This article deals with assessing the differences between the values of the shading factors for windows with overhangs calculated under extraterrestrial and terrestrial conditions. It has been shown that the shading factor can be estimated under terrestrial conditions employing the available algorithms for the monthly average daily tilt factor for beam radiation. The present results have been validated by comparing with the values obtained by hour-by-hour calculations employing solar radiation data. The shading factor values evaluated under terrestrial conditions can differ by 25% compared to the extraterrestrial value for non-south facing windows shaded by overhangs.