Fenestration, like windows and doors, can save energy in buildings by using natural light. By analyzing different factors like climate and window size, researchers found that using daylighting can lower energy use in all climates. To make this work, it's important to control how much sunlight comes in. By designing windows and lighting carefully, buildings can use less energy overall and reduce peak electricity demand. The best window setup depends on where the building is, which way it faces, and how much artificial light is used.