A developmental version of the building energy analysis computer program BLAST was used to perform simulations of a prototypical, single-story office building. Total annual energy consumption was computed using Typical Meteorological Year (TMY) [1] weather data from three locations in the United States. For each location, two electric lighting designs were tested on the baseline building (no roof monitors) to compare the energy requirements of current-practice and more efficient electric lighting designs. Then roof monitors were added to evaluate their energy saving potential for each of the electric lighting designs. The roof monitors had highly diffusing, vertical glazings facing southeast and southwest. The results show that improving electric lighting system efficiency and adding roof monitors for daylighting both have the potential for substantially reducing lighting electricity and the energy cost of operating the building. The potential benefits of daylighting are substantially lower for a building outfitted with a more efficient electric lighting system, although still significant. To determine the limits of validity of the simulations, a number of sensitives studies were performed. Among the issues investigated were: dirt deposits, snow accumulation, glazing optical properties, interior design, luminous efficacy of admitted sunlight, and thermostatic controls.