The details: It’s a familiar idea that shade can lower your utility bills in the summer, but this is the first study of its kind to look at the actual impact on real homes and real utility bills, says study author Dr. Geoffrey Donovan, a research forester with the U.S. Forest Service. Donovan took aerial photographs of 460 homes in the Sacramento, Calif., area and analyzed tree placement, crown size, and how far each tree was from the house. Then he and a coauthor compared that with each homeowner’s electricity use and utility bills.
He found that trees placed on the south and west side of a house, and no more than 60 feet away from the actual building, could lower a homeowner’s utility bills by about 5 percent between May and September. However, trees on the north side actually raised their bills by 1 percent. Why? Because on that side of your house, trees act more like blankets than shade. “They slow radiant cooling of the house at night,” Donovan says, “and they don’t cast any house-shading shadows.” So they cost you some cooling after the sun goes down, without sparing you some heat during the day.
What it means: A 5 percent decrease in utility bills may not leave you flush with cash, but that’s on top of the aesthetic value trees add to your property, the exercise you get planting and caring for them, and don’t forget: Trees absorb carbon dioxide and so help slow global warming. Because trees on the south and west sides of a home allow you to reduce your (greenhouse-gas emitting) fossil-fuel energy use, and absorb greenhouse gases at the same time, “they have double the carbon benefits than if they were out in the woods,” Donovan adds.