Computer programs which simulate the heating and cooling loads in buildings usually require the artificial lighting load to be part of the data, generally specified by hour in units of power per unit area. Consequently, no provision is made for the degree to which outside light could satisfy the illumination requirement in a perimeter zone. We show in this paper that there is a severe energy penalty for that mode of operation. The paper begins by describing the atmospheric model and radiation datasets from which we have calculated the direct and diffuse solar radiation in the visible band impinging upon horizontal and vertical surfaces. Next, a new model for calculating the natural light in a perimeter zone is described. Lastly, the paper gives two examples which demonstrate the significant degree to which natural light can satisfy the illumination requirements, and the large cost savings which result.