Total Harmonic Distortion (THD)
Total harmonic distortion, or THD, is the measure of how much a ballast can distort the waveform of that building’s power supply and interfere with other equipment. Electronic ballasts today are rated at less than 20 percent, 15 percent, or less than 10 percent THD. The magnetic ballast is rated in the 20 to 28 percent range. The lower the THD the better and more efficient the ballast, and the less likely it is to interfere with other electronic equipment.