Calculate glass G value, incident solar radiation, or total solar gain from any two inputs, with W/m² and BTU/hr·ft² units for windows and glazing.
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Glass G Value Formula
The glass G value compares the solar energy that passes through glass to the solar radiation that hits it. When G value is entered as a percentage, divide it by 100 before using it as a multiplier.
G = (TSG / ISR) * 100
TSG = ISR * (G / 100)
ISR = TSG / (G / 100)
- G = glass G value, expressed as a percentage
- TSG = total solar gain through the glass
- ISR = incident solar radiation hitting the glass
If you enter incident solar radiation and total solar gain, the calculator finds the glass G value. If you enter incident solar radiation and glass G value, it finds total solar gain. If you enter total solar gain and glass G value, it finds the incident solar radiation needed to produce that gain.
The calculator supports W/m² and BTU/hr·ft². Both radiation values are converted to W/m² during the calculation, then converted back to the selected output unit.
Typical Glass G Value Ranges
G value depends on glass type, coating, tint, and glazing system. Lower values mean less solar heat passes through the glass.
| Glass or glazing type | Typical G value | General meaning |
|---|---|---|
| Clear single glazing | 75% to 90% | High solar heat gain |
| Clear double glazing | 60% to 80% | Moderate to high solar heat gain |
| Tinted glazing | 35% to 65% | Reduced solar gain |
| Low-e solar control glazing | 20% to 50% | Lower heat gain, often used for cooling control |
| Highly reflective or specialist solar control glass | 10% to 30% | Very low solar heat gain |
Radiation Unit Conversion
| Conversion | Factor |
|---|---|
| BTU/hr·ft² to W/m² | multiply by 0.176116 |
| W/m² to BTU/hr·ft² | multiply by 5.67826 |
Example Calculations
Example 1: Calculate glass G value
You have incident solar radiation of 800 W/m² and total solar gain of 480 W/m².
G = (480 / 800) * 100
G = 60%
The glass G value is 60%.
Example 2: Calculate total solar gain
You have incident solar radiation of 700 W/m² and a glass G value of 45%.
TSG = 700 * (45 / 100)
TSG = 315 W/m^2
The total solar gain is 315 W/m².
FAQs
What does glass G value mean?
Glass G value is the percentage of incident solar energy that becomes solar gain through the glass. A G value of 60% means 60% of the incoming solar radiation passes through as solar gain, while the rest is reflected, absorbed, or otherwise not transmitted as useful gain.
Is G value the same as solar heat gain coefficient?
They are closely related. G value is commonly shown as a percentage, such as 60%. Solar heat gain coefficient, or SHGC, is usually shown as a decimal, such as 0.60. A 60% G value is equivalent to an SHGC of 0.60.
Is a lower glass G value always better?
Not always. A lower G value reduces solar heat gain, which can help reduce cooling demand. A higher G value allows more solar heat in, which can be useful in colder climates or during winter. The better value depends on climate, window direction, shading, and heating or cooling needs.