An air flux differential calorimetric system for the experimental determination of power losses in converters is presented; the system has been used to evaluate power losses in transistor inverters of different sizes. The experimental results have been compared with the results yielded by practical relations commonly used to evaluate the lossses in the various components of power converters. The results of this comparison are reported and discussed; they point out both the simplicity of the experimental method and the actual reliability of the practical relations used.