If you can get roughly 50 watts of input power per square foot of growing space, you can use that to get a large crop with a lot of secondary metabolites.
Conventional wisdom is 30 watts/sq ft or 40 watts. That can get you a large crop but it makes is hard to get high levels of secondary metabolites.
The reason?
You can reduce the hang height of say, a 30 watt light so that you're getting 1000µmol on your canopy. The problem wit that is that you're going to have to drop it to roughly 10-12" and, when you get a light that close, you're heating the flower tops to the point where the levels of THC and terpenes will drop.
If you check out my older grow journals*, you'll see that I was getting large yields, some as high as almost 900gm/sq meter. Huge plants with lot of flower. That wasn't because I have a green thumb-until I started growing cannabis, the only thing I grew was old. A big part of getting those yields is that I grow in hydro (I suspect hydro makes is easier to get better growth than soil but I don't have any data on it) and the other is because I give my plants "lotsa light". My approach is to get my plants to their light saturation point as fast as possible and keep it there for the duration of the grow. That approach is based on my reading "the research", primarily the published results of Bruce Bugbee and his students, but a number of other sources thrown into the mix.
What the research shows is that crop yield and crop quality (the ration of flower to above ground mass), tends to increase as light levels increase. But I've suspected that I wasn't getting high levels of secondary metabolites and, it was only a few months ago that I learned the reason why that's probably the case.
Plant metabolism increases as ambient temperatures increase up to about 85°F. After that, net photosynthesis starts to suffer (Chandra). With that in mind, my practice has been to keep my grows between 80 and 85°F. A few years ago, Mitch Westmoreland, then a student under Bugbee, released a video about "hemp" growing and he made an interesting statement that, when temps exceeded 78°, secondary metabolites suffered. I watched the video, took notes, and but didn't heed the comment about higher temps. It was the only mention of that type that I could find and, heh, I was getting pounds of weed so I blew it off.
Early this year, Bugbee was interviewed by Shane at Migro (check for the You Tube video) and he threw out a teaser light about temperature being a huge factor in growing cannabis. Shane took the bate but Bugbee just said that "more information would be coming out soon", or words to that effect. I thought he meant that he had a paper coming out but it was, in fact, Mitch Westmoreland sharing information from the research he's done for his PhD thesis.
Earlier this year, Westmoreland released two YT videos on four topics that are in his thesis. They're the best summary of information I've seen in the years (and many hundreds of hours) of learning about grow lighting. I strongly recommend that any grower who wants to up their game watch at least one of them.
The great reveal re. temperatures that secondary metabolite levels drop when the flower tops are > 78°F, as he said in his "hemp" video a few years ago but he lays is out in some detail.
OK, great info from the pointy-headed guy but what does that have to do with 30 watts vs 50 watts per square foot.
Input wattage is only a general guideline but what counts is µmol hitting the canopy. Assume that 100 watt light isn't going to put out as much light as a 200 watt light. That's not a big stretch to accept that postulate. Both will get, say, 1000µmol on your grow. However, the 100 watt light will have to be at 12" whereas the 200 watt light can be at, say 18". (These numbers hold true for lights in my 2' x 4' tent - I'm using them to illustrate the concept.)
The problem is that the 100 watt light at 12" will tend to heat your canopy more than the 200 watt light because of the low hang height. The trick is to get the light away from the canopy so as to reduce the temperature of the flowers. You can only do that with a bigger light.
If you look at my grow journals, you'll see that I use a 330 watt light. It was an expensive light and 330 watts in a 2' x 4' tent is just over 40 watts/sq ft. That's on the high side for conventional wisdom but I've found that I can't get 1kµmol on my canopy and keep my plants at the <=78° level.
The issue is to reduce the temperature at the canopy. Until a few weeks ago, my only option was to go to a more powerful light and I've decided on a 430 watt light. That light will generate well over 1kµmol at 12" which means that I can raise the hang height to about 18" while getting 1100±µmol on the canopy while keeping the canopy temperature down.
If you're interesting in maximum yield as well as retaining secondary metabolites, I'd go with the larger light.
As of a few weeks ago, there is another option. AC Infinity has released an air conditioner/environmental controller for grow tents. It's a superb idea because, in theory, it will keep temperature, RH, and/or VPD in range. It's a ≈ $700 device but it looks like it will give a grower excellent control over "the weather" in the tent so you are getting value for your $700.
Between those two choices, the 200 watt light is cheaper and, if you want to increase yield as well as get better quality cannabis, the 200 watt light is a very cost effective approach.
*My last two grows haven't had great outcomes - one was root rot of a really nice plant and my last grow was self-induced, total dumbshit mistake.
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