This paper presents another way to assess power quality delivered by transformers. It has been shown that is possible to measure the wave form irregularity of electrical signals radiated by electrical systems by means of the sample entropy algorithm. The electro-magnetic signal entropy reveals the presence of harmonics in the industrial and domestic electric power supply. It will show magnetic radiated wave measurements taken in the closest point to the transformer PCC, through a non-invasive and indirect measurement (using wireless magnetic sensor).