An electric energy storage apparatus can generate an AC output in a low-loss and low-noise manner without using a DC-DC converter or an inverter. The electric energy storage apparatus comprises: an electric energy storage module group formed by connecting in series electric energy storage modules each comprising one or more electric energy storage elements; a balancing circuit electrically connected to the electric energy storage module group and configured to adjust a voltage to be applied to each of the electric energy storage modules; a first switch group comprising switches each in a path connecting a first terminal and a terminal of one of the series-connected electric energy storage modules; and a second switch group comprising switches each in a path connecting a second terminal and a terminal of one of the series-connected electric energy storage modules. The electric energy storage apparatus may perform a switch changeover in the switch groups.