The Solar Heat Gain Coefficient (SHGC) is the percent of solar energy incident on the glass that is transferred indoors both directly and indirectly through the glass. The direct gain portion is the solar energy transmittance, while the indirect is the fraction of solar energy incident on the glass that is absorbed and re-radiated or transmitted through convection indoors. The SHGC ranges from 0 to 1 as a dimensionless(explanation 1.) number. Therefore, when the glass has a lower SHGC, it can resist the solar heat entering the room, effectively reducing the energy consumption of the A/C.
(explanation 1.)Dimensionless quantities in physics are those that require no unit.