Vertical farms grow crops on stacked trays inside a building, under lamps rather than sunlight. Because the climate indoors is controlled, harvests do not depend on the season or the weather, and the same floor area can produce many times what a field would. The catch is electricity: lighting and cooling account for most of the running cost, so the economics of a vertical farm depend less on farming than on the local price of power. Where electricity is cheap and land is scarce, the arithmetic works.