The most common use of thermal solar energy has been for water heating systems; this use has been commercialized in many countries in the world. This paper presents a model of a forced circulation solar water heating system for supplying a hot water at a certain temperature for an aquaculture system. The main components of the system are flat plate collector, storage tank, and a biogas auxiliary heater. The optimization problem is carried out using a genetic algorithm, which is one of the modern optimization techniques because of its evolutionary nature; it can handle any kind of objective function and constraint. Genetic algorithms don’t have mathematical requirements about the optimization problem; Also genetic algorithms are very effective in performing a global search (in probability), and provide a great flexibility. The optimal design of flat plate collector area using genetic algorithm is used to optimize the objective function considering the constraints required for the system. As the genetic algorithm is a discrete optimization tool, the number of variables in principle is unlimited. The economic analysis of such system is evaluated with the life cycle cost method. The collector area is equal to 63 m 2 , at this value the solar fraction reached 98% which is a very high value. Also, sensitivity analysis to solar radiation variation, air temperature variation, and interest rate has been carried out.