This paper presents a model of a distributed energy storage resource and its actuation signal. Many previous studies of distributed energy storage systems have assumed that the system has no energy storage limitations and that the actuation signal has no net or instantaneous energy content. This model includes these conditions so as to enable the design of automatic generation control signals whose characteristics maximize the device-level and system-level reliability of distributed energy storage systems. Analytical and numerical results show that the reliability of the DES device can be characterized using a set of non-dimensional parameters. These device-level reliability results are then translated into system-level reliability. These results are novel in that they can guide the balancing authority in constructing an automatic generator control signal specific to the capabilities of distributed energy resources. (C) 2013 Elsevier B.V. All rights reserved.