Method and practical use system for measuring the imbalance potential of an electrical installation or system comprising: i) acquiring instantaneous values of voltage (vA, vB, vC) and electrical intensity (iA, iB, iC) of each of the phases A, B, C of the installation, and breaking them down into their fundamental frequency components (vA1, vB1, vC1), (iA1, iB1, iC1); ii) obtaining effective voltage and intensity values and angles for initial phase difference between voltage and intensity, and, starting from these effective values obtaining active powers (PA, PB, PC) and reactive (QA, QB, QC) potentials for each of the phases; iii) from the active and reactive potentials, obtaining (4) a phasor imbalance potentials (ĀU) according to the following expression: ĀU=√{square root over (2)}( p·|PA+a2PB+aPC|+ q·|QA+a2QB+aQC|) where a=1|120° and p and q are orthogonal unit phasors. The invention also relates to a device for calibrating (21) instruments for measurement instruments of this imbalance power.