  Flexible loads are a resource for the Balancing Authority (BA) of the future
to aid in the balance of supply and demand in the power grid. Consequently, it
is of interest for a BA to know how much flexibility a collection of loads has,
so to successfully incorporate flexible loads into grid level resource
allocation. Loads' flexibility is limited by all their Quality of Service (QoS)
requirements. In this work we present a characterization of capacity for a
collection of flexible loads. This characterization is in terms of the Power
Spectral Density (PSD) of the reference signal. Two advantages of our
characterization are: (i) it easily allows for a BA to use the characterization
for resource allocation of flexible loads and (ii) it allows for precise
definitions of the power and energy capacity for a collection of flexible
loads.
