In supercapacitor technology, the determination of the largest voltage range useable for the charge storage is a key parameter to optimize their energy. For this purpose, “acceptable” coulombic and energy efficiencies are commonly used as cut-off values to delineate a safe operational potential window in opening-window charge-discharge experiments. Because of the arbitrariness of such an approach, attempts have been made over the past three decades to rationalize the determination of the stability window, but differentiate between capacitive and faradaic currents remains challenging. Assuming that it’s better to examine what is lost during the charge period to investigate the degradation of electrochemical interfaces, part of the problem is that we have not the correct perspective on stability with coulombic and energy efficiencies, since they give an indication of the way in which performance are retained during the charge process, as they represent the capacitive fractions of the measured quantities. Here, we propose a new formalism based on non-capacitive fractions to focus on what is lost during the charge period, in order to place the heterogeneous kinetics of the electrochemical degradation processes at the center of the debate on stability of the electrode-electrolyte interfaces in supercapacitors.