Grams per watt

Yeah, but that is so easy? So it can't be that in 200W each of my plant will get idk 70-80g? It's possible to get for example 100g each X4 in 200W.?

That would be 4 * 20g = 80g and 100w : 80g = 0.8gpw

the example i used would be accurate for a high end HPS vs high end LED light, all other factors the same.

beside properly filling out a space, and not tying to cover too much area relative to light provided... it's about efficacy of your light. If you do things well, that's what it'll tell you the most about... how efficient your light is at producing umol/s of PAR. (photosynthetically active radiation) example.. a 200w light that produce 400umol/s vs a 140watt light that produces 400umol/s... both will cover and grow the same size plant. you will obviously get a much better ratio with the more efficient light. few lights are as low as 2umol/J efficacy as this example.. but the high-end lights will be 2.8-3.0umol/J... i used "2.8" to calculate roughly 140watts (142.9w actually) in any larger scale, this starts to save you a lot of money per month.. even if compared to mid-range stuff at 2.4-2.5umol/J efficacy. 12.5% less watts on the largest single chunk of wattage expenditure, i.e. lights. This will often cover the price difference between over the course of 2-3 years on your utility bill.. then for the net 2-3 years it's saving you money by comparison to that once cheaper light.