Windows are rated in several categories which measure their energy efficiency. If you are shopping for windows for your house, more emphasis should be placed on various ratings based on the climate the home is located in. For example, if you are looking for replacement windows in Phoenix, Arizona, a sunny and warm location, the Solar Heat Gain Coefficient is almost certainly the most critical rating to look at. Other characteristics, such as the orientation of a window to the sun and the amount of shade which is present need to be considered.

The U.S. Government's Energy Star program is specially designed to make it convenient for people to identify energy efficient products and practices which will save them money and safeguard the environment. The Energy Star program scores over 60 products, including windows. Energy Star certification makes it easy for shoppers to be certain they are getting energy efficient replacement windows.

All Energy Star replacement windows are scored in five areas however Energy Star qualification is solely determined on the U-Factor and Solar Heat Gain Coefficient. These ratings are also the categories used to determine if a window qualifies for the IRS Energy Tax Credit.

U-Factor
Measures the pace of heat transfer and indicates how effectively a window insulates. The lower the U-Factor rating, the more efficiently a window insulates. The window U-Factor incorporates the thermal properties of the frame together with the glazing.

Solar Heat Gain Coefficient (SHGC)
Measures the solar energy transmitted by the window and tells us how effectively a window inhibits heat caused by the sun. The lower the SHGC, the less solar heat a window transmits.

Visable Transmittance (VT)
Measures the amount of light a window lets through. The higher the Visable Transmittance, the brighter a room will be.

Air Leakage (AL)
Measures the pace at which air passes through joints in a window. Air Leakage is determined in cubic feet of air that passes through one square foot of window area per minute. The smaller the Air Leakage value, the less air leakage.

Condensation Resistance
Measures how efficiently a window resists water build-up. The greater the Condensation Resistance factor, the less condensation build-up a window allows.

Other Replacement Window Ratings
There are some additional ratings that are not presented on the NFRC Energy Star label however they are beneficial and show up on some windows.

Light to Solar Gain
The ratio of Visable Transmittance to Solar Heat Gain Coefficient measures the amount of light that is transmitted without adding heat. A greater number signifies that more light is transmitted without adding heat.

R-Value
R-Value was created as a way to measure the insulation value of something. It calculates conductivity and illustrates the resistance to heat flow. A high R-Value implies a better insulated window. Most windows today are measured using U-Factor (see above).