Systems and methods for managing power supplied over an electric power grid by an electric utility and/or other market participants to multiple power consuming devices, each of which having a Power Supply Value (PSV) associated with its energy consumption and/or reduction in consumption. Power flow to the power consuming devices is selectively enabled and disabled, or power-reduced thereto, by one or more controllable devices controlled by the client device. Power control messages from a controlling server indicate amounts of electric power to be reduced and an identification of at least one controllable device to be instructed to disable or reduce a flow of electric power to one or more associated power consuming devices.