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Plants are around 2% efficient.

Ok, so they're going to be 2% efficient at using that 20% that was gathered by PV cells. So it still seems like using 2% of all the light would be more efficient than using 2% of a fifth of the light.

I suspect that by 2% efficient, that means they use 2% of the energy of all light. Since they don't use green at all, going through the PV and then generating artificial light of the frequencies they need does seem to improve things. But probably not by so much as to make up for the 20% efficiency of PV.

Every plant has a slightly different need but generally speaking you can grow plants very efficiently if you use the entire red spectrum and 10-20% of the blue spectrum. That allows for significant energy savings.

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