A new version of the ENERGY-10 computer program simulates the performance of photovoltaic (PV) systems and evaluates a wide range of opportunities to improve energy.,, efficiency in buildings. This paper describes two test cases in which the beta release of, ENERGY-10 version 1.4 was used to evaluate energy efficiency and building-integrated. photovoltaics (BIPV) for two federal building projects: an office and laboratory building at the Smithsonian Astrophysical Laboratory (SAO) in Hilo, Hawaii, and housing for visiting scientists at the Smithsonian Environmental Research Center in Edgewater Mary, land The capabilities of the software, the design assistance provided by ENERGY-10, and, a synopsis of results are given. Estimates of annual energy delivery by the five PV arrays, of the SAO are compared to F-Chart to help inform a validation of ENERGY-10. Results. indicate that, by simulating both the building electrical load and simultaneous PV performance for each hour of the year ENERGY-10 facilitates a highly accurate, integrated analysis useful early in the design process. The simulation is especially useful in calculating the effect of PV on the building peak load, and associated demand cost savings.