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Your window on the world could be your ticket to an extreme electric bill.

The Environmental Protection Agency says energy-efficient windows could slash household energy costs by up to 15 percent.

For the last several years in Dallas, code has required low-E windows for all new construction. According to Jim Benney, executive director of the nonprofit National Fenestration Rating Council, low-E (which stands for low emission) windows were first introduced in the 1980s as a way to retain heat in cold climates. A microscopic metallic film, invisible to the naked eye, enables the glass to keep cool air inside and warm air outside, and vice versa.

"With a clear piece of glass, 89 percent of the heat hitting it comes inside the house," says Mr. Benney. "With low-E, it's only 4 percent, which makes a big difference."

Another energy-saving solution is a double-paned window. Ten-year-old models were manufactured with a quarter-inch of air space between the panes. New models, explains Martin Gates, president of Dallas-based J&M Glass Co., have a half-inch or 5/8-inch space, which gives the window units optimal thermal efficiency.

Many companies tout the increased efficiency of inserting colorless argon or krypton gas between the two panes to reduce heat loss. But that's a technology more suitable for northern states, explains Mr. Gates, whose company manufactures thermal windows.

Texas houses need new windows with double glazing and spectrally selective coatings to reduce heat gain.

To retrofit a typical house with vinyl or hybrid aluminum window frames providing the latest technology, Mr. Gates says it would cost $3,000 to $12,000, or $350 to $500 per opening. The cost variance is due to considerations such as window size, energy-efficient features, decorative style and how much construction work has to be done to the opening.

Ready-made wood windows can be found in that price range, too, says Mr. Gates. But they only work if their off-the-shelf measurements exactly fit the window opening in the house, which is not likely in an older house.

Adds Mr. Benney: "Retrofitting a home can cost a fortune, especially when you're doing custom windows in wood. A homeowner isn't likely to ever recoup those costs in their electric bills, but most are happy with the look and delighted with how much quieter a two-pane window is as opposed to just one."

Unless you're the most confident (and talented) do-it-yourselfer, consider hiring a professional to install your new windows. Here are a few questions to ask when interviewing contractors for the job:

•Ask for a list of references, and make sure they're from the past two or three years. A dozen names isn't an outrageous number. Call a few at random from the list. And always ask this question: "Would you hire this person again?"

•While you're interviewing the professional, make sure you feel comfortable with him or her. Good communication is key to a successful outcome.

•Talk money: When soliciting estimates and bids, make sure you're comparing apples to apples. Do the estimates include the same quality of materials? What about warranties of the work?

•Verify that the contractor has both workers' compensation insurance and general liability insurance. Ask to see current certificates.

Sources: The U.S. Small Business Administration and www.contractors.com

Low-E A low-emissions coating consisting of a microscopically thin metal bonded to the glass that prevents UV rays and heat from penetrating.

U-Factor The rate of heat loss of a window. The lower the U-Factor, the better it insulates.

Solar Heat Gain Coefficent A fraction that indicates how much solar heat a window transmits. The closer to 0, the better.

Air Leakage Cubic feet of air passing through a square foot of window. The lower the number, the less the window is leaking.

Source: The National Fenestration Rating Council (www.nfrc.org)