The decision to use a desalination system as well as the selection of the most appropriate combined energy source depends on several parameters. The main are the low investment, the low maintenance requirements, a simple operation and a high reliability. This paper discusses the costs dependence from such parameters in the case of an RO desalination system coupled to a combined cycle power plant supplying both electricity and heat. An economic analysis is made for specific site data using DEEP code and the technical designs available in the program. The sensitivity of the levelized water cost from construction water plant cost, discount rate and operation and maintenance costs is established, taking into consideration that these parameters constitute the major constraint to the development of desalination water plants; water costs are determined for adequate economic hypothesis used in Morocco or recommended by international organizations such as IAEA and OECD.