The present article explores the importance of developing a new methodology for the optimal sizing of a power generation system. To this aim, the proposed methodology is based on the analysis of the life cycle cost, which is composed by a set of differentiating costs. These differentiating costs will depend mainly on the variables of the equipment that make up the system, such as the cost of power generation, reliability of the equipment and the complete system, including the possibility of incorporating backup equipment. This analysis will allow to obtain an optimal number of such equipment. The size of the power generation system, in addition to assuming a minimum cost, have to fulfil the annual required power generated by the system itself. The proposed methodology consists of five stages: (i) Study of technical factors, (ii) Modelling and reliability analysis of the system, (iii) Application of the LCCA technique, (iv) Analysis of total costs and evaluation of scenarios to find the optimum n system size, complying with the requested generated power, (v) Sensitivity analysis and obtaining final results.