ISWEC (Inertial Sea Wave Energy Converter) is a gyroscopic effect based wave power extraction device. The aim of this paper is to present a tool developed for the design process of this WEC. The main purpose of the software is to allow to the system engineer to compute the annual electrical productivity using a simplified model, taking into account several considerations such as system geometrical and electrical configurations, the hull properties or the installation site. To obtain this result, an automatic optimization of control parameters is implemented. A cost function is minimized under various system constraints (e.g. PTO torque and power, flywheel speed) and the absorbed power of the system is evaluated. Output of the software is the device productivity, along with considerations about structural loads, PTO utilization, floater and mass properties. The software will help the system engineer to take preliminary decisions and to verify the adaptability of this WEC to the considered sea site. The methodology here proposed can be easily generalised to other applications, especially in wave energy field, where severe constraints and time consuming simulations are common, and the use of a tool like this can reduce develop time, giving different information from the very start of the project.