Trying to talk people out of using grams/watt, or even getting them to understand why it isnt valid, isnt easy. I know. Ive been trying to do that for a while now with little to no success. Its been in use so long, everyone just assumes is has to be good. Id bet at this stage maybe 90% of the growers on this board learned about grams/watt on their first grow. Plus, its in a video so it must be true!
Here is a different way to look at it that may be easier to understand why trying to rate grow yields based on light fixtures or watts isnt valid.
I have a single 320 watt fixture in a 7.3 sq ft tent. Lets start with one of my smaller yields on my most recent grow - 1156 grams. Just to make the math easier, lets call it 300 watts, and 7 sq ft and 1000 grams.
Now imagine I make that single fixture twice as wide and with two times the elements, so its now a 600 watt single fixture. But its twice as wide so its now covering 14 sq ft. Is there any reason not to expect to get 2000 grams now?
Grow #1 has a single fixture and gets 1kg.
Grow #2 has a single fixture and gets 2 kg.
There is no reason why this cant keep going until you have a single 3000 watt fixture over a large area yielding 10,000 gms = 10kg on a SINGLE fixture.
You can ignore the rest of this post unless you really want to hear why Gm/Watt is also meaningless number just like gms/fixture 
Grams per watt is almost the same thing except that WATTS is the fallacy instead of the number of fixtures.
Take that C99 grow I was talking about earlier. I got around 2 kg off that single 320 watt fixture. Thats a hard to believe, whopping, amazing 6.25 grams/watt!!! How you ask??? āTimeā is the answer.
Iāll let you do the math on the over 3kg yield I had from the same 320 watts.
That grow was over 5 months long. I spent much longer vegging that plant than any other I have done so far. That allowed me to grow a tree of a single plant. Combine that with a lot of scrog training, and aeroponics and there you go.
My point is, grams/watt is meaningless unless you include the length of the grow AND either the number of plants or the sq ft of the space. And, watts is still almost completely irrelevant.
A few watts one way or the other has virtually no effect on yield. Plus, how do you measure it?
For example, my fixture is rated at 320 watts, but I almost never turn it up to 320. During the early growth, the light is turned down under 100 watts. I go to maybe 200-220 in veg, then up to a max of maybe 300 in flower. This last grow I never got over about 280.
Lately, I have been lowering the lights rather then increasing the watts. It saves on heat in the tent, which makes it easier to cool.
So would you use 300 for my watts/gram or the lowest number or some average? Most people use the rated value I guess, but do they actually use it at that value?
Plus, because Im running LEDās at well below their max rating, and they are running relatively cold, I can get my ā300 wattā fixture much closer to the plants than someone running an MH bulb for example. That means I get a lot more photons on the plant for the same number of āwattsā.
For my own personal records, I use grams/day. I cant use that to compare to anyone elses grow, but it works for me to track how well Im doing from grow to grow.
We need to just leave light - watts and fixtures - out of it all together 
EDIT:
Sorry man. I am cluttering up your thread. I will gladly remove these posts if you want. I have super thread powers <<<<<<(insert evil laugh here)
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