Recently, a consensus emerges on the relevance of using the energy efficiency (EE) as a metric to evaluate the operationaJ potentiaJ window (OPW) of the elec-trochemicaJ energy storage systems. Unfortunately, EE alone is not comprehensive enough to provide a full understanding of the electrochemical degradation at the electrode/ electrolyte interface since both the coulombic efficiency (CE) and the voltage efficiency (VE) have an impact on EE. Here it is proposed to deconvolute EE into its CE and VE contributions by ploning the variations of CE with VE after their Neperian logarithmic transformation (Ln) in a polar coordinate system (p, a) where the modulus p can be approximated to Ln(EE) and the argument a represents the relative importance of the individual contributions of CE and VE. ln the present case, resuJts obtained with YP80F based electrodes in 1 M H 2 S0 4 electrolyte demonstrate that the decrease in EE at both ends of the OPW is different in nature depending on whether the electrode serves as positive or negative electrode.