The window where you stand to watch the birds feed or your roses bloom could be the biggest energy drain on your house.

Windows are huge thermal holes, places where you can lose as much as 30 to 40 percent of your heating and cooling costs, according to industry experts.

In fact, replacing all residential windows in the United States with more efficient models would save $7 billion over the next 15 years, according to the National Fenestration Rating Council, a nonprofit group that developed a window energy-rating system based on whole-product performance.

Choosing replacement or new windows for your home is a daunting task because there are more than 300 manufacturers making them.

Each product has its own bells and whistles -- blinds or grilles between panes, tilt capabilities for easy cleaning, storm-protection features -- and you pay for those niceties.

"When you compare the prices of windows, you can't make a blanket statement," said Robert T. Criner of Criner Construction Co. in Yorktown, Va.

"All have different lines within their banner of manufacturing. And it's all in a state of evolution constantly."

There are a lot of nationally recognized brands that contractors seem to favor: Andersen, Certainteed, Caradco, Lyf-Tym, Marvin, Norandex, Pella, Peachtree and Simonton. Each company offers several lines with features that fit most pocketbooks. For a mid-range replacement vinyl window, you can expect to pay about $350 to $400 per standard-size, double-hung window, installation included, depending on construction costs where you live.

There is, however, some rhyme and reason to selecting a window for your needs.

First, consider your environment and the architecture of your home. If you are replacing or adding windows, you need to match or complement what's already on your house.

You also need to choose a window that's right for your climate, depending on whether you live in a northern or southern state and whether cooling or heating is more important to your comfort.

Evaluate what kind of maintenance you want to do on the windows. Wood needs paint or stain, vinyl requires a gentle cleanser.

There are also some new improvements available in windows. Fiberglas-frame windows are expected to make a major impact, just as the strong Fiberglas doors have done.

And if you hate cleaning windows, you'll like PPG's SunClean, a self-cleaning glass that can be used in residential windows.

Regardless of brand or style, look for the Environmental Protection Agency's Energy Star rating on the label because multi-pane glass is no longer the main measure of a good window, according to the EPA. Welded seams, low-emittance solar-protective coatings, gases between the panes, good weatherstripping and glass spacers made from steel, foam, fiberglass or vinyl -- not aluminum -- are some of the hallmarks of a quality window.

And, if you live near water with strong winds and salt spray, you may want to consider thicker glass panes, maybe even laminated ones, which some new building codes may now require.

But, there's one all-important characteristic you want to make sure you get with any window you select -- a good, reputable warranty. Any warranty is only as good as the company that backs it, so be sure to deal with a company that has a history of taking care of its customers.

"I go for looks, how they operate, but mainly who is going to stand behind them," said David Cross of Cross Remodeling Inc. in Hampton, Va.

Dennis VanCamp with Hatchett Home Improvement in Newport News, Va., agreed that you should carefully evaluate the warranty you get with your purchase.

"In the past, there have been several contractors selling windows with lifetime warranties, but after a couple years when a problem arose, neither the contractor nor the manufacturer were to be found," he said.

When you shop for windows, look for the label that gives you energy and other performance ratings, including:

* U-Factor. Measures how well a product prevents heat from escaping. U-Factor ratings generally fall between 0.20 and 1.20. The insulating value is indicated by the R-value, which is the inverse of the U-value. The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value.

* Argon is an inert, nontoxic gas used in insulating glass units to reduce heat transfer.

* Low-emittance (Low-E) coating. Microscopically thin, invisible, metal or metallic oxide layers are deposited on a window to reduce the U-factor by suppressing heat flow. Low-e windows cost about 10 to 15 percent more than regular windows, but they reduce energy loss by 30 to 50 percent.

* Solar Heat Gain Coefficient (SHGC). Measures how well a product blocks heat caused by sunlight. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat-gain coefficient, the less solar heat it transmits, which ultimately affects your cooling bills.

* Visible Transmittance (VT). Measures how much light comes through a product. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted.

* Air Leakage (AL). Indicated by an air leakage rating expressed as the equivalent cubic feet of air passing through a square foot of window area (cfm/sq ft). Heat loss and gain occur through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly. Optional rating.

* Condensation Resistance (CR). Measures the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the CR rating, the better that product is at resisting condensation formation. This rating cannot predict condensation, but it can provide a credible method of comparing the potential of various products for condensation formation. CR is expressed as a number between 0 and 100. Optional rating.