The H-BUS is a joint project of National Research Council of Italy and two supplier companies to develop a range extender Fuel Cell/Battery Hybrid Electric city bus. Within the project, CNR ITAE Institute is involved in determining the optimal level of hybridization assessing all boundary conditions (mission, performances, hydrogen consuption, range, etc...). The paper reports the characterization results of the hybrid system which allowed the identification of the power and energy consumption. These data were the starting points to define the size of Batteries and Fuel Cell (FC) system and to optimize the batteries State of Charge (SoC). PEM (Polymer Electrolyte Membrane) and ZEBRA® (Zero Emission Battery Research Activities) technologies have been selected for the fuel cell system and batteries, respectively.