A battery system includes a battery, a control section that controls the battery to charge electric power supplied from the electrical grid and to discharge electric power to the electrical grid; and a system controller. The system controller controls the control section such that the battery system operates in a first mode where electric power that is generated by a pre-designated distributed electric power source associated with the electrical grid is supplied preferentially to a user's load or in a second mode where electric power that is charged in the battery is supplied preferentially to the user's load.