Primary solar heat gain: the solar heat directly transmitted through a glass in its original solar radiation form.
Secondary solar heat gain: the solar heat absorbed by a glass and further transferred to the indoor space as heat and via all 3 heat transfer modes (conduction, connection and radiation)
The primary solar heat gain component is just the solar energy transmittance of the glass.
The secondary solar heat gain component is calculated as the solar energy absorptance of the glass multiplied by its inward flowing fraction. The solar heat absorbed by the glass causes a temperature increase of the glass. The absorbed solar heat flows to either the indoor side or the outdoor side. The fraction flowing to the indoor side is the inward flowing fraction.
For example, for a glass with 30% solar energy transmittance, 20% solar energy absorptance and 0.25 inward flowing fraction:
Its primary solar heat gain is 30%: 30% of the overall solar energy is directly transmitted to the indoor space
Its secondary solar heat gain is 20% × 0.25 = 5%: 20% of the overall solar energy is absorbed by the glass and 0.25 fraction of it is transmitted to the indoor space;
Its SHGC is therefore 30% + 5% = 35% or 0.35.
SHGC = primary solar heat gain + secondary solar heat gain
Primary solar heat gain = Solar energy transmittance Secondary solar heat gain = solar energy absorptance × inward flow fraction
Solar energy transmittance and SHGC are different. Solar energy transmittance is the primary solar heat gain component of SHGC only. The SHGC of a glass is always greater than its solar energy transmittance.