The growth of Renewable Energy Sources (RES) requires more reserve for generation-demand balancing. Recently, energy storage system such as battery is expected as additional reserve power source. The authors have proposed Battery-Assisted Load Frequency Control (BALFC). The method makes it possible to compensate reserve deficiency by providing excess components of LFC signal over ramp rate limits and upper/lower limits of LFC units such as thermal power plants. This paper illustrates that the BALFC can save required battery power capacity by effective use of battery's high responsiveness compared with a conventional method. Moreover, it is clarified that frequency control reserves can be saved by using battery with the BALFC instead of increasing LFC units.