Solar panel technology moves faster than most buyers realise. A panel specified in 2020 would be considered mid-range by 2026 standards. The technology that defined the market for most of the 2010s — PERC — is being systematically retired as factories retool for TOPCon, and heterojunction (HJT) is establishing a premium tier that was barely commercially available three years ago. Understanding these technologies lets you decode manufacturer specifications and make a choice you will not regret over a 25-year panel life.
The Cell Technologies
PERC (Passivated Emitter Rear Cell)
PERC added a passivation layer to the rear of conventional monocrystalline silicon cells, capturing light that would otherwise be wasted. Introduced commercially around 2016, PERC drove panel efficiency from around 17% to 20–21% at module level and became the global standard by 2020.
In 2026, PERC is a mature, proven technology available at the lowest price point. Average module efficiency is now 20.5–21.5%. If you are looking for the lowest cost per watt with a reliable, tested technology for a simple south-facing roof with no shading, PERC remains a credible choice — though the price gap with TOPCon has narrowed to the point where TOPCon is frequently the better value.
TOPCon (Tunnel Oxide Passivated Contact)
TOPCon places a thin tunnel oxide layer and doped polysilicon film on the cell rear, dramatically reducing recombination losses. Module efficiency has climbed to 22–23% for mainstream commercial TOPCon panels in 2026, with laboratory cells from Fraunhofer ISE and LONGi reaching over 26%.
TOPCon accounts for over 55% of global panel shipments in 2026 (BloombergNEF data). Major producers include LONGi Hi-MO 7, Jinko Tiger Neo, Trina Vertex S+, and Canadian Solar TOPBiHiku6. Key advantages over PERC: lower temperature coefficient (panels lose less efficiency on hot days), better low-light performance, and higher energy yield per square metre.
Temperature coefficient: PERC typically -0.35 to -0.40%/°C. TOPCon: -0.28 to -0.32%/°C. In a climate with 35°C roof temperatures, this difference alone contributes 1–2% more annual energy yield from TOPCon.
HJT (Heterojunction Technology)
HJT sandwiches amorphous silicon layers around a crystalline silicon wafer, combining the properties of both materials. The result is the highest commercially available cell efficiency — 23.5–25% at module level from leading manufacturers including REC Alpha Pro, Panasonic EverVolt, and Huasun Himalaya.
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HJT panels also have the lowest temperature coefficient of any commercial silicon technology (-0.24 to -0.26%/°C) and outstanding low-light performance. In diffuse-light climates (northern Europe, overcast tropical regions), HJT's low-light advantage is measurable.
The trade-off: HJT is more expensive to manufacture — the amorphous silicon deposition process requires different equipment from standard c-Si production lines, and manufacturing scale is smaller. Per-watt prices for HJT run 15–30% above TOPCon in 2026. For most residential installations, the efficiency premium does not justify the cost premium unless roof space is severely limited.
Glass-Glass vs Glass-Backsheet
This refers to the panel encapsulation, not the cell technology — both surfaces can be paired with either construction.
Glass-backsheet: The traditional construction. A tempered glass front, a polymer backsheet rear. Lighter (typically 18–20 kg for a standard 400W panel), lower cost, and the industry default for most residential and commercial installations.
Glass-glass (dual-glass): Both surfaces are glass. Heavier (22–25 kg typically), and more expensive, but offers significant advantages:
- Superior moisture barrier: ideal for humid coastal environments
- Better fire resistance (Class A rating easier to achieve)
- Essential for bifacial panels (rear glass allows rear-side light capture)
- Longer expected lifetime: many glass-glass panels carry 30-year product warranties vs 25-year for backsheet
For most residential pitched-roof installations with no special environmental concerns, glass-backsheet is appropriate. Glass-glass is worth the premium for coastal installations, flat-roof commercial projects where bifacial rear-side gain is significant, or any installation prioritising maximum lifetime.
Monofacial vs Bifacial
Monofacial panels generate power only from the front surface. They are simpler, lighter, and cheaper. Appropriate for most pitched-roof installations where the rear surface faces the roof and has no access to reflected light.
Bifacial panels capture additional light reflected from surfaces behind and around the panel (ground, white membranes, light-coloured walls). Rear-side gain depends entirely on the albedo of the surface below:
- Dark asphalt ground: 2–4% rear gain
- Concrete or gravel: 6–10% rear gain
- White membrane roof: 10–18% rear gain
- Snow-covered ground: 15–25% rear gain
Bifacial panels require glass-glass construction (glass rear allows light transmission). They are the recommended choice for ground-mount, flat-roof ballasted, and agrivoltaic installations where rear-surface light is available. For standard pitched roofs, bifacial is generally not worth the premium — the rear surface faces dark roofing material.
Technology Comparison Table
| Attribute | PERC | TOPCon | HJT |
|---|---|---|---|
| Module efficiency (2026) | 20.5–21.5% | 22–23% | 23.5–25% |
| Temperature coefficient | -0.35 to -0.40%/°C | -0.28 to -0.32%/°C | -0.24 to -0.26%/°C |
| Low-light performance | Moderate | Good | Excellent |
| Relative cost per Wp | Lowest | Low-medium | Medium-high |
| Bifacial availability | Limited | Yes | Yes |
| Typical product warranty | 25 years | 25–30 years | 25–30 years |
How to Choose
Standard pitched roof with good orientation: TOPCon at mainstream brand pricing. The efficiency and temperature coefficient advantages over PERC now come with minimal cost premium. Brands: Jinko Tiger Neo, LONGi Hi-MO 7, Trina Vertex S+.
Space-constrained installation: HJT for maximum watts per square metre. Accept the cost premium in exchange for more power from limited roof area. Brands: REC Alpha Pro, Panasonic EverVolt.
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Hot climate: TOPCon or HJT, both of which outperform PERC in high-temperature conditions due to lower temperature coefficients.
Coastal or harsh environment: Glass-glass construction, TOPCon or HJT, with IEC 61701 salt-mist certification.
Ground-mount or flat roof: Bifacial TOPCon or HJT with glass-glass construction. Model rear-side gain using realistic surface albedo values before committing to bifacial premium pricing.