Lumens are the measurement of how humans see light. Plants don't see lumens the same way people do since they don't absorb as much green light as our eyes see. So watts is a perfectly reasonable measurement in my eyes. 1000w of LED won't even compare to 600w of HPS in terms of lumens, because LEDs don't use the spectrums people see the best. Which is how HPS are rated. Watts is a measure of how much energy you're turning into light. That's why I decided to ask 120w for 1 plant instead of..
Will 10000 lumens be enough for 1 plant? There are many more factors that come into play. Such as side lighting, total surface area, etc etc. So I stand justified in my choice to measure in watts and not lumens.
Don't mean to come off as an asshat, just trying to explain how I see it
*edit* Did a little more digging to show what I mean. In the attached pictures, you can see that the human eye picks up most of 550nm wavelength. This color happens to be green, the color which plants don't absorb as much of. It's a common misconception that they absorb no green light. But they do actually, in very minute amounts. You can also see in the human sensitivity graph, once you get lower than 400nm and above 600nm it drops off substantially. So what exactly does this mean? It means that once you get closer to IR(far red, how snakes are able to sense heat in prey) and UV(deep purple) spectrums, the human eye doesn't pic up most of those colors. Now if you look at the plant sensitivity graph, you can see that at 600-650nm and 400-450nm are most effective. You can google absorption spectrums for chlorophyll A and B and find out that these numbers are relavitely close.
So how do lumens count in there? Well like I said before, lumens are a measure of light the way HUMANS perceive it. So theoretically, anything that emits green light will be considerably brighter than something that is blue or red. If you look at the HPS graph, you can see the most noticeable spike is near 550nm. That's why HPS lumens are sky high. Because they appear really bright to the human eye. Granted HPS will always be intense for plants too. But if you look at that graph you'll see only a small portion of the spectrum given off by HPS is between 600-650 nm. That's the part that the plant is absorbing the MOST. Doesn't mean the other wavelengths aren't used, it's just that they aren't as important.
This is why I prefer LED over HPS. It's more efficient in terms of usable wavelengths. And HPS create wayy too much heat for me to properly vent. CFLs have the advantage of giving off minor heat and the proper spectrums for veg/flowering. I hope this helps you to understand my reasoning for using watts over lumens.