Grow lights may be used to supplement natural light if there's not enough sun for your plants to thrive, as the sole source of lighting, or as an aid for starting seeds. Not all grow lights are created the same; the red and blue elements of the lighting spectrum are critical for compact foliage growth, while the orange-red portion of the spectrum simulates noontime sun and is most useful for encouraging flowering. Never use incandescent lights as grow lights; because they put off so much heat, they are likely to damage the plants.
Decide how big the area you're going to light is. This tells you how powerful a light you need; the bigger the area, the greater the wattage you'll need. As a general rule, a 250-watt grow bulb will light a 2-foot-by-2-foot area; 400-600 watt bulbs will light a 4-foot-by-4-foot area, and a 1,000 watt bulb will light an 8-foot-by-8-foot area.
Make sure you can hang your grow light from ceiling hooks or a suitable fixture above the surface you've placed your plants on. Also, use a flexible measuring tape to gauge the distance between the light fixture and the electrical plug-in. If the distance is longer than the cord on the light fixture, you'll need an indoor extension cord to help make the reach.
Hang your grow light based on how powerful it is. Less powerful High Intensity Discharge (HID) lights should be hung 2-3 feet above the tops of the plants, while more powerful systems should be hung four to six feet above the tops of the plants. Conventional fluorescent grow lights or shop lights and compact fluorescent bulbs should hang closer to the plant, one to four inches away, since they put off less heat but also less light.
Use a basic light timer, available at any home improvement store, to run your grow light on a regular schedule. Most plants will require about 12-16 hours of light per day. They also require at least 8 hours of darkness each day. Start by setting the timer for 12 hours of light and 12 hours of darkness and adjust the "light time" upwards if your plant does not appear to thrive.