I may be able to help here with some real world numbers. I am just finishing running a SOG with a plant density of 3 plants per sq ft under 2 600 watt HPS lamps. I harvested one plant that was a runt today (one week early). The runt is about 1/3 - 1/2 the size of the rest of my plants.. Keep in mind, this is with 3 days veg after transplant and that is it.. Otherwise this is just straight 8-9 weeks flowering depending on strain. And all bud produced is top cola super large dense nugs... I just cut and pasted this from a thread I have on my grow.. Hope this helps..
So I harvested the one runt I had a week early (today). This plant was less than 1/2 the size of the rest easily.. I had to pull it out as it never kept up with the canopy.. So I kept and flowered it to finish.. So I harvested today.. It was one cola but it was so dense that I broke it up as I was afraid of mold. I couldn't get inside the buds and remove the stems of the fan leaves and I was afraid they would rot and be a start for mold. so I broke it down.. These buds are SO DENSE!! I weighted this out wet and it was 52 grams.. So I am expecting just over 1/2 ounce. If that holds true and I estimate conservative side I get.. 30 plants that are at least twice this size in an area of 9 sq ft under 1200 watts.. So 30 plants at 1 oz per plant (twice the small one I just harvested and this is extremely conservative), So that gives me 30 oz's plus this 1/2 so 30.5 ounces 869.25 grams, so I get .724 grams per watt. Now this is an extremely conservative estimate. But even then, this is all top cola bud!! I think a more accurate estimate will be using 1.3 oz's per plant which will give me: 1126.5 grams or .939 grams per watt. So if this is what I wind up with next week I will be pretty pleased. I want to run another crop under the same conditions and this time give each plant 1 sq foot and allow for a bit of branching and compare the harvest to see which SOG method is more efficient and which one produces better overall quality.. I am thinking that this method with tighter plant spacing does as all the buds are simply top center colas that are the most dense swollen nugs you have ever seen.. I am pretty happy with this as this plant I just harvested was not even 1/2 the plant of the rest of the crop..