You wouldn't buy a car without knowing its gas mileage. Why not do the same when choosing energy-efficient ovens or even supermarkets and hotels? Energy Star, a rating system by the Environmental Protection Agency, will help you find them. Approved products can be pricier, but they cost less to power. Commercial buildings account for nearly 18% of U.S. greenhouse-gas emissions, but those with the Energy Star label consume 35% less energy than the average. By using Energy Star appliances at home, consumers can reduce their utility bill as much as 30%.