One Kilowatt is not a 1000th of a watt but 1Kw=1000 watt. 1 Kw is also what you call 1 unit in the US and at most other parts of the world.
If knowing how much watt a bulb consumes (in Kw/h) and approximately how much lumens it produces, you can in fact calculate both:
1. consumption per hour, day, month etc. and
2. if knowing the surface you need to cover, how many watt per lumens per square ft a specific setup consumes and costs. Yes you can, but you need to know how.
Example with
metal halide = 70-115 lumens per/watt and assuming that 5000 lumens per/sq ft are appropriate and/or sufficient for the purpose.
Let's assume that one 150 watt Metal Halide bulb would produce (for simplicity as an average from 70-115) 100 lumens per watt and thus producing 15000 lumens. Hence be sufficient for a surface of 3 sq ft. And to cover a surface of 21 sq', you'd need 7
bulbs of 150 Watt, which corresponds to 1050 watt and brings the total consumption of these 7 bulbs close enough to 1 Kw/h.
Making things as even as we can for the sake of simplicity while still keeping the maths accurate enough for the purpose, we can safely say that with 7 MH bulbs, we cover 20 sq ft, produce close to 100.000 lumens and are consuming 1 Kw/h.
Now we can easily and actually put watt in direct relation with lumens or square foot and vis versa and make our own equation for future projections and estimates.
Of course that other premisses and/or choices (of bulbs and lumens per sq ft needed) would also need a different "mathematical spadework", - and most probably not end up in such even numbers! But any 5th grader should be able to take over from here and transfer the described method to another model... without a doubt and without arguing, - right?
