Floating photovoltaic (FPV) plants in reservoirs can contribute to reduce water evaporation, increase power generation efficiency, due to the cooling process, and reduce competitiveness in land use. Based on this motivation, we propose a new methodology for sizing FPV plants in dams of semi-arid regions using the flood duration curve. The methodology innovations are no use of commercial software, the possibility of choosing the reliability level, the application in reservoirs of semi-arid areas of the world, and the use of a graphic analysis of the reservoir hydrological behavior. The case studies in the Brazilian and Australian semi-arid consider two scenarios: high reliability level (90%, scenario 1) and low reliability level (70%, scenario 2). The reliability level is linked to the electricity production; the evaporation reduction is proportional to the FPV plant area.