An energy-efficient home is truly a "system". One of the components of this system is energy-efficient windows. If you're planning to add or replace windows you may find the labels to be a bit confusing.
The NFRC label (National Fenestration Rating Council) appears on ENERGYStar rated products and allows for reliable comparisons. Energy Star requirements for windows vary by climate zone and are based only on the U-Factor and Solar Heat Gain Coefficient (SHGC).
U-Factor measures the window’s rate of heat loss. In Texas, ENERGY Star requires a U-Factor ≤ .40.
Solar Heat Gain Co-efficient (SHGC) measures how well a product blocks heat caused by sunlight. In Texas, the ENERGY Star requirement is ≤ .40.
Visible Transmittance (VT) measures how much light comes through the window in the range between 0 and 1. To maximize daylight and view, select windows with a higher VT. ENERGY Star has no required VT rating.
Air Leakage (AL) rating is expressed as cubic ft of air passing through a square foot of window area. The lower the AL, the less air will pass through cracks in the window assembly. Windows ≤ .30 or less are recommended. ENERGY Star has no required rating.
Condensation Resistance is scored on a scale of 0 to 100. The higher the resistance factor, the less condensation build-up the window allows. Again, there is no ENERGY Star requirement.
Here are a couple of links if you want to do more research:
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