The majority of the fenestration systems are available in the market today are of the type whose solar gain properties can be determined through the use of a computational procedure. However there is a class of fenestration systems (or attachments) that cannot be handled with the conventional procedures like solar path diagrams and shading masks. This class includes the daylight systems. These systems are an innovative technology for sunlight redirection or sunlight exclusion. A new formula for the solar heat gain coefficient (SHGC) is introduced, which can help in the discussion process concerning the optimization of solar gains through the above mentioned systems. This formula is solar positioned and radiation weighted value and can provide an accurate way to estimate the energy flux through the window and can be used as a performance rating methodology for the above mentioned systems.