Evaluating the improvement in efficiency and heat transfer rate in heat exchangers using additives has been the centre of attraction for many researchers. In this study, a solar flat plate water heater is modelled using ANSYS Fluent, and the simulated results are compared with experimental data. For the simulation, a single-phase model is used with equivalent correlations for the thermo-physical properties of the nanofluid. Using water as the working fluid, the performance was evaluated for a set flow rate and solar insolation. The study is extended by modelling the same in ANSYS Fluent and using water and CuNP mixture as working fluid to analyse the enhancement of heat transfer in thermal absorbers. The pressure drop in the system is analysed and the data is used to justify the usage of nanoparticles in solar thermal absorbers. The efficiency of the system for various volume fractions of nanoparticles is optimized by introducing an efficiency index. © 2021, Springer Nature Singapore Pte Ltd.