Two versions of the DOE Cool Roof Calculator are on the Internet to assist in the selection of an energy-saving surface for low-slope roofs. The CoolCalcEnergy version, for small and medium-sized commercial facilities, gives savings based on costs per unit of energy. This paper documents how the CoolCalcPeak version was added to include estimates of savings in demand charges that large facilities often incur for electricity. The database for the Cool Roof Calculator was reworked. Differences in peak deck heat fluxes with and without solar radiation control were obtained over the cooling season for varying locations (as characterized by average solar insolation), R-value of the low-slope roof, and solar reflectance and infrared emittance of the roof surface. Peak solar insolation is relatively constant over the U.S. Thus, lower demand charges save about the same amount of annual operating costs for a particular set of circumstances in all U.S. climates that have significant cooling requirements.