A method of controlling a network system is provided. The network system includes a metering device measuring energy supplied from a power supply source, a communication device in which energy information related to the energy communicates, and an energy management device recognizing the energy information, the energy management device controlling an operation of an electric product. The metering device or the energy management device checks whether communication of the communication device is possible. Also, when it is determined that the communication device is in a communication failure state, the communication failure state is displayed on the outside thereof. When a communication modem mounted on the electric product is in a communication failure state, the electric product may normally perform a power management program, based on power or operation information stored therein.