The factors on which depends preciseness of determination of induction motors efficiency are presented in the work. A classification of efficiency is given. It is discussed direct and indirect methods of efficiency determination with suitable classification of indirect methods. Dependencies for uncertainty of determination of unconditional and conditional efficiency are drawn. In calculating of uncertainty of conditional efficiency it is necessary to calculate differential power losses over set of parameter characterised the conditions of measurement. For example a structure of algorithm for calculation of differential losses over voltage supplied investigated motor is presented.