An apparatus for controlling a battery pack and an energy storage system including the apparatus are disclosed. In one embodiment, the battery pack includes at least one battery tray each including one or more battery cells. The apparatus may include an open circuit voltage (OCV) calculator and a state of charge (SOC) estimator. The OCV calculator may receive OCV measurement values of the battery cells of each battery tray when the power of the battery pack is turned on, and calculate a final OCV of the battery cells based at least in part on the OCV measurement values. The SOC estimator may extract an SOC value corresponding to the final OCV from an SOC table and estimate the extracted SOC value as an initial SOC.