An electric storage device includes a first electric capacitor, a second electric capacitor, a converter, and a processor. The converter converts electric power transmitted between an external power system external to the electric storage device and at least one of the first electric capacitor and second electric capacitor. The processor is configured to control the converter to operate in at least one of a first mode and a second mode, the first electric capacitor continuously discharging electric power to the external power system in the first mode, the second electric capacitor discharging electric power to the external power system and being charged by the external power system, intermittently, to stabilize frequencies in the external power system in the second mode.