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.
The nationally recognized rating method by the National Fenestration Rating Council (NFRC) is for the whole window, including the effects of the frame. Alternately, the center-of-glass SHGC is sometimes referenced, which describes the effect of the glazing alone. Whole window SHGC is lower than glass-only SHGC, and is generally below 0.8.
Solar heat gain can provide free heat in the winter but can also lead to overheating in the summer. How to best balance solar heat gain with an appropriate SHGC depends upon the climate, orientation, shading conditions and other factors. ENERGY STAR provides simplified guidance on recommended SHGC values for your climate; additionally, compare average simulated energy costs for your location based on various window types.