This paper deals with some problems concerning the evaluation of uncertainty depends on harmonic voltage distortion for precision electric power measurements and its further influence on evaluation expanded uncertainty of electric power measurements on National standard of electric power and power factor units of Ukraine. In experimental study cases the authors have modulated harmonic voltage distortions and evaluated the uncertainty depends on it. The paper presents the theoretical framework of the problem as well as the results of measurements, which illustrate the presence of the disturbing voltage harmonics in the low harmonics order such as 3rd, 5th, 7th, 9th, 11th and 13th.