Solar Helper. The company best known for its lineup of solar-powered attic fans now offers a way to expel hot air even after the sun goes down. The Solar Controller powers a fan for eight minutes every half-hour until the temperature drops below 80°F or the humidity goes below 75 percent. The maker says it plugs into a standard 110-volt receptacle and requires only $5 of extra electricity a year. It sells for $100. U.S. Sunlight Corp., 866/446-0966, ussunlight.com.