Solar gain

The solar gain represented by the "g" value is mainly of interest for transparent components. The "g" value is also called TSET (Total Solar Energy Transmittance), SHGC (Solar Heat Gain Coefficient) or more simply solar factor.
This expresses the share of solar energy that is transmitted through the element to the inside of a building.
When solar irradiation intercepts an element, part of the incident energy is directly reflected back outwards, a part is transmitted and a part is absorbed by the element itself.
The latter, by becoming heated will re-transmit part of the energy outwards

and another part inwards. The "g" factor therefore consists of two contributing parts. The first is represented by the energy which is directly transmitted inwards that is the solar energy which passes through the glass (τ). The second is given by the radiation which is re-transmitted inwards by the element itself (q, g=τ+q).
The ideal "g" value for a window glass should be sufficiently high so as to allow thermal gain during the winter, thus reducing heating requirements but at the same time sufficiently low so as to avoid overheating in the summer. In general, a system with a high "g" value is chosen for colder climates and for small glass surfaces. For warmer climates and larger surfaces, a system with a low "g" value is chosen.

Comparison between different glazing and shading systems with a double glazed insulating thin film module (a-Si)

  SHGC (g-value)
      Single glass pane ~0.80
      Double glazed with uncoated glass ~0.80
      Double glazed with solar control coating 0.30-0.70
      ASI THRU double glazed unit 0.10
      sc-Si photovoltaic module with insulated glazing 0.15
      a-Si/a-Si photovoltaic module with insulated glazing 0.12
      a-Si photovoltaic module with insulated glazing 0.18
      Micromorph photovoltaic module with insulated glazing 0.15
      External Venetian blind (white)* 0.12
      External fabric canopy* 0.09
      Internal roller blind (white)* 0.40
* at values in combination with double glazed windows with a SHGC=61% and a U-value of 1.4W/m2K (0.24Btu/hr ft2 F)

Source: Schott Solar AG & ISAAC-SUPSI