Quote Originally Posted by Roughrider
No. 50w/sq. ft. is not ā??in the ballparkā? for HID. Take a 400w HPS light that puts out 50,000 to 58,000 lumens. That will cover a 10 to 12 sq. ft. area very well. That's 33 to 40 watts per sq. ft. ... and that's giving you a very good grow. That amount drops as you get bigger bulbs...when you're at 600w (80,000-95,000 lumens) for instance, it's down to 31 to 36 lumens per sq. ft. So you can easily get by with 20-30% less than 50 lumens per sq. ft. with some HPS lights.

CFLs, on the other hand, almost uniformly put out 65 to 70 lumens per watt. They don't become more efficient at larger sizes like CFLs do. A 600w CFL is twice as efficient as a CFL, so using watts means you'll be off by 100%. That's a lot. 35w per sq. ft. will give you a terrific grow with a 600w HPS. To get comparable lumens, you'd need 70 watts of CFLs per sq. ft. (And, honestly, lumens are more available on CFLs than kelvins...because CFLS like to note incandescent equivalents, they usually put lumens on their packaging)

So, again, using watts is a very, very poor way of judging how much light you need. You can do VERY well with 40w sq. ft. with a 400w or larger HPS and have fewer accompanying heat and ventilation issues. On the other hand, 50W per sq. ft for a CFL is 3500 lumens per sq. ft.; very mediocre. You need to bump the amount of lumens by 40% to be in the very solid area. Different lights = different efficiencies. Don't use watts.
Very interesting, mostly true, but irrelevant.
I personally know dozens of patients right here in Oregon who are growing fine medicine right now using >50W/sq.Ft.