Your making me wonder if you've ever grown anything under artificial lighting.
The lumens output you speak of is at the bulb, not at the plant,
I actually thought this was understood. I am not disagreeing with your stated lumens,
a 1000 watt = 150,000 lumens.....at the bulb, agreed?
a 400 watt = 55,000 lumens.....at the bulb, agreed?
As you move further & further away from the light source, you loose lumens.
Again....
the "usual safe distance" for a 400 watt is 12"
the "usual safe distance" for a 1000 watt is about 24"
a 1000 watt light emits 11k lumens at about 24 inches
a 400 watt emits almost 17k lumens at one foot
17k x 2 400 watt lights= 34000 lumens
Again....you'd need 3 1000 watt lights to equal 2 400 watt lights.
Silly, silly, silly.
Peace
i agree with you to a point. 2 - 400watt lamps will give better light coverage than a 1000watt lamp + they can be hung much lower than a 1000watt lamp. the light from a lamp 1foot from the bulb will only be 1/2 as bright as it is at the source. light loses intensity very fast the farther away it is. ( see pic at the bottom)
recommended distance a 1000watt lamp should be hung from the top of the plants is around 2 foot, a 10000watt lamp at 2 foot will only shine 1/4th of the light from the source ie bulb.
so a 1000watt lamp that is 150,000 lumen is only shining 37500 lumens
at 2 foot away from the bulb
the recommended distance a 400watt lamp should be hung from the top of the plants is around 1 foot for a 400watt lamp
1 foot will shine 1/2 the amount of lumens of light from the bulb
so a 400watt lamp that is 50,0000 lumen is only shining 25,000 lumens at 1 foot away from the bulb.
now for the good part. you have 2 - 400 watt lamps that are in the same size room hung at 1/2 the distance of the 1000watt lamp, not only is it lower, the coverage of the plant canopy is much better than what the 1000watt will cover.
remember that light losses intensity from the bulb
the plants on the out side of the grow room under a 1000watt will only be getting (4x4 grow room) 1/16th of light or 9,375 lumens.
where as 2 - 400watt lamps in the same space will be hung over 2x2 space each. lets use the 25,000 lumen only from just the 1 - 400watt lamp the lumens will be more than 25,000 if you use the 2 - 400s it will not be double ,but it will be more than 25,000 lumens,
1 - 400watt lamp over 2x2 foot of room will shine 25,000 lumen at one foot above the plant canopy and the plants on the out side of the canopy will be getting light 2 foot from the bulb because the 400watt is hung at only 1 foot above, the plants on the end will be getting 1/2 of the light from the original 25,000 lumens 25,000 divided 2 = 12,500 lumens
even though the 400's look the better option on paper it does not always work like that, the penetration under a 1000 watt bulb can grow heavier buds under the canopy than the 400s its that extra 12,500 lumens under the bulb and that massive bud production which makes up for the better coverage that the 2-400s offer.
the 1000 watt is the better option because after 1 crop the light of both bulbs diminish and after a few crops the 400s will not be very bright where as the 1000 will still grow fat ass buds.
over the long run its more economical to replace 1 bulb than it is to replace 2.
i used to use 5-400's but now i only use 2-600s and 1-1000 in the 2nd room the 2 600s do not yield much more than the 1000watt on its own.