Windows, similar to constructions, are also dependent on a set of values known as solar heat gain coefficient (SHGC) values. These values determine the heat gain at a given time of day (and therefore the incident angle of the sun). At this time, the engine is unable to derive SHGC values given properties of a window and is therefore dependent on having them sent in directly. Like a construction's CTS values, the SHGC values are used in conjunction with the U-value for cooling loads. Only the U-value is used during winter for heating calculation.