A new computer program has been written for comparing the annual energy and economic performances of different window systems in non-residential buildings. The program, called CSHADE, performs a sideby-side hourly energy performance simulation of a test window system compared to a reference one. A comprehensive report is printed out at the completion of each simulation. The program is much simpler and easier to use than large building simulation programs such as DOE-2 or BLAST, and has relatively more modest computer requirements. Results are reported on the use of the new program to compare the performances of seven different window systems installed in buildings in Miami and Chicago.