It's often hard to look through windows and see more than what's outside. You can see light coming in, but it's hard to see the ultra-violet radiation that is heating up your home or heat that is already in your home escaping through the glass. But just as insulation impacts how much heat enters or leaves your house, windows can be just as important.
Windows that actively seek to reduce heat flow are regarded as Low-E, or Low-emissivity windows. These windows have coating on them (metal or metallic oxide) that is transparent to visible light but acts as a barrier to heat. The Efficient Windows Collaboration has a great review describing Low-E. Remember, Low-E just means that a window has a glazing that is actively trying to reduce heat transfer, but the term Low-E doesn't tell you how well the window performs. For that, you need to look at some ratings.
When investigating which Low-E windows will work in your home and climate, there are two main Energy Performance ratings to look for, U-Factor and Solar Heat Gain Coefficient (SHGC). The National Fenestration Rating Counsel (NFRC) produces a label that producers of Low-E products can choose to put on their product. They summarize the two main Energy Performance Ratings as follows on their website.
U-factor measures how well a product prevents heat from escaping. The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. U-Factor ratings generally fall between 0.20 and 1.20. The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value.
Solar Heat Gain Coefficient
Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat caused by sunlight. The SHGC is the fraction of incident solar radiation admitted through a window (both directly transmitted and absorbed) and subsequently released inward. 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 in the house.
It's important to realize that which U-Factor and SHGC that will work for you is highly dependent on where you live! For instance, in Northern cooler climates, you want a low U-Factor, because you don't want all that heat to escape in the winter. However, if you lived in Miami, you wouldn't be too worried about heat escaping in winter, but you would be worried about heat entering your house. Therefore, you would care more about SHGC. The lower the SHGC, the less solar heat enters your house. Cold climates need a low U-factor (so heat doesn't escape) and higher SHGC (they want the Sun's heat to come in) while Hot climates don't need a low U-Factor (they probably aren't heating much, so they don't need it) but a low SHGC (they don't want the Sun's heat to come in).
Don't know which climate you fall in? Trying to figure out whether you need a low U-Factor or high SHGC? Don't worry, EfficientWindows.org has a great selection tool that does this work for you by using a map of the country!
Other factors the NFRC call out on the label are the Visible Transmittance, Air Leakage, and Condensation Resistance. These three ratings don't rely as much on geographic location but tell a consumer more about how well the window is made and performs. The NFRC defines these as follows (from the same link as above).
Visible Transmittance (VT) measures how much light comes through a product. The visible transmittance is an optical property that indicates the amount of visible light transmitted. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted.
Air Leakage (AL) is 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 by infiltration through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly.
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. While this rating cannot predict condensation, 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.