How much light is needed for growing?
The answer depends on if you are growing small plants, in a small space, or large plants, and have a larger space to grow in. I'll try to answer this "in general" instead of being specific to one size plant.
Light seen and perceived with the human eye is measured in Lumens. There is an ideal amount of lumens for growing and a minimum amount of lumens. The very minimum amount of light required for smaller sized plants grown in the SH System is around 3000 lumens per square foot. Let me put emphasis on "minimum amount" of light. However, that's not 100% exactly accurate, since although you may have a 10,000 lumen light, the amount of light that reaches the plant varies with the distance between the light and plants, and the reflectivity of the grow area. The ideal amount is somewhere around 7000-10,000 lumens per square foot for average sized plants. As long as the plants do not show burn, as much light can be used as you want to use. (Note, the sun produces about 10,000 lumens per square foot, on a sunny mid summer day).
Determining lumens for your grow area:
First determine the square footage of your area (example in a 4 foot by 4 foot area, there is 16 square feet) If you have a 1000 Watt High Pressure Sodium Light Bulb, that produces approximately 107,000 lumens. Divide this by 16 (your square footage) 107,000 divided by 16 = 6687 lumens per square foot. So just divide the total amount of Lumens, by the total amount of square feet, and that's your lumens per square foot.
How far away from my plants do the lights go?
The lights in your grow room should be as close as possible to the plants without burning them. There is no such thing as too much light, unless there is overly sufficient heat to dry out and burn the leaves. A good rule is to put your hand under the light, if its too hot for your hand, chances are that the plants will be too hot too, so move the light up until your hand feels more comfortable. For seedlings or sprouts, I keep them a little further away from the light, because they are very susceptible to burning and drying out, at these young stages.
How do I decide which lights to use?
Efficiency is very important when choosing a type of light. The wattage is not the most important thing, different types of light produce different amounts of lumens per watt. For example, a 300 watt incandescent will produce about 5100 lumens. (not that you can grow with incandescent bulbs) While a 300 watt Metal Halide (just an example, they do not come in 300 watts), will produce 27,000 lumens. Obviously far more efficient for growing, while still using the same amount of electricity.
Approximate light production:
Incandescents: 17 lumens/watt
Mercury vapor: 45-50 lumens/watt
Fluorescents: 60-70 lumens/watt
Metal halide: 90 lumens/watt
High pressure sodium: 107 lumens/watt