Proton exchange membrane (PEM) fuel cells show characteristics of high power density, low operating temperature, and fast start-up capability, which make them potentially suitable to replace conventional power sources (e.g., internal combustion engines) as auxiliary power units (APU) for on-board applications. This paper presents a methodology for a preliminary investigation on either sizing and operating management of the main components of an on-board power system composed by: (i) PEM fuel cell, (ii) hydrogen storage subsystem, (iii) battery, (iv) grid interface for the connection to an external electrical power source when available, and (v) electrical appliances and auxiliaries installed on the vehicle. A model able to reproduce the typical profiles of electric power requests of on-board appliances and auxiliaries has been implemented in a computer program. The proposed methodology helps also to define the sizing of the various system components and to identify the fuel cell operating sequence, on the basis of the above mentioned load profiles. Copyright © 2006 by ASME.