Rheem. With an energy factor of 2.0, the HP-50 Heat Pump Water Heater offers more than twice the energy efficiency of a standard electric storage water heater, the maker says. The 50-gallon unit meets Energy Star and federal tax credit requirements. At just 21 inches in diameter and 75.5 inches tall, the HP-50 has a slimmer profile for installation in tight spaces. The heat pump water heater pulls available heat from ambient air, intensifies it, and transfers it to stored water for more efficient water heating. Ideal for homes in climates with average year-round temperatures above 40º F and in rooms with at least 1,000 cubic feet of surrounding air space. 800.621.5622. www.rheemhpwh.com.