This paper investigates the discrepancy between double-layer supercapacitor voltage and the maximum electrochemical window voltage in an alkaline electrolyte. The electrochemical window is determined by the potential difference between hydrogen evolution and oxygen evolution. As the specific capacitance of a negatively polarized carbon material may be different from its positively polarized counterpart, the hydrogen evolution and oxygen evolution in the electrode may not occur at the same time. To achieve the electrochemical window voltage, the two electrodes in a double-layer supercapacitor need to be balanced and have the same capacitance to ensure that oxygen and hydrogen are generated at the same time.