The efficiency estimation of electric motors has become a key subject due to industrial energy policies, environmental concerns, and the cost of electricity. It is of major interest to test a machine in-situ, as the installation conditions and the local grid voltage unbalance deeply affect the efficiency of the electric motor. To address this issue, this paper presents a methodology to estimate the efficiency of three-phase induction motors in-situ. The proposed method only requires a direct start-up of the motor to acquire the stator voltage and current waveforms during the acceleration process. The proposed methodology was validated experimentally using two distinct NEMA design induction motors with loading ranges between 30% and 120%. The efficiency results were compared with the IEEE Std. 112-B results, With a maximum deviation of 1.4% between the two methods. The results demonstrate the advantages of the proposed test for in-situ applications, as it is accurate, easy and fast to perform on energy audits.