  We present a method to find the maximum magnitude of any supply-shortfall
service that an aggregator of energy storage devices is able to sell to a grid
operator. This is first demonstrated in deterministic settings, then applied to
scenarios in which device availabilities are stochastic. In this case we
implement chance constraints on the inability to deliver as promised. We show a
significant computational improvement in using our method in place of
straightforward scenario simulation. As an extension, we present an
approximation to this method which allows the determined fleet capability to be
applied to any chosen service, rather than having to re-solve the
chance-constrained optimisation each time.
