Homeowners can knock 20 to 40 percent off their electricity bills with a new wind turbine that can be mounted directly on a building. The ­seven-foot-wide plastic turbine has a ring around its rotors that diffuses noise and limits vibration; the company claims that the turbine is no louder than a whisper. In windy locations, its power output should be about 2,000 kilowatt-hours a year.

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