How is window energy efficiency rated?
Windows are rated by U-factor and Solar Heat Gain Coefficient (SHGC). The lower the U-factor, the greater the insulating value of the window and the more heat you retain indoors during the winter months.
A lower SHGC means less solar heat will pass through the glass, which comes in handy during summers in the south. This can significantly reduce your air conditioning costs.
So, spending more for what’s called “low-emissivity glass” or “low-e” glass is usually worth the extra price. It reflects or absorbs heat in the form of infrared light, improving both the U-factor and the SHGC.