A network system includes at least one unit selected from between an energy consumption unit receiving energy and an energy management unit managing the energy consumption unit, wherein an energy usage amount or an energy usage cost of the energy consumption unit are controllable, and the energy usage amount or the energy usage cost when the unit is controlled based on information associated with at least an energy cost is smaller than an energy usage amount or an energy usage cost when the unit is controlled without using the information related to at least the energy cost, and a display device detachably coupled to the energy consumption unit, wherein availability of the display device is determined according to a communication signal transmitted to the energy receiving unit.