At the operation analysis of the electrical machines many doubts can arise as to which methodology to determine the efficiency is selected. IEC 60034 shows how the efficiency can be determined using different methodologies (i.e. direct measurement, summation of losses and other) To study the influence of both measurement system and each term of losses the static sensitivity must be evaluated. Sensitivity functions analysis allows the estimation of output error in front of parameter variation, as well as its influence on the global system response.