Flat Plate Solar Collectors are commonly used devices for the conversion of solar energy into heat, mainly for water and building heating. Their popularity is attributed to their simple design, a flat, rectangular structure covered with a transparent material (usually glass or plastic) that allows sunlight to pass through. Inside, an absorbing plate, usually made of black-painted metal, maximizes the absorption of solar radiation. However, their efficiency is often limited by the thermal properties of conventional heat transfer fluids such as water or thermal oils. To overcome this limitation, nanofluids, consisting of metal or metal oxide nanoparticles dispersed in a base fluid, offer an innovative solution. Thanks to their superior thermal conductivity and large specific surface area, nanofluids significantly improve heat transfer within the collector. This improvement allows for more efficient absorption of solar energy and reduces heat losses. In addition, nanofluids promote better temperature homogeneity on the absorber plate, thus increasing the overall efficiency of the flat plate solar collectors. The study of the impact of nanofluids on the efficiency of flat plate solar collectors, therefore, represents a crucial research area to optimize the energy performance of solar thermal systems and contributes to the transition to more efficient and sustainable energy sources. This paper analyzes the effect of Cu-H2O nanofluid on the efficiency of the flat plate solar collector through two parameters: the mass fractions and the nanoparticle sizes.