Want to save money on your energy bills? Buy new windows.

"If you switch from single-pane to double-pane windows, you could see as much as a 25 percent reduction in your annual energy bill," said Sam Heer, the general manager of Renewal by Andersen, the window-replacement division of Andersen Windows. "Our advice is always to look at long-term solutions."

The concept is simple: Energy-efficient windows serve as barriers to outside temperatures and help keep homes cool in the summer and warm in the winter. The barrier enables air conditioners and furnaces to work less hard, operate on less energy and consequently cut energy costs.

Energy-efficient windows are treated with something called a low-E coating that reduces the amount of ultraviolet light that penetrate the glass. Double-paned windows are more efficient than single-paned windows and a little less efficient than triple-paned windows. The efficiency is enhanced when an inert gas - typically argon, though Andersen uses a blend of argon and nitrogen - fills the space between the panes, acting as insulation, Heer said.

Windows are rated for their energy efficiency by the National Fenestration Rating Council based on four complicated-sounding properties: U-factor, which is the rate of heat loss of a window assembly; solar heat gain coefficient, the fraction of incident solar radiation admitted through a window; visible transmittance, the optical property that indicates the amount of visible light transmitted; and air leakage, heat loss and gain occurring by infiltration through cracks in the window assembly, according to the Efficient Windows Collaborative.

The EWC, a joint effort between the Environmental Protection Agency and the Department of Energy, gives guidelines and recommendations for window properties in all regions of the United States. For more information, visit www.efficientwindows.org and click on "window selection tool." Choose your state from the color-coded map.

The federal government is encouraging the use of energy-efficient windows by offering a $200 tax credit for replacing old windows in existing residences with windows that have earned the Energy Star rating for high efficiency windows.

The EWC did a cost comparison of different types of windows and the energy cost savings they could provide. In the case studies, the total energy costs of a 2,000 square foot home with 300 square feet of window area using the least-efficient window - double-paned, clear glass in an aluminum frame - were $1,079.17. The same house with triple-paned windows with low-E coating, argon gas-filled inside an insulated vinyl frame was $844.84, an annual savings of $234.

There are many benefits of high-performance windows, according to the EWC.

– Cooling and heating season savings: Low-E coatings, gas-fills and insulating spacers and frames can significantly reduce winter heat loss and summer heat gain.

– Improved daylight and view: New windows with low-solar-gain and low-E coatings can reduce solar heat gain significantly with a minimal loss of visible light (compared to older tints and films).

– Improved comfort: In summer and winter, occupant comfort is increased; window temperatures are more moderate and there are fewer cold drafts. Discomfort from strong summer sunlight is reduced.

– Reduced condensation: Frame and glazing materials that resist heat conduction do not become cold, and this results in less condensation.

– Reduced fading: Coatings on glass or plastic films within the window assembly can significantly reduce the ultraviolet and other solar radiation that causes fading of fabrics and furnishings.

– Lower mechanical equipment costs: Using windows that significantly reduce solar heat gain means that cooling equipment costs may be reduced.

Heer said the right window frame can be just as important in determining a window’s energy efficiency as the glass.

"It’s not a place where you want to cut corners, because it will cost you in the long run."