Window Solar Heat Gain Calculator

How it works

Q = A × I_peak × SHGC × shading fraction; BTU/h = W × 3.412; tons = BTU/h ÷ 12,000; daily kWh ≈ Q × sunny hours × 0.7 ÷ 1,000; AC electricity = heat ÷ COP

Sunlight through glass is often the largest single component of a home’s cooling load — a few square metres of unshaded west glass on a summer afternoon can equal the output of a space heater. The solar heat gain coefficient is the fraction of incident solar energy a window lets through, from 0.86 for clear single glass to 0.2 for spectrally selective coatings; multiplying it by the glass area and the irradiance for that orientation gives the watts. West and east windows take the low, direct sun that overhangs cannot block, and skylights see the full noon sun, so those are the priorities for solar-control glass or exterior shading; south glass under a properly sized overhang is largely self-shading in summer while collecting free heat in winter. Interior blinds help only modestly, because the heat is already inside the glass line.

Worked example

Six square metres of west-facing double low-e glass (SHGC 0.55) with no shading: 6 × 650 × 0.55 = 2,145 W at peak — 7,300 BTU/h, 0.6 tons of AC. Over five sunny hours that is about 7.5 kWh of heat, 2.1 kWh of electricity at COP 3.5, roughly $41 a season; solar-control glass (0.27) would halve it and exterior shades cut it 75%.

Frequently asked questions