Wow guys,
Alot of arguing about this.
50w/sq.ft=best yeild
Thus a 1kw is perfect for a 5x4 tray but will work for as much as 5x5 or 6x4.
600w is perfect for a 3x4 area but will work well for a 4x4 tray.
On a light rail you have to figure another formula.......par watts/hour or something like that (Al has mentioned it in his threads in the past) He is not a big fan of rails. I use rails with my 1kw lights but I do not make it travel the length of the rail just the center few feet to offer a bit more light on the ends of my trays as well as casting light via different angles to the very bottom of my clones in flower.
The true test of a grow is yeild....what you are shooting for, and what is reasonable from a well run op is 1 gram per watt. That being said i believe with 1200 stationary watts I would yeild around three pounds (1350+,- g) if I harvested a SoG from under both lights at once-or 1/4 of that harvest every 2 weeks. With my 1000 watt lights on movers I am getting about the same-1.25-1.33grams per watt.
So ultimately it comes down to the fact that 1200 watts (2 lights) should always yeild more than 1000 because harvests are based on grams per watt. Mo watts mo weight. Just don't exceed 50w/sq.ft or you will be wasting electricity.