A method of calculating a charging capacity of an energy storage system, the method comprising: a current application step (S110) of applying a predetermined current having a constant value to the energy storage system; a first voltage measurement step (S120) of measuring a voltage of the energy storage system while the predetermined current having a constant value is applied to the energy storage system; a second voltage measurement step (S130) of measuring a voltage of the energy storage system after breaking the predetermined current having a constant value applied to the energy storage system; and a resistance (R) and capacitance (C) calculation step (S140) of calculating a resistance (R) and a capacitance (C) of the energy storage system based on the voltage of the energy storage system measured in the first voltage measurement step, the voltage of the energy storage system measured in the second voltage measurement step, and the predetermined current having the constant value; a voltage state variable calculation step of calculating a state variable for a voltage of the energy storage system based on a resistance and a capacitance of a lookup table; and a charging capacity state variable calculation step of calculating a state variable for a charging capacity of the energy storage system using a current integration method for integrating the measured output current, and an energy storage system charging capacity calculation step of substituting the voltage state variable and the charging capacity state variable into an extended Kalman filter to calculate a charging capacity of the energy storage system, characterized in that the predetermined current having a constant value applied in the current application step is a current between a minimum current value and a 0.1c-rate, which is capable of measuring a resistance and a capacitance value of the energy storage system. a current application step (S110) of applying a predetermined current having a constant value to the energy storage system; a first voltage measurement step (S120) of measuring a voltage of the energy storage system while the predetermined current having a constant value is applied to the energy storage system; a second voltage measurement step (S130) of measuring a voltage of the energy storage system after breaking the predetermined current having a constant value applied to the energy storage system; and a resistance (R) and capacitance (C) calculation step (S140) of calculating a resistance (R) and a capacitance (C) of the energy storage system based on the voltage of the energy storage system measured in the first voltage measurement step, the voltage of the energy storage system measured in the second voltage measurement step, and the predetermined current having the constant value; a voltage state variable calculation step of calculating a state variable for a voltage of the energy storage system based on a resistance and a capacitance of a lookup table; and a charging capacity state variable calculation step of calculating a state variable for a charging capacity of the energy storage system using a current integration method for integrating the measured output current, and an energy storage system charging capacity calculation step of substituting the voltage state variable and the charging capacity state variable into an extended Kalman filter to calculate a charging capacity of the energy storage system, characterized in that the predetermined current having a constant value applied in the current application step is a current between a minimum current value and a 0.1c-rate, which is capable of measuring a resistance and a capacitance value of the energy storage system. <EOS>