For most purposes I agree with you! However, in this case, the alternatives I am considering are:
1. Build a 10000 m² greenhouse full of, say, lettuce, perhaps on multiple shelves ("vertical farming").
2. Build a 10000 m² solar park full of solar cells, then use the energy produced by the solar cells to illuminate lettuce being grown inside an opaque concrete box, of some arbitrarily variable size.
In this comparison, the efficiency of solar cells and of LEDs matters very much indeed! Because of their inefficiency, you get 30 times as much lettuce in case #1. That's the reason I think this scheme is uneconomic except in unusual cases. In another part of the comment thread, I agreed that abundant wind energy is a case where it might make sense.
It's true that there are solar cells in commercial production that are 30+% efficient instead of 16%. However, those are specialty solar cells designed for use in things like spacecraft (we used them on our satellites at Satellogic, for example.) Consequently, they are eye-wateringly expensive and not getting cheaper, and so nobody is building solar parks with them, particularly since non-arable land is abundant and will remain so for a couple of decades, while the prices of low-cost 16%-efficient solar cells are dropping like a hafnium pellet.
> The whole point of LEDs is that they are supposedly very efficient; around 40-50%.
No, LEDs are nowhere close to 50% efficient. LEDs have many wonderful attributes, including tunable color spectra, directionality, the possibility of being scaled down to submillimeter scales (hard to do with an incandescent bulb!) and, indeed, very good efficiency --- compared to other light sources, that is. The problem is, all light sources are shitty when it comes to efficiency; LEDs are just less shitty. That's why we charge our cellphones wirelessly with induction coils, not LEDs and expensive multijunction photovoltaic cells.
It's hard to get your hands on good efficiency numbers, because LED vendors don't quote any kind of absolute energy efficiency number in the datasheets, because they only publish luminous efficacy (because that's what people normally care about). In theory, we can derive the absolute energy efficiency using a luminous-efficiency curve: https://en.wikipedia.org/wiki/Luminosity_function. I'll see if I can do that in a separate comment.
> heat is not actually energy loss in a vertical farm.
This argument turns out to be wrong; I've explained why in detail in https://news.ycombinator.com/item?id=20906210, but in brief, 87% of the energy we're talking about gets lost in the solar park, not the hothouse, and artificial illumination to crop-growing levels produces so much heat that you need to air-condition the hothouse rather than heating it. Moreover, produced heat is always energy loss, because you can always reduce it further even by adequate insulation.
> Meaning that the reason vertical farming is getting a lot of attention is that the cost of energy has been dropping by rather a lot and is projected to continue to drop. Effectively this dominates variable cost in a vertical farm.
It's true that the cost of energy dropping, and to levels that would make people in the Space Age gasp. I still don't see how that justifies building a 30-hectare solar park to grow the same lettuce you could grow in a one-hectare greenhouse. I mean, how big is your armored vault hothouse going to be?
> You seem to be arguing this cost is too high. That seems to be countered by the many people actually growing stuff in greenhouses for decades this and making plenty of money.
No, man, that's not what I'm saying, man. I'm saying that if you're going to build a hothouse, make it a greenhouse. Daylight it, with skylights and/or lightpipes. Maybe supplement with artificial lighting some of the time. Lighting it with a solar farm that's thirty times as big is going to be more expensive, unless solar cells are thirty times cheaper than glass per square meter, and lighting it with fossil fuels is more expensive still. Fuck, thirty times cheaper than plexiglass. Thirty times cheaper than the shitty transparent plastic wrap we used to make greenhouses in Ecovillage Velatropa. If you're right and, against all odds, LEDs are now 50% efficient, exceeding the theoretical ideal luminous efficacy maximum Wikipedia gives, the threshold becomes fifteen times cheaper instead of thirty --- still improbable!
While I'm calculating the efficiency of LEDs for you, would you mind undoing your downvote, please, now that you know you entirely misread the main thrust of the comment you were downvoting?