Is there a rule of thumb on wattage per plant?

Well its a minimum of 50-100 watt per plant but if you want a bigger yield the more lights the merrier. =)

 

i've heard 100w per plant but it all depends on how any plants you are growing in what environment. if you have 16 clones in two square feet you certainly don't need 1,600w.

you could probably do 6 smaller plants under a 400w. do you mean 3' wide 4' deep or what?

 

I would try and get 3000-5000 lumens per plant (3000 at a minimum I would personally go with more), so if you get enough light to have that much lumen output per square foot and then figure out how many plants can be fit in that area (With reasonable wiggle room of course). For an example, I believe the 26 watt cfl lights give off 1850 lumens right off the bulb, but drop off a lot shorter than HID lights, like 6 inches.

 

There is a rule of thumb, but it has been bastardized and misquoted so much that there is a lot of misinformation out there.

It depends what type of light you are using -- this is critical, yet many quote a rule of thumb without regard for type of lighting.

With HID (MH and HPS) the rule of thumb is 50-100 watts per plant. Then, CFLs came along, and they are only about half as efficient as HIDs at converting electrical consumption into light. Since light, not watts, is what the plant needs, this difference in efficiency makes all the difference. Also, CFLs are advertised according to something called "equivalent" watts, which confuses the hell out of some people. With CFLs you want a bare minimum of 100 actual watts per plant, completely ignore the "equivalent" watts.

 

sounds like a nice little spot. 6 should fit in there fine so long as you flower them around 8" or just under a foot.