The centerpiece is a photovoltaic umbrella dome that collects roughly 90 percent of the house's energy needs. Made of a common plastic, the pillowy dome traps heat like a greenhouse. That hot air warms water in a tank tucked under the roof, turning out a daily average of 80 bath-ready gallons, even on the darkest days of December. At the umbrella's apex, a generator-equipped turbine produces electricity and, in chilly months, drives heat into the house. Photovoltaic cells studding the 484-square-foot dome floor create additional electricity.