Vertical Farming Doesn't Stack Up
by Stan Cox and David Van Tassel

A note on calculations:

The following is a very rough estimate of the amount of power needed just for lighting vertical farms to grow the U.S. wheat crop. Note this is under ideal conditions for nutrients, temperature, and other productivity factors. Under excellent conditions, wheat has radiation use efficiency of 2.8 grams of biomass produced per 106 joule of photosynthetically active radiation (PAR). So to produce one metric ton (106 g) of wheat biomass requires [106 g / (2.8 g/10⁶ J)] = 3.6 × 10¹¹ joules of PAR over a season under ideal conditions.

Suppose an excellent 50% harvest index (ratio of grain mass to total biomass), so that a metric ton of wheat grain requires 7.1 × 10¹¹ joules (actually more because the protein and oil in the grain require extra energy to produce, but ignore that.) The US produced 60 million metric tons of wheat grain in 2009, so that required 6 × 107 times 7.2 × 10¹¹, or 4.3 × 1019 joules of intercepted light energy to produce. A metal halide greenhouse light (which provides light rich in the wavelengths needed for photosynthesis) requires 2.9 joules of electricity input to produce one joule of photosynthetically active radiation, so to produce 4.3 × 10¹⁹ joules of PAR would require 1.2 × 1020 joules of electricity at the socket. One kilowatt-hour is 3.6 × 10⁶ joules, so 3.4 × 10¹³ kWh of electricity would be required to run those lights. 

Total delivered US electricity supply from all sources in 2007 was 4.2 × 10¹² kWh. So the entire US electricity supply would have to be increased eightfold just to substitute for the solar radiation converted to biomass by the annual wheat crop.

(We acknowledge that the vertical crop would capture small quantities of sunlight through the glass walls of the vertical farm, and that the top floor could be a fully functional greenhouse or garden. That would reduce the artificial lighting requirements, but recall that all of the assumptions of the above calculations err in favor of optimism. Deviations from optimum crop growth, harvest index, and health, or less than ideal distribution of light fixtures very close to the crop canopy, reflection of a small portion of PAR from leaves (which does occur), waste of light falling on bare earth between plants in their early growth stages, or any other unforeseen negative impacts on yield will have the effect of increasing the amount of artificial light provided, and electricity consumed, per unit of production calculated above. If anything, the calculated requirement of an eightfold increase in U.S. electricity supply is an underestimate.)