This article proposes a method to optimize the design of a small fixed-voltage wind energy conversion system (WECS). The system is composed of commercially available elements which are: a small horizontal axe wind turbine, a onestage gearbox, a permanent magnet synchronous generator, a diode bridge and a battery bank. As there are no controlled devices on the system, the design must be carefully done to find the configuration that maximizes both, the system utilization and the system's output power. Using the mechanical and the electrical power equations, an optimization problem is proposed. This problem is aimed to find the optimal combination of the gearbox ratio and the battery voltage in order to extract the maximum amount of energy form the WECS. The mechanical power is modelled using a new proposed power coefficient function approximation. The constrained optimization problem is then solved by a MATHEMATICA© genetic algorithm based routine. Results are shown and discussed.