The research presented in this paper is part of a project whose aim is to develop a flexible system to help industrials to efficiently install marine energy farms in a suitable area. We introduce a methodology and a decision-aid system for marine farm design. The developed framework will help, for a given marine area, to find the most relevant sites and marine technologies using multi-criteria analysis. The approach is oriented toward marine current energy but this methodology can be extended to others marine energies. Amongst the criteria involved in the decision process, a model is developed to evaluate the quantity of electrical energy produced by the farm, and the cost of the system during its entire lifetime. These two parameters allow us to derive the cost of the produced energy, which is one of the more important criteria to evaluate the economic feasibility of a marine energy project. The energy produced is evaluated taking into account both technological possibilities (turbines technology, generator type, underwater cables, offshore substation etc.) and site characteristics. The cost is estimated thanks to a specific cost model of each component and the farm layout, and also includes a first order evaluation of the installation/dismantling and maintenance costs.