Addressing the mismatch between the intermittent solar irradiation and the time-varying load demand, the optimal energy management principle with its corresponding control scheme for stand-alone PV - H 2 system is investigated to achieve high energy efficiency. Based on the proposed optimal energy management strategy, a systematical system sizing method is developed for determining the minimum capacity of the system components. Therefore the system hardware cost can be well assessed using the developed system sizing method. Case studies are carried out to verify the effectiveness of the proposed energy management strategy and system sizing method. Simulation results show that the investigation in this article provides a good straightforward solution to the pre-design and operation of stand-alone PV - H 2 systems.