  We consider the problem of dispatching a fleet of heterogeneous energy
storage units to provide grid support. Under the restriction that recharging is
not possible during the time frame of interest, we develop an aggregate measure
of fleet flexibility with an intuitive graphical interpretation. This
analytical expression summarises the full set of demand traces that the fleet
can satisfy, and can be used for immediate and straightforward determination of
the feasibility of any service request. This representation therefore
facilitates a wide range of capability assessments, such as flexibility
comparisons between fleets or the determination of a fleet's ability to deliver
ancillary services. Examples are shown of applications to fleet flexibility
comparisons, signal feasibility assessment and the optimisation of ancillary
service provision.
