A method for responding to a change in electric power demand includes (1) charging an energy storage subsystem from an electric power grid, (2) discharging the energy storage subsystem into the electric power grid at a discharge rate that is less than a maximum rate of discharge of the energy storage subsystem, and (3) adjusting the discharge rate in response to a signal selected from the group consisting of a signal to provide a regulation up service and a signal to provide a regulation down service. An energy storage system includes an energy storage subsystem for storing electric power, an interface for interfacing the energy storage subsystem with an electric power grid, and a controller configured to control operation of the interface in response to a signal to provide a regulation up service and a signal to provide a regulation down service.