Pretty much goes by lumens/sq ft (or better yet PAR). Depending of the efficiency of your 68 watt cfl
(which I'm guessing typically is 60-85 lumens/watt) you are getting about 4,080-5,440 lumens.
That's just about enough for 1 sq ft coverage per light. 3,000 lumens/sq ft is minimum, 5,000/sq ft is a
good number to strive for. High output would be considered 7,500-10,000 lumens/sq ft. If you can reach
the magical 1 gram/watt mark with your lights it'd be about 68 grams or 2.4 zips total yield. More plants
will give less zips/plant basically or vice versa.
I'd ignore the "equivalent" factor as that's compared to an incandescent bulb - iow marketing hype.
What I'm looking for are bare bulb T5 fixtures (somewhat more efficient at 92.5 lumens/watt) that I can
hang vertically. If I can't find one maybe the bare bulb cfls can compete w/the T5s. Happy growing!