How to use the window solar heat gain calculator
Solar gain through glazing is window area × SHGC × incident irradiance × shading factor. The NFRC solar heat gain coefficient rates the whole window, glass and frame together, on a 0 to 1 scale, so swapping a 0.60 SHGC unit for a 0.30 one cuts the gain in half at the same area and orientation.
Worked check: a 100 ft² west window with SHGC 0.40 at 222 BTU/hr·ft² and no shading admits 8,880 BTU/hr, which is 2.60 kW, 88.8 BTU/hr·ft² of glass, and 0.74 tons of cooling from that one window. The orientation defaults represent peak clear-sky summer irradiance on a vertical surface at mid latitude, close to the long-published ASHRAE peak values, so enter hour-by-hour irradiance from local climate data when you need a real design number.
Treat the result as one component of the cooling load, not the equipment size. A full calculation adds envelope conduction, ventilation and infiltration, and internal gains from people, lighting and equipment, then applies the time lag from thermal mass using a method such as the radiant time series. The shading factors here are representative values that vary with product, color and mounting, so confirm final numbers against product test data with your mechanical designer.
Frequently asked questions
SHGC is the fraction of incident solar radiation admitted through the whole window, glass and frame together, rated from 0 to 1 by NFRC. The older shading coefficient used 1/8 inch clear glass as its reference of 1.0, so the two scales differ and convert roughly as SHGC equals 0.87 times SC.
No. This is only the solar gain through the glazing. A real cooling load adds conduction through walls and windows, ventilation and infiltration, and heat from people, lights and equipment, and it applies the time lag caused by thermal storage in the building.