The outfit produces 4,000 heads of leaf lettuce a week, every week, apparently immune to weather variations. There may be additional outputs; it's not particularly clear.
The lettuce in grown in 2 fancy-smancy greenhouses, very high tech with computers and stuff, which cover 12,000 square feet, which is a tad over .25 acre. They're planning to add another greenhouse, some 20,000 square feet, which would bring them up to .75 acre. Although they're greenhouses, consider this quote:
A computer regulates everything: the 43 high-pressure sodium lights and heater that maintain summerlike light and temperature; the shade cloths that come down at night or when it’s too sunny outside; the pH, nutrient balance and flow of the water and the water system; and carbon dioxide emitted into the air to boost growth.They have 12 part-time employees (retirees and housewives paid over minimum wage, plus 3 relatives of the owner-manager.
The lettuce with roots still attached sells in a clamshell for $5 a pop!!! (I'd assume they're selling to K street lobbyists, not to poor underpaid Feds.) Not clear how much the grower gets.
So, if we assume 5,000 a week for 50 weeks, that is 250,000. Assume $2 to grower is $500,00; assume $4 and it's a million. Assume the equivalent of 6 full-time employees paid $30,000 each is $180,000, leaving $320,00 for operating expenses and profit, or more.
If a population of 1 million uses a head per person per week, then it would take 200 such operations to supply, or 50 acres. So rooftop gardens could indeed supply greens for the city, assuming the residents were very well-paid.