Because plants can be one inch to seven plus feet wide lets talk in terms of square feet. A son agro HPS is one of the most effiecent HID bulbs on the market producing 53,000 lumens at the core of the bulb. If we could take this core and make it into a big ball of dough then make a large pizza of sorts out of it not taking into account that lumens decrease per distance travelled like crazy. Are strange pizza core would cover at 10000 lumens per square foot 2.25 about square feet. You are all crazy. I can grow fine with using on average about a fraction of the lumens per square foot with use of a track. This is why production uses smaller plants with a thinner canopy. Yes 1500 is the recommended amount I would tell people to use. If any one can break down lumen decrease in regards to area and output feel free to tell this new cat why they should be wasting time and money on electricity, discouraging a motivated grower from not producing mother plants because they think it will cost them 3000 dollars a month to run a decent size grow. Neva my friend I can cover 100 square feet with 1000 watts using a track with no problems at all. thats 10 watts per sq foot. the average 1000 watt say has 113,000 lumens. If lumens did not decrease that would be 1130 lumens per square foot if you are skilled enough to make the magic pizza dough design, which I have only seen once high up in the Andes mountains, deep in a cave where the munchcin people live. Lets say because are lumens are travelling on average more 3 feet we can take that 1130 and make it 130 lumens per square foot. Some greedy growers who use chemicals for weight may not want competition, but I would like to see every street corner with a 4 foot mound of organic nuggets on it so I am going to accually teach you so you can grow.