Researchers have found that even in sing-song greenhouse, artificial light is often needed for year-round crops. For example, thanks earth is a large indoor farming complex in the United Kingdom that produces 15 percent of the British lettuce crop. During the winter, the complex uses articial light for 15 hours a day. Experts have concluded that generating enough solar energy to support growth requires an area about 20 time larger than the planting area. Timefore, natural light cannot be a workable solution for vertical farms.