Hybrid renewable systems increase electricity production by reducing the randomness of sources without greenhouse gas emissions. Literature indicates that further research is still required for large systems connected to the grid. The energy storage capability of hydroelectric plants provides an opportunity to develop new renewable energy generation. This work presents the results of a hybrid preliminary design between an existing hydropower plant (HPP) in Brazil, Santa Branca, and a simulated floating photovoltaic plant (FPV), occupying only 2.8% of reservoir's surface. The study analyzed 20 years of HPP historical data, the methodology proposed the injection of the FPV full power into the system during daytime and the HPP adjusting the production. As this process changed the HPP generation profile, operational limits such as reservoir level needed to be verified. In the period, while HPP generated about 4 TWh, the FPV could add 2 TWh without significantly changing the daily generation of HPP. This increase of 50% in production was possible since there was space available in the reservoir to store water during the day and use it during the night, working as a free virtual battery for the FPV. A 50% increase of the grid connection capacity factor was also observed.