Fluorescent Grow Lights and Compact Fluorescent Grow Lights use substantially less energy than traditional MH and HPS grow lights. T5 High Ouput (HO) bulbs are the most popular fluorescent bulbs for plant lighting, and they're the most efficient. T5 fluorescents are more efficient than T8 and T12 fluorescent tubes, and they are smaller in diameter. In fact, "T" refers to the diameter of the bulb. T12 has a diameter of 12/8", T8 = 8/8", T5 = 5/8". Typically, the more narrow the lamp, the more efficient it will be.