A simple equivalent circuit model for electrochemical (EC) capacitors can be established based on electrochemical impedance spectroscopy. The circuit consists of an ohmic resistor and a finite-length Warburg element in series. The EC capacitor's performance including the transient/pulse response and energy density as a function of power density (Ragone plot) can be stimulated by the equivalent circuit model with three useful parameters including an ohmic resistance, total ionic resistance, and total capacitance of the electrodes.