Tempered glass generally has higher strength and breaks into smaller fragments than heat-strengthened glass. Heat-strengthened glass has lower surface compression and forms larger fragments, but it can still be used in solar modules. The choice depends on glass thickness, module construction and test results—not the glass name alone.
Key Differences
Both types undergo heating and controlled cooling. Faster cooling produces the higher surface compression associated with fully tempered glass. That compression helps prevent small surface cracks from opening under stress.
| Property | Fully tempered glass | Heat-strengthened glass |
|---|---|---|
| Surface compression | Higher, providing greater resistance to crack opening | Lower, providing less strengthening |
| Strength at the same thickness | Generally withstands higher stress before breaking | Generally breaks at a lower stress level |
| Breakage pattern | Typically shatters into many small fragments | Typically forms larger fragments |
| Resistance to bending | Heat treatment does not substantially change elastic stiffness | Similar elastic stiffness at the same material, thickness and dimensions |
| Solar-module suitability | Must be assessed within the complete module | Must also be assessed within the complete module |
Smaller fragments do not make a broken panel safe. IEA PVPS identifies compromised insulation, moisture entry and possible hot spots or arcing as consequences of module glass breakage. Larger heat-strengthened fragments are not guaranteed to remain in place either; retention depends on the laminate and support system.[1]
Glass Thickness in Actual Modules
There is no single thickness that identifies a good solar panel. Tongwei’s published module specifications illustrate three different constructions:
- G12R-66 monofacial, up to 635W: 3.2mm tempered front glass, a white backsheet and a listed module weight of 29.0kg.
- G12R-66 bifacial, up to 630W: 2.0mm tempered front glass, 2.0mm “semi-tempered” rear glass and a listed weight of 32.5kg.
- G12R-48 bifacial, up to 455W: 1.6mm tempered front glass, 1.6mm “semi-tempered” rear glass and a listed weight of 22.4kg.
These are manufacturer-listed examples, not independent durability rankings. They show why neither “all double-glass panels use fully tempered glass” nor “glass below 2.8mm is a defective specification” is a reliable buying rule.
Thin glass also needs closer scrutiny. IEA PVPS reports that achieving the surface compression of thicker, fully tempered glass is more difficult with glass at or below 2mm. Its 2025 review also identifies limitations in standardised quality assessment for thin PV glass. A datasheet’s word “tempered” should therefore not be treated as proof that every thickness has the same strengthening level.[2]
What Hail Tests Prove
A 25mm hail test is not a guarantee against every hailstorm. In research reviewed by IEA PVPS, both 2mm glass/glass and 3mm glass/backsheet modules survived the IEC 61215-2 benchmark using 25mm ice balls. Their resistance at higher impact energies was different.[2]
For a hail-prone site, request the actual test report. Check the ice-ball diameter, impact speed, impact locations and the electrical tests performed after impact. A certificate alone does not tell you how much impact resistance remains beyond the tested conditions.
Wind and Snow Ratings
Tongwei’s application specifications list wind/snow loads of 2400Pa/5400Pa for the examples above, including those with semi-tempered rear glass. These published figures do not establish a universal requirement for every solar module.
Before comparing load ratings, confirm whether each figure is a test load or an allowable design load. Also check the approved rail spacing, clamp positions and mounting orientation. The U.S. Department of Energy treats module selection, racking, attachments and foundations as parts of the same weather-resilience design—not separate purchases.[3]
Documented Installation Failures
IEA PVPS’s field-oriented failure sheets include examples of glass broken by over-tightened screws and poor clamp design. They also identify clamps that are too short or narrow, incorrect clamp positions, module twisting and edge impacts as potential causes. Thin rear-glass cracks can be harder to spot during a front-only inspection.[1]
At delivery, check corners, edges and rear glass. Before installation, match the clamp dimensions, support positions and tightening torque to the exact module manual. Do not substitute a universal torque value or panel gap.
Avoid labelling every unexplained crack “spontaneous breakage.” Photograph the crack origin, clamp locations and frame condition before removal. Those details help the manufacturer distinguish impact damage, mounting stress and possible manufacturing defects.
What to Check Before Ordering
- Exact construction: Confirm front and rear glass thicknesses and treatment types. If the supplier uses “semi-tempered,” ask which heat-strengthening specification it follows.
- Model-specific certification: Match the quoted model to the certificate’s covered model family and construction. Check IEC 61215 design qualification and IEC 61730 safety qualification, along with local requirements.
- Supporting documents: Obtain the current datasheet, installation manual and relevant hail and mechanical-load reports. Tongwei’s product-document download centre provides the starting point for its module documentation.
- Warranty terms: Separate product-defect coverage from the power-output warranty. Neither is determined by glass treatment alone.
IEC 61215 explicitly states that qualification results are not a quantitative prediction of module lifetime. Passing its tests does not, by itself, prove 25 or 30 years of service or a particular annual degradation rate.[4]
Facades and Overhead Glazing
A facade, canopy or glazed roof needs more than a standard module specification. The design must address falling fragments, retention after breakage, structural loads, electrical safety and applicable building requirements. Neither fully tempered nor heat-strengthened glass alone guarantees that a broken panel will remain overhead.
Do not adopt a generic “6mm glass plus 1.52mm PVB” build without project assessment. Glass thickness, interlayer, panel span and fixing details must suit the actual application.
Cracked Panel Glass
A panel may still generate power after its glass cracks. That does not confirm electrical safety. Keep people away from damaged glass and have a qualified technician assess the module and isolate it where required. IEA PVPS recommends replacing damaged modules; adding resin or replacing only the glass does not establish that the original safety performance has been restored.[1]