In article , I wrote:
How
do you propose to illuminate your greenhouses if not by direct
sunlight? You need either something like 10 MW/hectare if you do it
with efficient electric lighting

The Hublitz et al. paper referred to elsewhere
http://www.marshome.org/files2/Hublitz1.pdf
gets 2.3 kW/m^2, a factor of two worse than my estimate. Their
estimate was for high pressure sodium lights while mine was for LEDs,
which are more efficient.

Heat loss of a surface greenhouse won't be trivial, but it's only
about 30 kW/hectare

Sorry, I dropped a couple of decimal places there and also used too
low a temperature. The correct figure is about 400 W/m^2 _if_ the
greenhouse is a perfect emitter. That's still less than the power
needed for artificial lighting but not by as large a factor as I
found originally.

Bringing in sunlight from outside requires about a square centimeter
of light pipe or fiber per square meter of greenhouse; that part was
correct.

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