The integrated air-source HP-50 heat pump water heater has an energy factor of 2.0 and qualifies for a federal tax credit. Instead of heating stored water with a conventional electric element, the unit transfers available heat from the ambient air, intensifies the heat by concentration, and transfers it into the water. The 50-gallon unit measures 21 inches in diameter and 75.5 inches tall. Cost: $1,700 to $2,200. Rheem Manufacturing Co. 800-621-5622.