This paper presents the development of a sizing approach under reliability and energy performance indexes applied to fuel cell/battery (FC/BAT) hybrid power system. It deals with the interdependence between the components sizing process and the control strategy. The proposed approach is based on imbricated optimization loops and considers two important criteria, fuel consumption and reliability. This proposed approach involves multi-objectives optimisation to study the trade-off between reliability and energy saving to improve the optimal design relevance. Also, the implementation was performed using a low complexity process to offer compromise between computation time and results accuracy. This constitutes a very effective support tools to help design engineers in the early design stages.