The present work investigates the effect of the nanoparticles concentration on the optical and stability performance of a water-based nanofluid in solar photovoltaic/thermal (PV/T) systems experimentally and numerically. A novel nanofluid is formulated with the inclusion of the reduced graphene oxide decorated with silver (rGO-Ag) nanoparticles in water. Five different concentrations of nanoparticles in the range from 0.0005 to 0.05 wt% is suspended in water to prepare the samples. Optical properties are measured using UV-Vis. The UV-Vis absorption analysis reveals that all samples show consistent optical absorption coefficient (alpha) at higher value (more than 3 cm(-1)) in the range of 1.5 to 4 eV. The application of optical filtration (OF) using water/rGO-Ag nanofluid in hybrid PV/T system presented more solar energy absorption through the OF. The hybrid system shows better performance at concentrations less than 0.0235 wt% compared to the PV system without integration with optical filtration. The hybrid solar PV/T system with OF using water/rGO-Ag nanofluid is able to produce thermal energy with efficiencies between 24% and 30%.