For windows and skylights, the engine uses the following equations to calculate heat gain:
Direct beam solar heat gain qb:
(13)
Diffuse solar heat gain qd:
(14)
Conductive heat gain qc:
(15)
Total fenestration heat gain Q:
(16)
where
= window area, ft²
, 
, and 
= direct, diffuse, and ground-reflected irradiance
= direct solar heat gaincoefficient as a function of incident angle q; may be interpolated between values
= diffuse solar heat gain coefficient (also referred to as hemispherical SHGC)
= inside temperature, °F
= outside temperature, °F
= overall U-factor, including frame and mounting
Once this breakdown of all the space loads and coil loads is finished, the engine determines the month and hour for the maximum zone load (for the purposes of display, a zone load is always a constant volume load, or peak of the sums, regardless if the individual spaces are variable volume). The engine then moves on to deal with plenums and psychrometrics.