Solar gain is the most variable component of cooling load. It changes with orientation, time of day, season, and glass type.
The formula
Q_solar = Window Area × SHGC × Incident Solar Radiation
Where:
SHGC = Solar Heat Gain Coefficient (0 to 1)
Modern low-e glass: 0.22–0.40
Older single-pane: 0.65–0.85
Orientation makes a 7x difference
Same 100 sq ft window, same building, same day (July, 4pm):
Orientation Solar Gain (BTU/hr) Why
──────────────────────────────────────────────────────
West 15,000–20,000 direct sun at hottest hour
South 8,000–12,000 sun high, partially blocked by overhang
East 3,000–5,000 direct sun was morning (already passed)
North 2,000–3,000 diffuse sky radiation only
West is worst because peak solar and peak outdoor temperature hit at the same time.
The afternoon spike
8am–2pm: building runs fine
3pm: west offices start overheating
4pm: supply air cannot keep up
5pm: complaints, thermostat wars
Cause: solar gain was averaged in the calculation
system sized for noon average, not 4pm peak
SHGC comparison
For a 200 sq ft west-facing window at 4pm peak (100 BTU/hr/ft² incident):
Old single-pane (SHGC 0.75):
Q = 200 × 0.75 × 100 = 15,000 BTU/hr
Modern low-e (SHGC 0.25):
Q = 200 × 0.25 × 100 = 5,000 BTU/hr
Difference: 10,000 BTU/hr = 0.83 tons
Replacing the glass cuts solar gain by 67% on that wall.
Shading effectiveness
External shading (overhang, fin): blocks 50–90% of direct sun
most effective on south walls
ineffective on west (low sun angle)
Internal shading (blinds): blocks 10–30% of heat gain
solar energy already inside
blind absorbs and re-radiates as heat
External shading before the glass > internal shading after the glass. Always.
The quick check
If solar gain > 20% of total cooling load:
→ size system for 3pm–5pm peak, not noon average
→ check west-facing glazing separately
→ consider SHGC upgrade or external shading
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