Because if you are growing 1 acre of lettuce across 10 floors of a building (1/10th acre per floor) now you need to collect 1 acre of sunlight from a 1/10th acr

Because if you are growing 1 acre of lettuce across 10 floors of a building (1/10th acre per floor) now you need to collect 1 acre of sunlight from a 1/10th acre sized roof. Where do you get the other 9/10th of an acre of sunlight from?