This paper investigates the optimal integration of a hybrid renewable energy system consisting of wind turbines and/or photovoltaic solar panels with membrane-based desalination units used to obtain freshwater from seawater or brackish water with zero brine production. The minimization of the total annual cost is proposed as the objective function. In the first case study, no grid connection is considered and wind turbines and/or photovoltaic panels can be only used for providing the electrical power required by the desalination processes to meet variable freshwater demand. Then, in a second study, a grid-connected hybrid renewable energy system is investigated. The possibility of variable freshwater storage is considered in order to conveniently accommodate the renewable energy generation towards a cost-effectiveness process design. To show the model powerful, renewable energy sources available in the South of Argentina are assumed. Data taken from NASA are used to characterize the weather conditions, such as solar irradiance and wind speed. Also, data taken from catalogue are considered for the wind turbines. The proposed model has the potential to assess to what extent variable water demands can be covered by wind and solar energy in any site of the world with only changing the input information associated to the freshwater demand and meteorological wind and solar data.