A battery system including an assembled battery, a charge mode for charging the assembled battery and a discharge mode for converting electric energy charged in the assembled battery to AC power having the same or substantially the same frequency as a grid and supplying the AC power to a power supply line through which the grid and a load are connected further including a current detector for detecting current supplied to the load, and a control device for calculating an average power usage of past several days based on current being detected by the current detector, and converting the amount of electric power corresponding to the difference between the average power usage and a power usage based on the current value from the current detector to the AC power from the assembled battery to the load in the discharge mode to supply the AC power to the power supply line.