Large-scale reverse osmosis (RO) plants with productions of the order of several cubic meters per day are extensively used throughout the world for water desalination. These systems, using electricity for running the high-pressure pumps, are among the ones with the lowest cost per cubic meter of water produced. Nevertheless, in some cases, for instance small rural sites or during catastrophes where drinkable water is not available, small RO systems running on photovoltaic (PV) systems could also be used to obtain drinkable water for life support. These autonomous systems can be made using commercially available small RO compact units with typical daily productions of the order of 100-500 L and functioning with pressures as low as 5 bar. Running on PV modules from 50-100Wp, these units can produce drinkable water from brackish waters containing salt concentrations of the order of 5000 ppm. This paper describes a unit of this type that has been assembled at the Renewable Energies Department of INETI and presents the results of the first laboratory tests These first results are being used for tuning a mathematical model of the system based on the I-V characteristics of the PV modules. The model of the system will be used for predicting the yearly water production of the unit and obtaining a value for the cost of the water produced by this type of system.