In situ machine efficiency estimation techniques are increasing in importance due to pressures to improve performance and for demand-side management. Numerous techniques have been proposed to estimate the efficiency of an induction motor in situ. Only a few are actually practical considering the level of intrusion and the accuracy. Besides, very little has been done to make these methods compatible with the unbalanced supply condition that exists in most industrial applications. In this paper, a new evolutionary-based in situ efficiency estimation technique, capable of dealing with balanced and unbalanced power supply conditions without the requirement of the no load test, is proposed. The effectiveness of this method is verified by experimental results.