This paper presents an original sizing method for Energy Storage Systems (ESS) based on directly matching their capabilities – as specified by their energy-power Safe Operation Area (SOA) in the Energy-Power (EP) plane – with the energy and power demand required to accomplish their missions. Starting from the system requirements and from an energy management strategy, the power demanded by a set of representative operating scenarios and its associated energy are calculated and represented as trajectories in the EP plane. The objective is to size the ESS such as its SOA contains these trajectories. Comparison between different technologies of Energy Storage Devices (ESDs) is possible using this SOA characterization. Special attention should be paid to compare specific SOAs across devices. Diverse energy management strategies can be synthesized in the EP plane where they can be compared and analyzed. The sizing method converges extremely fast and is suitable for its integration in an optimization loop. The method allows to determine directly and efficiently the technology and the size most appropriate (in terms of indicators such as mass or cost) to a given EP demand. In the paper, three different technologies (SuperCapacitor, Li-Ion and H 2 ∕ O 2 batteries) are characterized and compared in terms of sizing synthesis.