Researchers have found that even in single-story greenhouses, artificial light is often needed for year-round crops. For examples, Thanet Earth is a large indoor farming complex in the United Kingdom that produces 15% of the British lettuce crop. During the winter, the complex uses artificial light for 15 hours a day. Experts have concluded that generating enough solar energy to support growth requires an area about 20 times larger than the planting area. Therefore, natural light cannot be a workable solution for vertical farms.