The measurement of the efficiency of power electronics converters is a special challenge, because it is difficult to perform it accurately using electrical methods (especially in the high efficiency range or in low power applications). Therefore in many applications, where accuracy is crucial, engineers resort to calorimetric approaches. These in turn are extremely slow and special conditions need to be created to obtain good results. In other applications, where the accuracy is not the primary goal, but rather the fast, on-line, comparative estimation governs the demand for realizing the final aim. As a result, a solution to effectively determine the efficiency of the converters while working, is still sought for. The current paper aims at presenting a possible alternative, calculating the efficiency on-line with satisfactory accuracy at a low cost.