The four warranty layers
Every commercial solar installation in the UK is covered by four overlapping warranty regimes. Understanding each is essential to comparing proposals on like-for-like terms.
1. Panel manufacturer warranty
Major UK-market manufacturers (JinkoSolar, LONGi, Trina, JA Solar, Canadian Solar, REC) currently offer 25-30 year linear power warranties on their commercial-grade modules. The standard guarantees:
- 98% of nameplate power at year 1
- 87-92% at year 25-30 (linear decline)
Materials and workmanship warranty (the physical module construction) is typically 12-15 years separately.
2. Inverter manufacturer warranty
Inverter warranties vary by manufacturer and product line. 2026 typical:
- Solis 100K-300K Pro+: 10 years standard, extendable to 20
- Huawei SUN2000: 10 years standard, extendable to 20
- Sungrow SG110CX: 10 years standard, extendable to 25
- SMA Sunny Tripower CORE: 10 years standard, extendable to 25
- Fronius Tauro: 12 years standard, extendable to 25
Inverter replacement at year 12-15 is typically planned into the 25-year economic model — modern inverters cost £35-60 per kW vs typical £900/kW system cost.
3. Workmanship warranty
Installer-provided warranty covering installation quality — typically 5 or 10 years. Covers mounting system integrity, cabling, switchgear, commissioning quality, and any issues arising from installer practices rather than component defects.
5-year workmanship warranty is standard from MCS-certified installers. 10-year warranties are available from premium installers and typically priced into the proposal.
4. Insurance-Backed Warranty (IWA)
The fourth layer protects against installer business failure. If the installer goes out of business, the IWA insurer steps in to honour the workmanship warranty. IWA is provided by specialist insurers (QANW, GPI, Cumbria Insurance) typically as 10-year cover.
For commercial buyers, IWA is the most important of the four warranties because it’s the one that protects against the failure mode (installer business failure) that the customer can’t directly assess at point of sale.
What good warranty looks like
For a £200k+ commercial office solar install in 2026, look for:
- Panels: 25-year linear power, 12-year materials/workmanship — from a top-10 global manufacturer
- Inverters: 10-year standard extended to 20-25 years — from an established UK-service brand
- Workmanship: Minimum 5 years, preferably 10
- IWA: 10-year insurance-backed cover from a regulated UK insurer
If any of these are absent or significantly below market norms, ask why.
The questions that catch bad proposals
Four questions reliably separate good warranty packages from poor ones:
1. “Who is the IWA insurer and what’s their A.M. Best rating?” Unregulated or low-rated insurers offer little real protection. QANW (rated A by A.M. Best) and GPI are the major UK PV IWA providers.
2. “What happens if the panel manufacturer goes out of business?” Brand-name manufacturers (Jinko, LONGi, Trina, JA, Canadian, REC) have UK-based or EU-based legal entities for warranty claims. Niche manufacturers may have no UK presence — making warranty claims practically difficult.
3. “Is the inverter warranty extension paid up-front or pay-on-use?” Up-front payments are baked into capex; pay-on-use means you might face a £15-25k charge at year 11 if you forgot to extend. Always confirm payment timing.
4. “What’s covered by workmanship warranty and what’s specifically excluded?” Bad installers exclude common failure modes (water ingress, cable degradation, switchgear faults). Good warranties cover all installation-related issues.
Our standard warranty package
For office solar projects we deliver:
- 25-year linear power on panels (Tier-1 manufacturers only)
- 12-year materials/workmanship on panels
- 10-year standard inverter, extended to 20 years (paid up-front in capex)
- 10-year workmanship warranty
- 10-year IWA via QANW (A-rated insurer)
- 5-year O&M service inclusive (annual inspection, monitoring software)
- Optional 25-year O&M extension at £8-15/kWp/year
Total warranty cover is meaningful — and 10-year IWA protection means our installations remain warranted even if SEO Dons Ltd ceases trading.
Request a feasibility study with full warranty documentation.
Panel degradation guarantees in detail: LID, LETID, and PID
The 25-30 year linear power warranty on solar panels covers output degradation over time — but the degradation mechanism matters, and different types of degradation are covered differently.
LID (Light-Induced Degradation). All crystalline silicon panels experience an initial power loss of 1-3% during the first 1,000 hours of operation, as boron-oxygen complexes form under light exposure. This is a normal, predictable phenomenon. Top-tier 2026 panels (LONGi Hi-MO 6, JinkoSolar Tiger Neo, Trina Vertex S+) now use n-type TOPCon cell architecture which eliminates LID almost entirely. The power warranty on n-type panels starts from a higher effective baseline as a result.
LETID (Light and Elevated Temperature Induced Degradation). A more complex degradation mechanism affecting certain p-type PERC cells, manifesting as power loss of 2-6% during high-temperature operation in the first 1-2 years. Well-characterised by manufacturers since 2018. Most major manufacturers’ current PERC products have LETID mitigation applied. Check whether the specific module model has LETID testing certification (IEC TS 63126).
PID (Potential-Induced Degradation). Caused by high system voltage driving ion migration through the encapsulant and glass, resulting in shunting of cell edges. PID is a significant concern for systems operating above 800V DC. Mitigation strategies: PID-resistant cell encapsulant (most major manufacturers now standard), negative-frame-to-earth grounding, and anti-PID modules (certified to IEC 62804). For commercial systems where DC bus voltage regularly reaches 900-1000V, PID resistance certification should be explicitly specified.
LeTID vs PID in warranty claims. Both LID and LETID degradation within the first 2 years of operation are covered by the power warranty. PID, however, can be complicated — if PID results from an installation error (incorrect grounding, undersized cable insulation) rather than a product defect, the claim may fall on the installer’s workmanship warranty rather than the manufacturer’s product warranty. Understanding this distinction is important when evaluating multi-layer warranty packages.
IWA warranty vs manufacturer warranty: the critical difference
The Insurance-Backed Warranty (IWA) and the manufacturer’s product warranty serve entirely different purposes and protect against entirely different risks. Understanding the distinction is essential to evaluating any commercial solar warranty package.
Manufacturer warranty. The manufacturer guarantees the product performs to its technical specification for the warranty period. Claims are made to the manufacturer’s UK subsidiary or EU entity. The manufacturer bears the obligation to repair or replace defective products.
Risk not covered: the manufacturer going out of business. Several PV manufacturers from the first solar boom (Solibro, Q.CELLS predecessor entities, Sanyo) have exited the market, leaving their warranties effectively unenforceable. Even large manufacturers can be acquired, restructured, or have UK operations wound down.
IWA (Insurance-Backed Warranty). The IWA is purchased by the installer and covers the installer’s workmanship warranty — meaning it protects the customer if the installer goes out of business and can no longer honour their workmanship commitment. The IWA insurer steps in to arrange remediation at no additional cost to the customer.
Risk not covered by IWA: product defects. A panel that fails due to manufacturing defect is still a manufacturer warranty claim. The IWA only covers installation-quality failures (poor mounting, cable degradation, switchgear faults attributable to installer workmanship).
What this means in practice: A complete warranty package needs all four layers because each covers a different failure mode:
- Panel product defect: manufacturer warranty
- Inverter product defect: manufacturer warranty
- Installation quality failure: workmanship warranty (backed by IWA if installer fails)
- Installer business failure: IWA insurer steps in to honour workmanship warranty
Without IWA, if the installer fails in year 3 and there is a workmanship defect causing system underperformance, the customer has no practical recourse. This is not a theoretical risk: the UK solar installer market has seen multiple business failures (Carillion Sustainable Solutions, Mark Group, Home Energy Scotland — all ceased trading).
What voids warranties
Four common scenarios that void or complicate warranty claims:
1. Unauthorised modification. Any change to the installed system — additional panels, inverter replacement with a different model, switchgear modification — that is not documented and notified to the relevant warranty provider can void coverage. The key test is whether the modification is “substantially different” from the warranted specification. Replacing a failed inverter with the same model (under manufacturer warranty) is fine. Replacing with a different brand’s model without notifying the IWA insurer may affect workmanship warranty coverage.
2. Improper maintenance. Panel warranties typically require that modules are maintained in accordance with the manufacturer’s cleaning and inspection recommendations. IWA workmanship warranties may include a maintenance obligation clause. Failure to carry out periodic EICR inspections, or allowing known faults to persist without remediation, can give insurers grounds to dispute claims.
3. Physical damage from excluded events. Most panel warranties exclude physical damage from hailstones above IEC 61215 test specification (25mm at 23 m/s), flood damage, subsidence, or fire. These events fall to building insurance rather than product warranty. Make sure your building insurer is aware of the PV installation to ensure these events are covered.
4. Transferring ownership without warranty transfer notification. On building sale, product warranties typically transfer to the new owner automatically (they run with the asset, not the entity). However, IWA policies usually require formal assignment notification to the insurer within 30 days of sale. Failure to notify can result in the new owner not being recognised as the named insured.
Verifying warranty in due diligence
For property acquisition or refinancing due diligence on buildings with existing PV installations, four documents are essential:
1. MCS Installation Certificate. Confirms the system was installed by a certified contractor to MCS standards. Available from the MCS national database using the site address.
2. IWA Certificate. Names the insurer, policy number, the assured (should be current building owner or transferable to purchaser), system specification, and coverage term. Should be from an insurer with an A.M. Best rating of B+ or above (QANW is A-rated).
3. Panel manufacturer warranty certificates. Confirming panel model, serial numbers, warranty term, and manufacturer entity. Serial numbers should match as-built documentation from commissioning.
4. IEC 62446-1 commissioning report. String IV curves, insulation resistance, earth continuity measurements at commissioning. This baseline data is essential for any future performance dispute — without it, proving the system degraded below warranty threshold is very difficult.
In practice, many pre-2018 commercial solar installations have incomplete warranty documentation. Where key documents are missing, they can sometimes be reconstructed from MCS database, manufacturer records, and installer records — but this takes 4-8 weeks and adds to due diligence cost.
Our standard warranty package
For office solar projects we deliver:
- 25-year linear power on panels (Tier-1 manufacturers only: JinkoSolar, LONGi, Trina Solar, JA Solar, Canadian Solar)
- 12-year materials/workmanship on panels
- 10-year standard inverter warranty, extended to 20 years (paid up-front in capex)
- 10-year workmanship warranty covering full installation scope
- 10-year IWA via QANW (A.M. Best A-rated)
- 5-year O&M service inclusive (annual inspection, monitoring)
- Full IEC 62446-1 commissioning documentation pack
- Warranty transfer protocol documentation for building sale events
Key takeaways
- Panel degradation warranties cover LID, LETID, and PID — but different degradation types have different warranty claim paths; understand which failure mode applies before claiming
- IWA protects against installer failure; manufacturer warranty protects against product defects — both are needed because they cover different risks
- Four scenarios void warranties: unauthorised modification, improper maintenance, excluded physical damage events, and failure to notify on building sale
- Due diligence on existing PV assets requires MCS certificate, IWA certificate, manufacturer warranty certificates, and IEC 62446-1 commissioning report
- For new installs, confirm the IWA insurer’s A.M. Best rating: QANW (A) and GPI are the established UK PV IWA providers