Qualifying for the $200-$500 federal tax credit on new windows and doors depends on two measurements, U-factor and Solar Heat Gain Coefficient.
Just because windows or doors are Energy Star-labeled doesn't mean they're eligible for a federal tax credit. And with costs running about $500-$1,000 per window including labor, it's wise to know something about the scientific lingo and numbers on the product labels you're likely to encounter. Here's your pro-level label-decoding guide so you can be sure you're buying qualified products.

Which Labels Matter?  
The two labels you should look for: The U.S. Department of Energy's blue-and-yellow Energy Star label, which specifies the climate zones the product is certified for, and the white National Fenestration Rating Council label. Nonprofit NFRC is the industry-recognized certifying body for windows and doors. It reports raw numbers only; Energy Star tells you whether those numbers constitute superior performance, putting its seal of approval on those products that meet its standards.
To confuse matters, DOE has issued a blue label that manufacturers can use to signify that a product qualifies for the tax credit. But DOE doesn't require that manufacturers include it.

What You Need to Get the Tax Credit
For windows or doors to qualify for the credit, two NFRC-supplied measurements must each be equal to or less than 0.3, regardless of climate: U-factor and solar heat gain coefficient (SHGC). You must also have the manufacturer's signed statement that the product complies with IRS requirements. This either comes with purchase or can be downloaded from the manufacturer's website.
Don't be swayed by ratings the manufacturer may post on its own label. A window or door's frame and other components (weather stripping, sidelights, transoms) can significantly affect its energy efficiency, so NFRC measures based on the entire unit, not just the window glass or door slab alone. Manufacturers, on the other hand, sometimes report values that don't take the entire unit into account, according to Energy Star.

A Guide to Measurements
The NFRC label typically lists five measurements, including the tax credit-critical U-factor and SHGC. The other three are somewhat less important to energy performance, according to Energy Star, but can help you judge how well a window or door will perform in a particular application—for example, whether it'll let in enough light. 
Where you live affects which measurements are most important, but the tax credit requirements are uniform across the country. There are four Energy Star climate zones, differentiated by whether heating, cooling, or a mix of the two is most critical to energy performance.
1. U-Factor
Range: 0.20 to 1.20
The lower the number, the better an insulator the window or door is.
Tax credit qualification requirement: 0.3 or less
Efficient Windows Collaborative climate recommendations:
  • Northern: 0.35 or less 
  • North Central or South Central: 0.4 or less 
  • Southern: 0.60 or less
A low U-factor means that less heat escapes in the winter, which makes it particularly important in cold northern climates, according to the Collaborative, a coalition of government agencies, research organizations, and manufacturers that promote efficient window technology.

2. Solar Heat Gain Coefficient (SHGC)
Range: 0 to 1
The lower the number, the less solar radiation—and heat—the window or door allows inside.
Tax credit qualification requirement: 0.3 or less 
EWC climate recommendations:
  • Northern: The highest you can find (paired with a low U-factor) if cooling isn't a significant concern; up to 0.55 if cooling is a significant concern.
  • North Central: 0.4 or less for climates with significant air conditioning; up to 0.55 for climates with moderate air conditioning.
  • South Central or Southern: 0.4 or less.
SHGC refers to the solar radiation a window or door allows inside. Seek the lowest possible SHGC rating in warm climates to minimize the use of air conditioning. Look for a slightly higher number in cooler climates so that the sun can help heat your home in winter, but be sure to balance SHGC with an efficient U-factor for your area.

3. Visible Transmittance 
Range: 0 to 1
Lower number means the room will be dimmer; a higher number means the room will be brighter.
Tax credit qualification requirement: none 
This number applies to windows or doors with windows only. Visible transmittance is the amount of light a window allows to pass through. With older window glazing techniques, VT and solar heat gain were basically the same; the brighter a room, the hotter it got. But new technologies allow windows to let in lots of light while the room stays cool. 
Consult VT numbers if you're looking to reduce glare in a room or fill it with natural light, but be warned that a very low VT may mean you have to use artificial lighting even during the day.

4. Air Leakage
Range: N/A, but .0.3 is standard building code
The lower the number, the more airtight the window or door.
Tax credit qualification requirement: none
This number, expressed in cubic feet per minute per square foot of window/door area, represents the amount of air that the window or door's frame allows to pass through. Energy Star standards don't consider air leakage because it's difficult to measure accurately and can change over time as frame materials expand, contract, or warp in place, according to the EWC. Still, this measurement can help you compare similar products, especially if they'll be buffeted by the elements.

5. Condensation Resistance
Range: 1 to 100
The lower the number, the more condensation the window or door allows to build up.
Tax credit qualification requirement: none  
Condensation resistance is a measure of how much moisture a window or door allows to build up on the surface (which can drip onto wood and cause mold or discoloration) or between glazing layers (which can't be clean and blocks your view). Energy Star-rated windows tend to resist condensation well, so this number won't likely affect your purchase decision.

Before Buying New Windows
You can recoup 78.7% of the project cost for midrange vinyl replacement windows, according to Remodeling Magazine’s annual Cost vs. Value Report.
But before you buy, check out these articles for more info:
What You Need to Know About Buying Energy Efficient Windows
Repairing Windows May Be the Smarter Option