An energy storage device comprising: a plurality of battery packs having a first connector, respectively; a network interface to exchange data with a server; and a connector to receive alternating current power from an internal power network or outputting AC power to the internal power network; a power converter to convert the AC power from the internal power network into direct current power based on information about the power to store when the information about power to store is received from the server, or, convert DC power stored in the battery pack into AC power based on the information about the power to output when the information about power to be output to the internal power network is received from the server, characterized in that the energy storage device further comprises: a case for containing the plurality of battery packs; a plurality of second connectors formed in the case and coupled with a plurality of first connectors, respectively; a controller to perform a control operation to turn off an electrical connection between a first battery pack and a second battery pack for a predetermined period when the second battery pack is additionally attached to the second connector under a condition that the first battery pack is attached to the second connector; and wherein the first connector includes the positive power terminal, the negative power terminal and the control signal terminal; wherein the second connector includes the positive power terminal, the negative power terminal and the control signal terminal, wherein the power connection terminal, negative power connection terminal and control signal connection terminal of the second connector are coupled with positive power terminal, negative power terminal and control signal terminal of the first connector of the attached battery pack, respectively, when the battery pack is attached, wherein the respective battery pack includes knobs (415a, 415b) formed at front side of the respective battery pack. a plurality of battery packs having a first connector, respectively; a network interface to exchange data with a server; and a connector to receive alternating current power from an internal power network or outputting AC power to the internal power network; a power converter to convert the AC power from the internal power network into direct current power based on information about the power to store when the information about power to store is received from the server, or, convert DC power stored in the battery pack into AC power based on the information about the power to output when the information about power to be output to the internal power network is received from the server, characterized in that the energy storage device further comprises: a case for containing the plurality of battery packs; a plurality of second connectors formed in the case and coupled with a plurality of first connectors, respectively; a controller to perform a control operation to turn off an electrical connection between a first battery pack and a second battery pack for a predetermined period when the second battery pack is additionally attached to the second connector under a condition that the first battery pack is attached to the second connector; and wherein the first connector includes the positive power terminal, the negative power terminal and the control signal terminal; wherein the second connector includes the positive power terminal, the negative power terminal and the control signal terminal, wherein the power connection terminal, negative power connection terminal and control signal connection terminal of the second connector are coupled with positive power terminal, negative power terminal and control signal terminal of the first connector of the attached battery pack, respectively, when the battery pack is attached, wherein the respective battery pack includes knobs (415a, 415b) formed at front side of the respective battery pack. a case for containing the plurality of battery packs; a plurality of second connectors formed in the case and coupled with a plurality of first connectors, respectively; a controller to perform a control operation to turn off an electrical connection between a first battery pack and a second battery pack for a predetermined period when the second battery pack is additionally attached to the second connector under a condition that the first battery pack is attached to the second connector; and wherein the first connector includes the positive power terminal, the negative power terminal and the control signal terminal; wherein the second connector includes the positive power terminal, the negative power terminal and the control signal terminal, wherein the power connection terminal, negative power connection terminal and control signal connection terminal of the second connector are coupled with positive power terminal, negative power terminal and control signal terminal of the first connector of the attached battery pack, respectively, when the battery pack is attached, wherein the respective battery pack includes knobs (415a, 415b) formed at front side of the respective battery pack.