This post will no where near cover everything you need to know about choosing grow lights. This post will give you some very helpful hints and considerations to take into account before spending money on an expensive system.
-Number of chambers : Will you have all your plants in one space for their whole life time. If so you may want to consider a Metal halide Bulb and Ballast for the grow cycle and a high pressure sodium/metal halide conversion bulb for the bloom phase.
-Amount of Space : If your grow spaces are large you will need high powered lights so that all the plants get light. Keep in mind 400 watt and higher MH and HPS bulbs will benefit greatly from air cooled hoods which require strong inline fans to cool. The cost of these items may raise your initial setup fee but will pay off by the first harvest by the success of the crop yield and quality. Without an air cooled hood a 400 watt HPS bulb must be 3-4 feet away from the plants to keep temperatures down. With an air cooled hood some plants can be nearly touching the glass. If you have a high powered inline fan the plants can most likely grow up to the hood. This makes a big difference in yield because light is diminished at higher rates at farther distances.
-2′ x 2′ or 3’x3′ gardens can bloom under 400 watts
-3’x3′ or 4’x4′ gardens could easily use 600 watts
-4’x4′ or 5’x5′ gardens can use 1000 watts of light
-Number of plants: Depending on your style of growing you may want to get a set of 2 lights instead of one big light. 3 x 600 watt lights can outperform 2 x 1000 watt lights in the exact same conditions. The three 600 watt lights can cover a greater area without losing light from distance.
-Height of Grow Space: If your grow room or grow box is not very tall and you are also concerned with heat, you may want to use compact fluorescent tubes from the hydroponic shop. You can get high powered tubes for grow and bloom phases. The height of the ballasted light set is only a few inches so it can fit in short places. Its great for the grow phase or a mothering chamber. The bloom tubes will flower plants but sometimes they are lanky unless you use a product like BushMaster.