The old term for term for this measurement was shading coefficient. SHGC values range from 0 to 1. The lower the SHGC number, the less solar gain or heat passing through the window. Many power utilities offer rebates to customers who upgrade their windows. They typically will give you a rebate based on the total SHGC number of your upgraded product.
Little known Secret
Most of these rebate programs will give you a rebate if your upgraded windows have a SHGC # of .4 or less.
Many times you can still qualify for a rebate if you add an exterior shading product that has a SHGC # of .4 or less. Be sure to check with your local power utility to find out the details of their specific rebate program.
The EZ Snap shading mesh has a SHGC of .1827. This SHGC number is based on a solar profile angle of 45 degrees (most common) and a clear, double pane window.