I haven't use COB LED, but I consistently got 0.8 to 1.2g per watt with low- to mid-grade LED (blurple-color UFO; white-light LED fixtures; ordinary household lightbulbs with the plastic diffusion dome cut off[1]).
For fun, I'll show six photos of my household LED lightbulb grow.
1. This photo shows a fixture I made (I documented how to make it. I could post a how-to). It's holding five Cree BR35 PAR38 9.5w floodlights (the center might be a spotlight). I don't think Cree makes this anymore. I haven't looked at their new BR30 or 40 model.
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The tent legs have LED lightbulbs attached too. (Something like Phillips 9w [60w equiv] in clamp-on reflectors from Home Depot. But, the clamp-on device is modified to attach directly to the tent leg, and can slide up/down the tent leg. I've got that documented too. I can post info showing how to do it. Again, the plastic diffusion globe is cut off[1].). The far corner pole has a PAR38 attached, like the top "fixture."
2. Same fixture, but with with BR35 PAR38
spotlights (or, at least illustrating how the fixture could do top-down lighting with greater distance). The blurple light was from Gotham Hydro. They're out of business now. I think it was 135w.
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So, that's six 9.5w floods (maybe two are spots) and two 9w globeless[1] household lightbulbs (Cree or Philips). It's in a 4x4 tent. Assuming that plant is in a 2x2 space, that's 18.75w/sq ft. The 2x2 space to the left of the plant was open. I had another couple floods attached to a tripod there. Later, I attached 2-3 more 9w globeless lightbulbs in reflectors to the tent legs. I ended up running 28-32w/sq ft.
3. This was the plant before harvest:
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4. The following was the largest bud (with a Caliber IV humidity meter, for scale):
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5 & 6. After dried and trimmed:
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I don't recall the total dried weight. But, it was from a 2x2' space. I typically got this kind of result from LED.
Caveats:
1. My impression is that grow style/skill has more to do with yield volume than the light does. I think there are many factors like soil (dense, slow drying vs light, fast drying -- I use the later), imbalanced nutrient ratios (I manage my ratios using a spreadsheet. I don't follow multi-bottle "lineup" schedules).
2. I think gram-per-watt has limitations as a measurement. It implies efficiency, but doesn't take into account the horizontal & vertical space you grow into. I think knowing the w/sq ft contributes to the meaning of g/w. (Maybe even w/cu ft would add more meaning if you're bathing the plant from all sides.). E.g., let's say you live in Tokyo where real estate is a premium (and, for the sake of argument: energy is cheap). You might prefer g/sq ft efficiency without regard for g/w. efficiency.
For example, the guy using 80w/sq ft of Mars LED (below). He might have got 1.5g/w (I don't know). That would be impressive, of course. But, if you can get 1g/w at 40w/sq ft, that would reflect upon what it costs to get 50% more g/w.
3. As you may notice in the photos above, the buds weren't very frosty, not sticky (felt dry like tobacco, not resiny. Not very strong aroma either.).
I have used a few different LED fixtures (expensive, cheap). They all produced this difference in finish quality (compared to T5HO at 40-45w/sq ft, or CMH at 38w/sq ft counting the ballast). I think that has something to do with UV (LEDs don't produce it). You can add reptile CFL bulbs to add UV to your LED grow. But.. it's a hassle. I never knew how long to run them. Or, their lumen/uv deprecation (when they should be replaced).
4. The LED market is
very treacherous compared to traditional/comoditized/componetized lighting. If you live in a hot climate and truly need cooler lighting, you can buy expensive lights that run at 20-25w/sq ft.
But, most of what's sold out there needs to be run at 40w/sq ft. There's no reason to buy it (instead of UV-producing T5HO and/or CMH).
Worse, those mediocre fixtures are often sold as expensive lights. Expense and polished web sites don't mean it's good. Threfore, It's hard to know what's worth the money because there's so little objective info out there. Secret-sauce spectrums, fudged PAR readings, overstated watt/coverage areas.
Also: brands tend to create a religious/political following. Just because someone gushes about a brand means *nothing*. I remember when TopLED/Mars was on 420mag. People posted photos of airy buds, thrilled with what they were doing. (If anyone suggested Mars was hyping the watts, and the grower should use stronger light... they risked being banned. Or, if Mars owners used more watts [and got better results], people would be banned for asking why LED was being used if it had to be run at that w/sq ft. I.e., why not stick with traditional lighting?).
That's a tough topic because growing is fun. If someone's having fun with low- to mid-grade LED... (or a predatory expensive brand) why rain on their parade?
But, when someone's making a decision about buying LED... presumably they want objective criteria, not cheerleading. I found that to be a *very* unpleasant realm of LED. It turned into a focus of making LED work. Not necessarily *why*. (For example, there was a guy finishing an autoflower under Mars. The grow journal was specifically about demonstrating a Mars grow. The buds were *amazing*. People were drooling over it. I'm sure people bought Mars fixtures because of what they saw. But, nobody mentioned that he was running 80w/sq ft. That's more than HPS!).
So, I'm just saying. Be skeptical. There really *is* a lot of flim-flam out there. And, many testimonials are more emotionally-driven than objective. For example, the guy growing autoflowers *loved* his light. But, he never mentioned
how much that love cost him (in energy, heat). If you saw the buds and the professed pleasure... you'd be immediately sold. But, there was more to the story. (I think it's normal human psychology called "investment factor." When you sink money into something, you need it to be a bargain, perfect, the best, etc. You need others to validate your choice and results. But, it seemed more toxic with LEDs. More us vs. them. More defensive. Impervious to objectivity. The lack of demand for better seemed to create a race to the bottom for the flim-flam sellers.).
[1] You must use a GFCI outlet to protect against electrocution. Open bulbs expose line voltage surfaces.
