at flow of 0.7 L per minute and 1.3 L per minute, respectively. Additionally, incorporating water enhanced the PVT mean daily thermal efficiency by 25.9% at a flow rate of 0.6 L per minute and by 28.3% with flow per minute of 1.2 L. The findings highlight that the inclusion of nano-colloid in PVT collectors can explosively boost both electrical and thermal efficiency, with optimal results achieved at lower flow rates and higher nanoparticle concentrations. The study underscores the promise of nanofluid-based PVT collectors as efficient and enduring technologies for renewable energy production.