A flat plate solar collector (FPSC) was analytically studied, with functionalized graphene nanoplatelets (f-GNPs) as its working fluid. Four samples (wt % nanofluids) were prepared in different base fluids such as ethylene glycol (EG), distilled water (DW):EG (70:30), and DW:EG (50:50). Experimental results (via DW) were used to verify the effectiveness of the analytical model. Some of the operating conditions were taken into account in this research, including temperatures, power, and mass flow rates. Experimental techniques were used to elucidate the modified nanofluids’ physicochemical properties, such as its particle sizes, stability, and morphology, involving electron microscopes (EMs), UV–VIS, and X-ray techniques. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were applied to test the thermal analysis. The findings confirmed that the use of f-GNPs nanofluids enhanced the performance of the FPSC relative to the use of base fluids for all testing conditions. The maximum enhancement of the collector’s effectiveness at a mass flow rate of 1.5 kg min−1 and a weight concentration of 0.1 wt %, increased to 12.69%, 12.60%, and 12.62% in the case of EG, DW:EG (70:30), and DW:EG (50:50), respectively. The results also confirmed an improvement in both the heat gain (FR(τα)) and heat loss (FRUL) coefficients for the f-GNPs nanofluid.