lighting energy savings (Apte and Arasteh, 2006). Advances have largely been made by reducing the heat-transfer coefficient (U-value) of windows through the use of low-emissivity (low-E) coatings and by reducing the solar-heat-gain coefficient (SHGC) via spectrally selective low-E coatings. The U-value is the primary determinant of winter heat loss; the SHGC is the primary determinant of summer cooling loads.

Two new window-technology advances, now available in niche markets, could have far-reaching implications if they became mainstream products and systems. The first is highly insulating “superwindows” that achieve U-values in the range of 0.1–0.2, as compared with a typical U-value of 0.5 for double-glazing and 0.35–0.4 for the ENERGY STAR® windows currently being sold in cold climates. The second advance is a new generation of dynamic products that reduce cooling loads and electric illumination when daylighting is available in commercial buildings. The full penetration of these advanced technologies into the building stock, which could take decades, might shift the role of windows in buildings to being approximately “energy neutral.”

Low-Energy and Zero-Net-Energy New Homes

It is possible to construct homes that combine high levels of energy efficiency—in the building envelope, heating and cooling systems, and appliances—with passive and active solar features in order to approach zero-net-energy consumption.9

The whole-building approach described earlier is being used by the DOE to reach a zero-net-energy consumption goal. For example, two highly instrumented homes were built with the same floor plan in Lakeland, Florida, in 1998 (Parker et al., 2000). Over 1 year, the control home used 22,600 kWh of electricity. During that same year, the experimental home used only 6,960 kWh, a reduction of 70 percent. The experimental home also had solar PV production of 6,180 kWh. When this production is accounted for, the experimental home’s net energy use was only 780 kWh, about a 97 percent reduction relative to the control home.10


A home with zero-net-energy consumption may at times produce more energy than it consumes (for example, through PV panels on the roof) and at other times consume more energy than it produces.


The full cost of providing backup power in the electrical supply system for those times when the PV array is not generating electricity would need to be included in any analysis of the overall systems cost of such low-energy homes.

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