The energy transmittance (also g-value) is a measure of the energy permeability of transparent components. This indicates which proportion of the solar radiation energy, for example, can get through and contribute to heating inside. The g-value as the total energy transmission degree is the sum of the direct transmission of solar radiation, as well as the secondary heat emission inwards from radiation and convection. Loss is caused by reflection or absorption from or by the transparent component. The g-value is expressed pro rata. Thus, a g-value of 0.5 corresponds to a gain of 50% by combining the energy transmitted from the outside and any energy radiated back from/returned by convection from the interior.