The present invention discloses an energy storage system. The energy storage system includes M cell strings, N energy storage converters, first ends of the N energy storage converters are coupled to at least one of the M cell strings, and second ends of the N energy storage converters are configured to connect to a power grid. A first end of a first energy storage converter is coupled to Q cell strings in the M cell strings, and the first energy storage converter includes a DC/AC conversion unit and at least one DC/DC conversion unit. A first DC/DC conversion unit is coupled to at least one of the Q cell strings by using the first end of the first energy storage converter, the first DC/DC conversion unit is coupled to the DC/AC conversion unit, and the DC/AC conversion unit is coupled to the power grid by using a second end of the first energy storage converter. The first DC/DC conversion unit is configured to perform adaptation between a voltage of the DC/AC conversion unit and a voltage of a cell string. Therefore, a cell capacity is fully used, and a waste of the cell capacity is reduced.