I dont have the text book formula of expoential lost of lumens per foot of distance in front of me, but knowing their is a loss i offer these thoughts. The 1000w can cover a greater square area, but at the same time, due to heat it must be placed further from the plants than the 400w. Though the 400w covers less square area, it can be placed closer, thus loosing less of its original lumen output than the 1000w placed further away. I would say these these two factors average eachother out. I prefer multiple lower wattage lights placed closer than one high powered light. I myself use 3 600w HPS on light movers for an even closer distance to plants than recommended without the light mover(no hot spots). The 600w bulb is actually the most efficient bulb on the market in terms of lumens per watt. While the 1000w bulb produces 115,000 lumens within a 1' distance, the 600w produces 90,000 lumens within a 1' distance. If you do the math, that's 115 lumens/watt versus 150 lumens/watt respectively. In other words the 600w gives you more lumens for your kilowatt dollar.