Quote Originally Posted by Zandor
The magic number is 50 watts per sqft
Disagree slightly. The number of watts per unit area will depend on the type of lighting, its efficiency and watts drawn per lamp. You want to shoot for about 10-15,000 lumens per square foot, which approximates typical direct summer sunlight. Lumens are more important than watts when it comes to grow lighting. Bear in mind that the lumes per watt figure for a given lamp will increase with higher wattages - in other words, the higher the wattage of a given lamp, the more efficient it generally is. I give typical lumens per watt figures for some common lighting types below, in order of efficiency:

Incandescent: 10-20 lum/w
Fluorescent: 35-100 lum/w
Metal halide: 80-120 lum/w
HPS: 90-150 lum/w
Beeblebrox.420 Reviewed by Beeblebrox.420 on . Square feet and Watts.............? What is the amount of watts you want to have per square foot? Example Question: Is one 400w HPS enough to grow two plants within 4 sq ft.? Rating: 5