Power transformer is one of the most efficient devices used in power systems. Modern electrical distribution systems typically supply a very high percentage of non-linear electronic loads. As a result, transformer losses increase, and energy efficiency decreases. The level of this deterioration is a function of harmonic voltage magnitudes at a transformers primary terminals, and load-generated harmonic currents. There are not yet recognized standards or methods for determining of transformers losses or efficiency under these non-linear load conditions. Conventional methods for measurement of transformer losses, open and short circuit tests, under nonlinear load do not apply, because measurements are possible only when the load is present, i.e, in real working conditions. The subject of this research is to determine the penetration of errors in measurements of power transformer losses and efficiency under nonlinear load conditions. We are not focused on digital process, A/D conversions, etc., we are focused on algorithms and methods of measurement and their response to the instrumentation errors. Significance of assessment of measuring errors is due to the fact that the errors of data propagate through calculations and produce errors in measuring results. Analyses are done with algebra of penetration of errors in calculations and measuring algorithms.