During daytime, sunlit windows transmit much more heat due to direct solar radiation than by conduction through the window which is caused by the difference in temperature between inside and outside.

But even accounting for windows in the shade, direct SOLAR radiation can easily contribute 80% or more of the total peak daytime window heat gain.

During the day, the total peak heat gain through the windows can be more than 10 times as much as through the walls and ceiling combined.