The solar transmittance of a surface is the fraction of the sun's radiation that is transmitted through the surface. Solar transmittance is a factor in the calculation of the PV array temperature. Because it is relatively unimportant, rather than having you enter it, HOMER uses the assumption suggested by Duffie and Beckman (1991) that the product of the solar absorptance and the solar transmittance is 0.9 or 90%.
See also
