When specifying solar film on international projects or reading a bilingual technical data sheet, two indicators often appear side by side without explanation: the g-value (European standard) and the SHGC (Solar Heat Gain Coefficient, American standard). Both measure the same physical reality, but their calculation methods differ slightly — which can lead to discrepancies when interpreting performance data.
Definitions: What Each Indicator Measures
The g-value (also referred to as the solar factor or total solar energy transmittance) is defined by European standard EN 410. It represents the fraction of total incident solar energy transmitted into the building — both directly (short-wave radiation) and as heat re-emitted by the glazing (long-wave radiation). A g-value of 0.30 means that 30% of incident solar energy enters the building. Values range from 0 to 1 (or 0% to 100%).
The SHGC (Solar Heat Gain Coefficient) is defined by American standard NFRC 200. It measures the same physical quantity: the fraction of total solar energy entering the building. Like the g-value, it ranges from 0 to 1.
As a first approximation, g-value and SHGC are numerically very close — within a few percentage points — for standard glazing. However, methodological differences do exist.
Methodological Differences
EN 410 uses a reference solar spectrum of type "air mass 1.5" (AM 1.5), suited to European weather conditions (temperate solar radiation). The NFRC standard also uses a spectral distribution, but with slightly different reference conditions (temperature, wind speed, solar flux).
The main practical difference concerns the treatment of surface emissivity and convective transfers within the glazing assembly. The NFRC calculation incorporates specific convection conditions that can slightly alter the final result. For standard monolithic glazing, the difference between g-value and SHGC is typically less than 3 points. For more complex systems (double glazing with Low-E coating or applied film), the gap can reach 5 to 8 points.
Practical Application for Solar Films
When a solar film is applied to glazing, its g-value (or its contribution to reducing the overall SHGC of the glazing-plus-film system) is measured against one standard or the other, depending on the testing laboratory.
Solar Screen technical data sheets report the g-value of the glazing-plus-film assembly (per EN 410), calculated on a reference base of 3 mm clear glass or 4-16-4 double glazing depending on the configuration. For example, a film with a g-value of 0.30 on clear double glazing means that total solar energy entering the space is reduced to 30% of incident radiation.
To convert a g-value to SHGC (or vice versa) in the context of an international specification, the following rule of thumb applies:
- For values measured on single glazing: SHGC ≈ g-value (±2%)
- For values measured on double glazing with Low-E: the gap can reach ±5% depending on boundary conditions in the standard
In practice, for routine technical decisions, the difference is marginal and does not justify systematic recalculation. However, for projects subject to strict energy certification (LEED, BREEAM, RE2020), compliance with the standard specified in the project brief is mandatory.
Reading a Bilingual Technical Data Sheet
Solar Screen data sheets present values in accordance with EN 410. If an American specifier or a design team working with NFRC standards requests values in SHGC, the recommended approach is:
- Identify the reference glazing used for the calculation (single or double)
- Apply the numerical correspondence (typically equivalent to ±3%)
- State in the submitted document that the value is derived from EN 410 and approximately converted
To avoid any ambiguity, some manufacturers — including Solar Screen — report both indicators on their data sheets, or offer custom simulations for the specific project glazing.
For a detailed understanding of how solar film performance is measured on commercial buildings, a Solar Screen guide covers the assessment methods used in professional projects.
Key Takeaways
| Indicator | Standard | High value = | Scale |
|---|---|---|---|
| g-value | EN 410 (Europe) | High heat transmitted | g = 0 → 1 |
| SHGC | NFRC 200 (USA) | High heat transmitted | SHGC = 0 → 1 |
| Equivalence | ~±2 to 5% depending on glazing | — | Approximate conversion |
To access the full technical data sheets for Solar Screen solar control films, including g-values measured per EN 410 by glazing configuration, visit the dedicated solar control range page.