Although I'm still kind of a new poster, I keep seeing a lot of the same comments--many of which are answered an Dr. Khronik's excellent El Cheapo Guide to Lighting. But there are a few things I'd like to add in regard to watts, lumens, cfl, hps, and efficiency and how much light you need. Maybe this should be stickied?
Watts have nothing to do with light or growth. Watts measure how much power a light fixture uses to produce light. You can tell how efficient a light bulb is by looking at watts. A 23 watt CFL produces as much light as a 100 watt incandescent...even though the incandescent has more watts, it's useless. See what I mean?
What you care about is light...more specifically, light energy. That's what plants use during photosynthesis/growth. Light is measured in lumens. In my experience and reading, lumen amounts per sq. ft./sq. m. look like this
2000 lumens sq. ft./21500 lumens sq. m. = Absolute minimum for growth. You won't get much from this, especially after the plant has grown a bit. Not really enough to flower well.
3000 lumens sq. ft./32250 lumens sq. m. = Pretty Good growth. Enough light for the entire light cycle, although your yields may be lower.
4000 lumens sq. ft./43000 lumens sq. m. = Very good growth. Once you pass around 3500, growth rate and ability goes up fast.
Over 5000 lumens sq. ft./53750 lumens sq. m. = Optimal growth. Dense growth in all stages.
Keep in mind that using reflectors, using mylar or having flat white walls, and keeping your lights close to your plants keep you from wasting lumens. It's not just about having light, it's about getting the light to your plants. IMO, people ofter overbuy lights. This creates more light, but the light isn't always hitting the plants. And that creates more heat and ventilation issues, which causes stress problems.
That's why it's still impossible to tell anything about growth or yield based on just lumens. A guy that has an HPS that is too far away from plants that have no walls near them and no ventilation may get poorer results than a grower with CFLs that uses reflectors and has a couple of lights under the canopy in a well-ventilated spot.
HPS lights are often said to generate more heat than CFLs. That's not really true...it's just that they are more efficient at producing light, and there's a smaller surface area on the bulb itself for the resulting heat to dissipate. That means more ventilation. But the higher amount of lumens per watt means you use less power and get greater light penetration through your canopy. Still, I'm a believer that well used CFL's can give you great grows with less ventilation and heat issues. If you're in a small to very small area (less than 4 sq. ft./.25 sq. m.), I'd consider the advantages of CFLs in that way.
But HPS is more efficient. A typical 250 watt HPS bulb/unit will produce about 27,000 lumens. I've seen people use a 250w in a 3' x 3' room and get good results. That's 9 sq. ft. which = 3000 lumens a sq. ft. (Really, a 250w HPS is better in a smaller area.) to give you an idea of the difference in efficiency of CFL vs. HPS, think of this.
23w CFL = 1600 lumens = 69.6 lumens/watt
30w CFL = 2000 lumens = 66.7 lumens/watt
40w CFL = 2600 lumens = 66.3 lumens/watt
150w HPS = 14000 lumens = 93.3 lumens/watt
250w HPS = 28000 lumens = 112 lumens/watt
400w HPS = 50000 lumens = 125 lumens/watt
600w HPS = 90000 lumens = 150 lumens/watt
So you can see that HPS is more efficient than CFL...and as you get into bigger HPS bulbs, it becomes a lot more efficient. There's also fewer hassles with multiple cords and saved money on your energy bill. If you've got a big area and/or you can deal with the heat and ventilation, HPS is the way to go in flowering. Still, I'm a believer in small HPS lights and combo HPS/CFL grows...if you've got a 2' x 2' room, you can use a 150w HPS and 4 23w CFLs from Wal-Mart and get a terrific grow with very few heat issues.
Hope this helps some people. And, yes, I wrote it all.