Solar panel technology shifted dramatically between 2024 and 2026. The monocrystalline panels that dominated Irish rooftops for years now come in three distinct varieties, and the differences matter more than the monocrystalline label itself. PERC, the previous industry standard, has been replaced by N-type TOPCon as the mainstream choice, while HJT offers premium performance at a higher price. If your installer quotes a system without specifying cell technology, you need to ask which one you are getting because the wrong choice costs you thousands in lost generation over 25 years.
This guide explains PERC, TOPCon, and HJT in plain language, compares their performance in Irish weather conditions, and tells you which specification to demand for your home.
The Three Solar Cell Technologies You Will Encounter in 2026
Modern monocrystalline solar panels use one of three cell architectures. Each has different efficiency, degradation rates, and performance in Ireland's cloudy, diffuse-light conditions.
PERC (Passivated Emitter and Rear Contact)
PERC was the dominant technology from approximately 2018 to 2023. It uses P-type silicon (boron-doped) with a reflective passivation layer on the rear of the cell to recapture light. Module efficiency ranges from 19% to 21%, and degradation runs at 0.50% to 0.70% per year. At the time of writing (September 2026), PERC is being phased out in favour of N-type technologies. If your quote includes PERC panels, you are being offered outdated technology.
TOPCon (Tunnel Oxide Passivated Contact)
TOPCon replaced PERC as the residential standard in 2024 to 2025. It uses N-type silicon (phosphorus-doped) with a thin tunnel oxide layer and polycrystalline silicon layer on the rear of the cell, reducing electron recombination and increasing efficiency. Module efficiency ranges from 21.5% to 23.5%, and degradation runs at 0.35% to 0.45% per year. This is what Jinko Tiger Neo, JA Solar DeepBlue 4.0, Trina Vertex S+, and LONGi Hi-MO X6 use. TOPCon is now the minimum specification you should accept for an Irish installation in 2026.
HJT (Heterojunction Technology)
HJT combines monocrystalline silicon with thin amorphous silicon layers, creating a hybrid of crystalline and thin-film technologies. Module efficiency ranges from 22% to 23.5%, and degradation runs at 0.25% to 0.30% per year, the lowest of any mainstream technology. HJT offers exceptional low-light performance and the lowest temperature coefficient (less output loss when panels heat up). All HJT panels are naturally bifacial, capturing reflected light from the roof surface. The premium over TOPCon is approximately 10% to 20% per panel. Canadian Solar HiHero, REC Alpha Pure, and Huasun are the main HJT brands available in Ireland.
Why N-Type Outperforms P-Type in Irish Conditions
The distinction between P-type and N-type silicon matters more than efficiency ratings. P-type PERC cells use boron-doped silicon, which suffers from light-induced degradation (LID) caused by the boron-oxygen defect. In the first hours of sunlight exposure, P-type panels lose a percentage of their rated output permanently. N-type TOPCon and HJT cells use phosphorus-doped silicon, which avoids this defect entirely.
Over 25 years, this shows up in three ways. N-type panels start closer to their rated output, lose capacity more slowly, and retain a higher percentage of original performance at year 25. A TOPCon panel rated at 22.5% efficiency and degrading at 0.40% per year retains approximately 88.8% of original output at year 25 (producing 19.98% efficiency). A PERC panel rated at 20.5% efficiency and degrading at 0.60% per year retains approximately 85.0% of original output (producing 17.43% efficiency). The gap widens every year the system operates.
For Ireland specifically, N-type panels deliver better performance under the diffuse, overcast conditions that dominate the Irish year. TOPCon and HJT cells respond more strongly to scattered light than PERC, generating more electricity on cloudy days when the sun is not directly visible. This advantage compounds across an Irish winter, when diffuse light is often the only available light for weeks.
Performance Comparison for Irish Rooftops
Here is how PERC, TOPCon, and HJT compare on the metrics that matter for Irish installations.
| Metric | PERC (P-type) | TOPCon (N-type) | HJT (N-type) |
|---|---|---|---|
| Module efficiency | 19-21% | 21.5-23.5% | 22-23.5% |
| Temperature coefficient | 0.34%/°C | 0.29%/°C | 0.24-0.26%/°C |
| Degradation per year | 0.50-0.70% | 0.35-0.45% | 0.25-0.30% |
| Low-light performance | Baseline | Very good | Excellent |
| Output at year 25 | ~85% of rated | ~88-89% of rated | ~91-92% of rated |
| Bifacial capability | Limited | Yes (standard) | Yes (natural design) |
| Relative cost (per panel) | Lowest | Moderate | Premium (+10-20%) |
*Figures correct as of September 2026. Exact values vary by manufacturer and model.
Temperature coefficient measures how much output drops per degree Celsius above 25°C. Irish summer roof temperatures rarely exceed 35°C to 40°C, so the temperature coefficient difference between TOPCon and HJT (0.29% versus 0.24%) is marginal in Irish conditions. This matters more in hotter climates.
Low-light performance measures how effectively a panel converts diffuse, scattered light into electricity. This is where N-type panels deliver the most meaningful advantage in Ireland. HJT's amorphous silicon layers perform exceptionally well in low-light conditions, followed closely by TOPCon. Both significantly outperform PERC under overcast skies.
What You Should Specify for Your Irish Home
For the vast majority of Irish homes, N-type TOPCon monocrystalline panels at 22% or higher efficiency are the correct specification. TOPCon is the current industry standard, competitively priced, widely stocked across major brands, and delivers everything most homes need. It outperforms PERC on every relevant metric and costs approximately the same or less per watt of output.
Upgrade to HJT if you want the best long-term performance and lowest degradation, particularly if you plan to own the property for 20 years or more. HJT makes the most sense when your roof is predominantly north-facing or partially shaded, where every available photon from diffuse light becomes more valuable. The 10% to 20% panel premium typically recovers over the system's lifetime through additional generation, but the payback period on that premium is long (10 to 15 years).
Avoid PERC panels in 2026. They are outdated technology that degrades faster, performs worse in low light, and offers no meaningful cost advantage over TOPCon. If your quote includes PERC panels, ask the installer to swap them for N-type TOPCon at the same price. Most will comply because TOPCon is now the standard wholesale specification.
How Much Extra Generation Does N-Type Deliver?
The real-world output difference between PERC, TOPCon, and HJT depends on your roof orientation, shading, and local weather patterns. Here is a typical comparison for a south-facing 5 kWp system in County Dublin over 25 years.
PERC (P-type, 20.5% efficiency, 0.60% degradation per year):
- Year 1 output: 4,615 kWh
- Year 25 output: 3,920 kWh (85.0% retention)
- Total 25-year output: 106,835 kWh
TOPCon (N-type, 22.5% efficiency, 0.40% degradation per year):
- Year 1 output: 5,055 kWh
- Year 25 output: 4,490 kWh (88.8% retention)
- Total 25-year output: 121,305 kWh
HJT (N-type, 23.0% efficiency, 0.30% degradation per year):
- Year 1 output: 5,165 kWh
- Year 25 output: 4,760 kWh (92.2% retention)
- Total 25-year output: 125,445 kWh
TOPCon delivers 14,470 kWh more than PERC over 25 years. At €0.42 per kWh avoided grid cost, that is €6,077 additional value. HJT delivers 4,140 kWh more than TOPCon over 25 years, worth €1,739. The HJT premium of approximately €500 to €800 for a 5 kWp system pays back in 10 to 15 years, after which the extra generation is pure savings.
SEAI Grant and VAT Treatment
The SEAI Solar PV Grant of up to €1,800 applies to all three cell technologies, provided the system is installed by an SEAI-registered contractor. The grant covers €700 per kWp for the first 2 kWp, then €200 per additional kWp up to 4 kWp, for a maximum grant of €1,800. A 5 kWp system receives €1,800 regardless of whether it uses PERC, TOPCon, or HJT panels.
Ireland applies 0% VAT to the supply and installation of residential solar panels, so the headline price difference between cell types is the panel cost itself, not tax. This makes the TOPCon premium over PERC negligible and the HJT premium transparent.
For more details, see our SEAI grant guide.
Bifacial Panels and Irish Roofs
Bifacial panels generate electricity from both the front and rear surfaces, capturing reflected light from the roof material beneath. All HJT panels are naturally bifacial, and most TOPCon panels are now available in bifacial configurations. PERC panels have limited bifacial capability.
For standard rooftop installations, the rear-side generation from bifacial panels is modest, typically adding 2% to 5% to annual output versus monofacial panels, depending on roof material and installation height. Slate roofs reflect approximately 10% to 15% of incident light; concrete tiles reflect 15% to 20%. The bifacial benefit is larger on ground-mounted systems with high-albedo surfaces (gravel, white membrane) beneath.
In most cases, the bifacial benefit is a bonus rather than a deciding factor. If you are specifying HJT for its low degradation and excellent low-light performance, the bifacial capability comes included. Do not pay a premium for bifacial TOPCon panels purely for the rear-side generation; the return on that premium is marginal on a typical Irish roof.
What About All-Black Back-Contact Panels?
All Back Contact (ABC) and Interdigitated Back Contact (IBC) panels place all electrical contacts on the rear of the cell, eliminating the front-side busbars visible on conventional panels. This increases efficiency by 0.5% to 1.5% and creates an all-black aesthetic. Aiko Neostar (ABC, 23% to 25% efficiency) and SunPower Maxeon (IBC, 22% to 23% efficiency) are the main examples available in Ireland.
ABC and IBC panels deliver the highest efficiency available from silicon cells, but the premium over standard TOPCon is significant, approximately 15% to 30% higher per panel. This is justified primarily when roof space is very constrained (10 to 14 square metres) and you need to maximise watts per square metre. For most Irish homes with 20 to 30 square metres of available roof, standard TOPCon delivers sufficient output at lower cost.
Final Thoughts
Solar panel technology moved forward in 2024 to 2026, and PERC is no longer the standard. N-type TOPCon is now the mainstream specification for Irish residential installations, offering 21.5% to 23.5% efficiency, 0.35% to 0.45% annual degradation, and excellent low-light performance at competitive pricing. HJT is the premium choice for homeowners who want the best long-term yield and are willing to pay 10% to 20% more per panel for 0.25% to 0.30% annual degradation and exceptional diffuse-light response.
If your installer quotes a system without specifying cell technology, ask which one you are getting. If the answer is PERC, ask for N-type TOPCon instead. The cost difference is minimal, and the performance difference compounds over 25 years. TOPCon is the minimum specification you should accept in 2026.
Ready to Specify N-Type TOPCon or HJT Panels?
WattCharger works with SEAI-registered installers who use current-generation N-type panels as standard. Get your free consultation today and specify the exact technology you want for your roof.
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Blog Author: Rowan Egan