About Daylight & Solar Gain Calculator
The daylight and solar gain calculator analyses glazing systems for solar heat gain, visible light transmittance, and daylight factor, then checks them against ASHRAE 90.1, the NCC, and EN limits. It is used by architects and facade engineers to select glazing and size shading.
Choose a glazing build-up and frame, set the window geometry, location, and shading device, and the tool returns whole-product SHGC and VLT, the light-to-solar-gain ratio, solar heat gain with shading, an average daylight factor, and a pass or fail against the chosen standard.
How It Works
- Compute whole-product SHGC, VLT, and U-value by frame-weighting the centre-of-glass values.
- Determine the solar position and angle of incidence, then the irradiance on the glazing.
- Calculate solar heat gain Q = SHGC * A * I * F_shading and apply the overhang shading factor.
- Estimate the average daylight factor with the BRE split-flux method and compare SHGC and U-value to code limits.
Worked Example
A 2.0 m by 1.5 m window (3.0 m2) with whole-product SHGC = 0.35 under 600 W/m2 incident irradiance and a shading factor of 0.8 gives a solar heat gain of 0.35 * 3.0 * 600 * 0.8 = 504 W.
Formulas
- Whole-product SHGC
SHGC_w = SHGC_c * (1 - f) + SHGC_frame * f- Solar heat gain
Q = SHGC * A * I * F_shading- Average daylight factor (BRE split-flux)
DF = (A_glazing * T_vis * theta) / (A_total * (1 - rho^2)) * 100- Light-to-solar-gain ratio
LSG = VLT / SHGC
Standards & References
- ASHRAE 90.1-2019
- NCC 2022 (BCA)
- EN 17037 (BRE split-flux)
Frequently Asked Questions
What is the difference between SHGC and VLT?
SHGC (solar heat gain coefficient) is the fraction of incident solar energy that passes through the glazing as heat; VLT (visible light transmittance) is the fraction of visible daylight transmitted. A high VLT with a low SHGC, giving a light-to-solar-gain ratio above about 1.25, indicates spectrally selective glazing.
How is the daylight factor calculated?
The tool uses the BRE split-flux method: the average daylight factor depends on glazing area, visible transmittance, a sky angle factor, room surface area, and the average internal reflectance.
Which compliance standards are checked?
It checks glazing SHGC and U-value against ASHRAE 90.1-2019 (US, by climate zone), the NCC 2022 (Australia), and EN limits, returning a pass or fail for each property.
Does it account for shading devices?
Yes. An external overhang is modelled by projecting the solar shadow over the window height, giving a shading factor that reduces the effective SHGC and solar heat gain.