GPW doesn't mean much IMHO, Gram/Watt/Day is the real way to calculate it.
A poster named pH had a website discussing this and had a calculator to this automatically.
I remember Lucas (yes the guy who is resposible for the formula who is a member here) saying that pH was a friend or maybe mentor.
It makes sense really because if growing a sativa over 90 days you can't compare it with a 60 day indica.
This system says more about the room efficienty also.
Say 360 grow days a year, the 60 day variety get 6 cycles a year, the 90 day strain only pumps out 4.
Both may have hit 1GPW but over the course of a year the 60 days crop will have beaten anual production of the first by 33% (2 divided by 6).
Now pH also factors in room size or watts per square foot, I am not sure how that factors in calculation wise but i makes sense.
I use a 400W in a 4 square foot room, technically it is good for up to 16 square feet, there is no way the 2 situations compare.
Also when you factor things in like the fact that letting buds rippen beyond their prime to gain extra weight it can sway things further, who is the better grower?
The one who got the weight or the one who produced a better quality product?
Food for though.
the square foot of the area gives you a variable to measure lite intensity.so many lumens per square foot.... Gots to quantifythat's a good point...
gram/watt/day is the true way to measure it
For example: 1 gram per watt x2000w /56 days = 0.01785 gram/watt/day
1 gram per watt x2000w /70 days = 0.01428 gram/watt/day
the less time it takes, the more efficient.
I think room size is completlely irrelevant... this isn't a measure of grams per sq ft it's grams per watt of electricity.
He got 1.3 GPW...
https://www.icmag.com/ic/showthread.php?t=185081&page=10
That's the most efficient grow I've ever seen...I doubt anyone is pulling more than 1.5gpw...Heath robinson claims 2gpw but I kinda have a hard time believing that...
I've been growing for 4 years...I still haven't hit GPW yet, hoping that changes with my new vertical room though...the closest I've come so far was 19oz under a 600w with 16 plants (0.88 GPW)
I agree with the size therory.The more headroom and area in which to streach out and get a better air exchange etc......Butidk, im a big fan of G/sqr ft.....because no mater how much light you have you are always limited by space....G/w is an indication of how efficient you are with your lighting and electricity.
put it this way, if i have a 10x10 room and use 2kw and fill the room wall to wall with plants. i might get 4-5 lbs....
but if i have a 4x8 room and use 2kw and fill it from wall to wall, i might get 2-3 lbs.....
granted that the buds in the larger room may be not as nice or compact, but the overall yield will be much greater than the smaller room.
i think in the end you have to use both G/sqr ft and G/W to get the optimal efficiency level