Hybrids systems benefits are strongly dependent on the system design and the control strategy. The strategy choice strongly affect sizing and vice versa. This phenomenon is not always taken into account which leads to a non-optimal sizing of the system. This paper suggests a new method to optimize the design of hybrid power generator system from the point of view of fuel consumption taking into account this interaction. The main idea is to calculate the optimal control strategy of the system energy flux. Then, we can perform, for a given load profile, an optimization of the whole structure to find the best sizing.