A good grow light is one that provides the correct amount and type of light for the plant being grown as well as its stage of development. Plants of course use light as an energy source, which it converts into chemical energy by means of photosynthesis. The amount that a plant grows and provides is directly dependent on the amount of light available to it.

Because light is so important to plant health (upon which everything else depends!), the purchase of grow lights requires a bit of thought and consideration. Fluorescent grow lights are very efficient and cool compared with incandescent bulbs, which are not a particularly good choice because of the heat generated. Fluorescent lights provide full spectrum light, which many indoor gardeners think to be a good thing. However, the fact is that plants do not use the full spectrum at all times.

Metal halide bulbs are a type of grow light that is useful for a plant during its growth stage - when the stem, branches and leaves are developing. During this stage, the plant uses primarily blue spectrum light. On the other hand, a plant which is producing flowers and/or fruit needs light on the red end of the spectrum. High pressure sodium, or HPS are generally a better choice when the plant reaches this stage.

The problem with both MH and LPS lighting is that both produce considerable heat, requiring the use of fans to keep the plant and/or the soil from overheating.

Light-emitting diodes, or LEDs, have been around since the early 1920s, but have only recently been used in hydroponic gardening as grow lights. This type of grow light has many advantages over fluorescent and metal vapor lights, and no real downsides other than the initial expense. While they do cost more in the beginning, LEDs are considerably cheaper in the long run because of their long life span (average life is about 100,000 hours, compared to an average 15,000 hours for other types) and energy efficiency (energy consumption is about 10 to 20% of comparable fluorescents and metal vapor lamps). In addition, LEDs are cooler, thus eliminating the need for fans and elaborate ventilation systems.

Regardless of the type of grow light used, certain accessories may be necessary in order to maximize plant growth and production. For example, a reflector increases light intensity, maximizing available light while at the same time eliminating any "hot spots."

Ballasts are devices that provide power to the grow lights. A fluorescent light fixture has a built-in ballast, but other types - including metal vapor and LED types - require an external, or "remote" ballast. Again, its worthwhile to spend some extra money here; new digital ballasts produced domestically are superior to imports from China or old-fashioned magnetic coil types. Although they are more expensive, they will extend bulb life. Better quality digital ballasts also have numerous built-in safety features as well, such as the ability to detect short circuits and defective bulbs.