But solar panels are only about 30% efficient, so that kinda kills any gains.

Thanks, this is exactly the comment I was looking for. In addition to the 70% loss due to the solar panel efficiency, I think we should also lose some efficiency in the conversion to light via leds (although I expect that’s much more efficient, perhaps at like 80%).

I’m curious what is physically possible, if we assume we can achieve the max possible efficiency. I’m guessing there’s behavior like a Carnot engine, and the energy transfer can only be up to ~86% efficient (but please correct me if I’m wrong!!). In that case, conversion from light to energy via solar panels -> conversation back to light via leds should be 0.86*0.86 = 73% efficient in best case. And the full effect should be (800/300)*0.73 = 1.94, about twice as good as growing plants with the sun’s direct light. I’m surprised that seems possible!

p.s. My efficiency guesses are probably wrong. Please correct me.

	▲	magicalhippo 3 hours ago \| parent [-]
		By using multiple junctions and stacking them, top one converting the most energetic photons, then the second-most etc, one can approach the theoretical limit of about 95% or whatever it is. However in practice it's very expensive and difficult as I understand. AFAIK the current state of the art is about 6 stacked junctions at around 60% efficiency, at great cost. And as you say the LEDs aren't 100% efficient either, though both deep red and bright blue are among the most efficient, about 85% there. So that leaves you with about 50% overall just from those two.