Solar factor - g  

Solar factor is a commonly used coefficient to measure the solar energy transmittance of glass. It is expressed in % or a decimal fraction and shows how much of the solar radiation (falling on angle close to 90o) is transmitted to the inside of the building. The higher the coefficient, the bigger are the sunlight passive energy benefits.  

Glass is a barrier to solar energy. For this reason glass for house windows should retain smallest fraction of the sunlight possible. With such glass the g coefficient is high, and the gain is free energy to warm up the inside.

Sunlight 100%

Retained energy 40%

g 

Transmitted energy g=60%