The emergence of Plug in Battery Electric Vehicles (BEV) is a process which will bring a large aggregate source of distributed energy storage into the electricity industry. The potential exists for this storage to bring benefits from the ability to shift net BEV demand (both charging and vehicle to the grid export) in response to electricity industry needs. The potential for BEV flexibility to act as a Distributed Energy Resource (DER) is however constrained by a range of factors including their mobility, need to serve transport energy requirements, and the locational/temporal availability of physical charging opportunity. This paper addresses the challenge of characterizing the availability of this new storage resource and aims to be of use to policy makers and electricity industry planners in developing strategies for maximizing the value of BEV integration for the electricity industry. In particular it: presents a general discussion of, and framework for understanding the manner in which BEV storage gives rise to charging/discharging flexibility, presents a method for simply characterizing the factors which constrain this flexible resource, introduces a method for empirically establishing a benchmark DER potential which fully accounts for relevant constraints, and applies these methods to a case study of vehicle use in the Sydney Greater Metropolitan area. Results show that, in respect of vehicle transport in Sydney, charging speed and the need to reserve energy in respect of transport needs is a lesser constraint than the impact of charging infrastructure availability. While the DER potential declines during the day in all cases, access to additional charging infrastructure minimizes this decline. Investment in additional non-residential charging infrastructure may therefore be particularly important in maximizing the DER potential arising from BEV storage flexibility, in particular for the opportunity to manage the integration of high future PV generation levels.