Offshore wind energy generation has developed rapidly especially in the last decade. In some countries, despite having good wind conditions for its development, it has not been possible to implement it. This is because its continental shelf is very short, that is, the land under water in shallow seas is not wide enough to install fixed foundations. Consequently, the development of floating platforms is awakening interest in this type of generation, as is the case in Spain. In this article, a methodology for calculation the generated electrical energy in floating offshore wind farms and the corresponding losses is developed. In this way, all necessary data involved in the methodology have been obtained from the sites destined for offshore wind development. These emplacements have been released by Spanish Government in recently approved Maritime Spatial Plan. The generation potentials in Spanish waters have been evaluated according to most interesting ratios as levelized cost of energy, capacity factor and cost of installed megawatt. Results show that most of emplacements are interesting enough for the development of OWFs. Deserve to be mentioned three main areas, such as, northwest waters, Canary Island zones and north part of Catalonia.