Summary

Solar panel technology has advanced significantly in the past decade. Polycrystalline panels (the blue panels common in UK installations from 2010–2018) are now largely obsolete for new specifications. The current UK market is dominated by monocrystalline half-cut cell panels from Tier 1 manufacturers, typically offering 400–450Wp per panel at 20–22% efficiency.

For installers, panel selection affects the yield estimate, string design, mounting system compatibility, and long-term warranty exposure. Specifying a lower-quality panel to save £20 per panel can result in performance degradation, warranty claims, and brand damage. Understanding the panel technology options helps defend the specification to customers and competitors.

Key Facts

  • Monocrystalline — solar cells made from a single silicon crystal; higher efficiency than polycrystalline; current standard; recognisable by uniform black/dark grey appearance
  • Polycrystalline — cells made from multiple silicon crystals; slightly lower efficiency; blue mottled appearance; largely superseded in new UK installs
  • Half-cut cell — the solar cells are cut in half, reducing resistive losses and improving shade tolerance; the panel is wired as two independent half-panels in parallel within the same module; now standard on most Tier 1 panels
  • PERC (Passivated Emitter and Rear Cell) — a cell architecture that adds a reflective rear layer to improve light absorption; standard on most modern monocrystalline panels; efficiency ~21–22%
  • TOPCon (Tunnel Oxide Passivated Contact) — the current high-efficiency cell architecture succeeding PERC; efficiency ~22–24%; becoming standard on premium panels
  • HJT (Heterojunction Technology) — high efficiency (~23–24%); excellent temperature coefficient; used by REC, Panasonic (formerly), and others; premium price
  • Efficiency — the percentage of incident solar radiation converted to electricity; typical current UK residential specification: 20–22% for monocrystalline PERC; 22–24% for TOPCon/HJT
  • Power output (Wp, watt-peak) — the panel's rated output under Standard Test Conditions (STC: 1,000W/m² irradiance, 25°C cell temperature, AM1.5 spectrum); typical UK domestic panel: 400–450Wp
  • Temperature coefficient (Pmax) — the reduction in output per degree Celsius above 25°C; typically -0.25 to -0.35%/°C; lower is better; relevant for hot summer days in the UK (panels can reach 60–70°C)
  • Tier 1 manufacturer — Bloomberg New Energy Finance's (BNEF) classification of bankable solar panel manufacturers based on production volume, financial stability, and track record; Tier 1 list includes JA Solar, Trina Solar, LONGi, Canadian Solar, Jinko Solar, REC Group, Sunpower
  • Product warranty — the manufacturer's warranty on the physical panel (defects, delamination, discolouration): typically 25 years for Tier 1
  • Performance warranty — guarantees output over time; typically: ≥98% at year 1; linear degradation to ≥80.7% at year 25 (LonGi standard); or ≥80% at year 25 (standard market)
  • MCS 012 approved products — panels used in MCS installations must be on the MCS approved product list or meet equivalent certification (IEC 61215, IEC 61730)
  • IEC 61215 — the international standard for design qualification and type approval of crystalline silicon photovoltaic modules
  • IEC 61730 — the safety qualification standard for PV modules; required alongside IEC 61215 for MCS compliance

Quick Reference Table: Panel Technology Comparison

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Technology Efficiency Typical Wp Shade Tolerance Relative Cost Status
Polycrystalline 15–17% 250–300Wp Poor Low Largely obsolete for new builds
Monocrystalline PERC 20–22% 380–440Wp Moderate Medium Current standard
Monocrystalline half-cut PERC 20–22% 400–450Wp Good Medium Dominant UK spec
TOPCon 22–24% 420–475Wp Good–Excellent Medium–high Increasingly common; successor to PERC
HJT 23–24% 400–450Wp Excellent High Premium installs; excellent temperature performance

Detailed Guidance

Why Half-Cut Cell Matters for UK Roofs

The UK's variable irradiance and frequent partial shading (clouds, chimneys, aerials) make half-cut cell technology particularly relevant:

Reduced internal resistance losses: By halving the cell, the current through each cell is halved (resistance losses scale with I²). At peak summer output, a standard full-cell panel loses 1–2% more to internal resistance than an equivalent half-cut panel. For a typical 10-panel string, this compounds across the string.

Improved shade tolerance (within the panel): A half-cut panel is wired as two independent sub-modules in parallel. If the top half of the panel is shaded (by a chimney, for example), only the top sub-module output is reduced; the bottom sub-module continues operating at full output. A standard full-cell panel with the same partial shade would lose most or all output for the affected string.

Reduced hot spots: Lower operating current reduces the severity of hot spots on partially shaded cells. Hot spots accelerate cell degradation and are a leading cause of panel failure in shaded installations.

Tier 1 vs Budget Panels: The Risk Assessment

Tier 1 manufacturers (JA Solar, Trina, LONGi, Canadian Solar, Jinko) have bankable balance sheets, established UK supply chains, and track records of honouring product warranties. Tier 2 and budget panels may have lower upfront cost but carry risks:

  • Manufacturer financial instability: if the manufacturer ceases trading, the 25-year product warranty is worthless
  • Performance warranty claims: a manufacturer with limited UK presence is difficult to pursue for a performance claim 10 years into the installation's life
  • MCS compliance: not all budget panels are on the MCS approved list (or have the required IEC 61215/61730 certificates)

The price difference between Tier 1 and budget panels is typically £15–£30 per panel (trade). On a 15-panel system, this is £225–£450. The risk-adjusted cost of a warranty failure far exceeds this.

Recommended approach: Specify Tier 1 panels as standard. If a customer insists on budget panels to reduce cost, document the discussion and the risks clearly in writing.

Panel Size and Roof Compatibility

Larger panels (450Wp+) are becoming standard because they reduce the number of panels needed for a given kWp, reducing installation time and cost. However, larger panels have implications:

  • Weight: larger panels are heavier and more awkward to handle on a roof; two-person installation is required for panels over 25kg
  • Roof fit: larger panels (typically 2,100mm × 1,130mm) may not fit neatly on smaller roofs with dormer windows or chimneys; check the panel dimensions against the available roof space
  • Mounting compatibility: confirm the mounting system is rated for the panel weight and dimensions; rafter hook spacing must suit the panel's mounting hole pattern

Performance Warranties and What to Check

The performance warranty is the most important long-term commitment from the manufacturer. Read the small print:

Linear warranty (preferred): A linear performance warranty guarantees a specific output at each year of the panel's life, not just a cliff-edge guarantee at year 25. Example: 98% at year 1; 0.45% degradation per year; ≥88.6% at year 25. This is superior to a binary "80% at year 25" guarantee because it catches accelerated early degradation.

Degradation rate: Standard market: 0.55%/year (80% at year 25). Premium Tier 1 (LONGi, REC): 0.40–0.45%/year (84–86% at year 25). The difference is small per year but significant over 25 years — a customer with premium panels earns meaningfully more energy over the lifetime.

Claiming on a warranty: Performance warranty claims require monitoring data showing output below the warranted level. This is why panel-level monitoring (via SolarEdge, Enphase, or similar) is valuable — it provides the evidence needed to support a claim.

Frequently Asked Questions

Should I specify TOPCon or PERC panels for a standard domestic install?

For a standard domestic installation, PERC half-cut monocrystalline is entirely adequate and competitively priced. TOPCon is the future direction and offers marginally higher efficiency and slightly better temperature coefficients; it is worth specifying where the customer wants maximum output from a constrained roof area (limited panel count, want maximum kWp). For a standard roof with no space constraints, the price premium of TOPCon over PERC is harder to justify to the customer.

Are black-framed (all-black) panels worth the premium?

All-black panels (black frame + black backsheet) are purely aesthetic — they blend better with a dark roof. They typically cost 5–10% more than silver-framed panels of equivalent specification and may run slightly hotter (black backsheet absorbs more heat), marginally reducing output. For listed building or conservation area installations where a discreet appearance is important, the premium may be justified. For standard installs, the performance is equivalent and the cost is unnecessary.

How do I handle a customer who wants the cheapest panels available?

Explain the warranty risk clearly: the 25-year product and performance warranty is only as good as the manufacturer's ability to honour it in year 15 or 25. A manufacturer that folds, or that supplies defective panels in a quality lapse, leaves the customer with no recourse. Tier 1 panels cost marginally more; the long-term risk-adjusted cost is substantially lower. Most customers understand this argument when it is presented clearly.

Regulations & Standards