The g-value gives the total solar energy transmittance, i.e. the fraction of solar energy (direct and indirect) that enters the building through a transparent element. The direct gains equal the total short wave transmissivity of the element. This fraction is zero with opaque elements. Indirect gains are obtained via absorption of solar energy, which is then radiated as heat. This fraction is > zero with both, opaque and transparent building elements. In steady state calculations, the g-value is usually only used with transparent elements, nevertheless. As input value for PHPP, the g-value needs to be assessed using EN 410.
The solar heat gain coefficient (SHGC) used in the United States and some other countries, for example New Zealand, describes the same physical property, but is assessed using deviating boundary conditions. Thus, g-value and SHGC can not be used interchangeably.