Civil Law and Solar Panel Efficiency Defect Disputes in Europe

1. Introduction

Solar panel efficiency defect disputes arise when photovoltaic (PV) modules fail to produce the level of electricity promised, specified in technical documentation, required by contract, or reasonably expected from the installation.

Typical disputes involve:

lower-than-warranted power output;

premature degradation;

delamination;

defective junction boxes;

defective soldering or electrical connections;

micro-cracking;

manufacturing defects;

incorrect technical specifications;

defective inverters;

installation defects;

failure to meet guaranteed performance ratios;

loss of electricity revenue;

replacement and repair costs;

loss of use of the solar installation.

The most important legal distinction is:

Low efficiency may constitute contractual non-conformity even where it does not constitute a “safety defect” for EU product-liability purposes.

This distinction is extremely important. Product-liability law traditionally focuses on safety, while sale-of-goods law and contractual warranty law focus more directly on conformity, performance and fitness for the agreed purpose. The recent French photovoltaic litigation demonstrates this distinction very clearly. (Cour de Cassation)

2. Basic Legal Formula

For examination purposes:

Solar Panel → Promised Output → Actual Output → Performance Deficiency → Defect/Non-Conformity → Causation → Contract/Warranty/Product Liability → Loss → Remedy

A simple formula is:

Guaranteed Efficiency – Actual Efficiency = Performance Shortfall

But the legal claim requires more:

Performance Shortfall + Contractual Promise + Causation + Legally Recognised Loss = Potential Claim

3. Main Causes of Solar Panel Efficiency Defects

A. Manufacturing defect

Examples:

defective cells;

defective soldering;

faulty junction boxes;

poor electrical connections.

B. Material degradation

Examples:

delamination;

corrosion;

degradation of backsheets;

micro-cracking.

C. Design defect

The panel design itself may produce:

excessive heat;

premature degradation;

electrical failure;

fire risk.

D. Installation defect

The manufacturer may not be responsible where poor installation caused the reduced output.

E. Maintenance defect

Poor cleaning, shading, damaged cables or inverter failure may affect production.

F. Performance misrepresentation

A seller may have promised:

“This system will generate X kWh per year.”

If actual production is substantially lower, the dispute may be primarily contractual rather than a product-safety claim.

4. European Legal Framework

A. Contract Law

This is often the primary route in commercial solar projects.

The contract may contain:

minimum power output;

degradation guarantee;

performance ratio;

annual energy yield;

product warranty;

replacement warranty;

EPC obligations;

availability guarantees;

liquidated damages.

For large solar farms, the EPC agreement can therefore be more important than general product liability.

5. Consumer Sale-of-Goods Law

For consumer purchases, EU conformity rules are important.

The seller can potentially be responsible when goods:

do not correspond to the description;

lack agreed characteristics;

do not perform as promised;

are not fit for their ordinary purpose.

The modern EU framework is found principally in Directive (EU) 2019/771 for consumer sales.

6. Product Liability

The traditional EU Product Liability Directive, Directive 85/374/EEC, imposed liability for damage caused by defective products.

A product is defective when it does not provide the safety that a person is entitled to expect.

This is different from saying:

“The product does not produce as much electricity as promised.”

Therefore, underperformance alone may be a conformity/warranty problem rather than a product-liability problem.

The new Product Liability Directive (EU) 2024/2853 updates this regime and applies to products placed on the market or put into service after 9 December 2026. It retains the safety-based concept of defectiveness and expressly regulates modern products and components. (EUR-Lex)

7. Case 1 — CSNSP 431 v GenSun and Avancis GmbH

French Cour de cassation, 28 May 2025, No. 23-20.341

This is one of the most directly relevant European authorities on solar-panel performance disputes.

Facts

A Portuguese company was involved in the development of a photovoltaic power plant in Portugal.

GenSun acquired thousands of photovoltaic modules from the German manufacturer Avancis.

Problems arose concerning:

manufacturing defects;

module characteristics;

insufficient performance;

degradation/delamination;

conformity with technical specifications.

The purchaser sought remedies including restitution and damages associated with the underperformance of the modules.

Legal Issue

The case involved:

cross-border sales;

photovoltaic modules;

manufacturing defects;

insufficient performance;

contractual warranties;

jurisdiction;

applicable law;

direct claims by a sub-purchaser.

The French Cour de cassation partially quashed the appellate decision and sent the relevant issues back for reconsideration. (Cour de Cassation)

Importance

This case demonstrates that solar-panel performance specifications can form the basis of a contractual conformity dispute.

It also shows that a purchaser downstream in a supply chain may face difficult questions concerning:

privity;

direct action against the manufacturer;

choice-of-law clauses;

jurisdiction clauses;

product warranties.

Key Principle

Photovoltaic underperformance can generate contractual remedies independently of ordinary product-safety liability.

8. Case 2 — CSNSP 431 / GenSun / Avancis: Montpellier Court of Appeal

The underlying Montpellier litigation provides even more detail.

The photovoltaic plant used approximately 18,360 photovoltaic modules.

Some modules:

were damaged during transportation;

showed manufacturing defects;

developed abnormal colour changes;

exhibited delamination;

experienced loss of power.

The operator complained about loss of power and delamination. The manufacturer initially argued that some visible phenomena were merely aesthetic and did not affect performance.

The dispute ultimately concerned whether the modules conformed to the agreed technical characteristics and whether their performance was sufficient. (Cour de Cassation)

Importance

This is an excellent example of the distinction between:

Aesthetic defect

and

Performance defect

and

Safety defect.

A defect that looks cosmetic may nevertheless become legally important if it causes:

degradation;

loss of output;

non-compliance with specifications;

reduced useful life.

9. Case 3 — CSNSP 431 v GenSun and Avancis: Nîmes Court of Appeal, 17 April 2026

This is particularly important because it is very recent and directly concerned photovoltaic module conformity.

The Nîmes Court of Appeal dealt with the litigation following the 2025 Cour de cassation judgment.

The court's discussion concerned major conformity defects, including:

module technical specifications;

NOCT values;

early degradation;

delamination;

safety standards;

non-conformity with contractual expectations.

The court recorded that the modules had significant conformity problems and considered whether those defects justified contractual resolution and restitution. (Cour de Cassation)

Importance

This case demonstrates that efficiency specifications can be legally enforceable contractual characteristics.

For example:

If the contract says that a module must have a particular technical specification, and the actual module materially differs from that specification, the buyer may have a conformity claim.

Exam Principle

Technical specification → contractual conformity → substantial defect → possible rescission/restitution.

10. Case 4 — Scheuten Solar / Alrack Photovoltaic Panel Litigation

Caen Court of Appeal, 20 May 2025, No. 22/02705

This is another direct photovoltaic defect case.

Facts

A residential photovoltaic installation consisted of 15 Scheuten Solar panels equipped with Alrack Solexus junction boxes.

The installation subsequently stopped producing electricity.

Expert evidence identified defects in the junction boxes.

The defects caused:

interruptions;

shutdown of the installation;

electrical problems;

potential arcing;

serious fire risk.

The court found that the defect was connected with the design/component of the photovoltaic panels. (Cour de Cassation)

Legal Issue

The case considered:

product liability;

hidden defects;

contractual liability;

insurance;

causation;

economic loss.

Important Finding

The defective junction boxes created a safety risk and made the equipment unsuitable for its intended use.

The court also recognised that the defects were not apparent or detectable by the purchaser at delivery because they manifested only after the installation was energised. (Cour de Cassation)

Importance

This is extremely useful for solar disputes because a hidden defect may remain invisible during installation but become apparent only after:

Electrical activation → operational use → degradation/failure → expert investigation

11. Case 5 — AIG Europe v Scheuten Solar / Alrack

French Cour de cassation, 4 June 2025, No. 23-19.724

This case arose from another photovoltaic installation involving Scheuten Solar modules and Alrack junction boxes.

Facts

A homeowner had photovoltaic panels installed on her house.

The system later malfunctioned.

Expert investigations established defects involving the connection boxes.

The homeowner suffered:

repair/replacement costs;

loss of electricity production;

operating losses.

The insurer sought recovery from the manufacturers' insurers. (Cour de Cassation)

Important Legal Question

Could product liability compensate for:

damage to the defective photovoltaic product itself; and

resulting loss of electricity production?

Principle

The traditional French implementation of EU product-liability law generally distinguishes:

Damage caused by a defective product to another property

from

Damage to the defective product itself and purely economic consequences of that damage.

The Court of Appeal had analysed the relationship between product liability, hidden defects and other contractual/tort remedies.

The Cour de cassation ultimately rejected the insurer's appeal in June 2025. (Cour de Cassation)

Importance

This case is especially important for efficiency-loss claims.

A claimant should not automatically assume:

“My solar panels underperformed, therefore EU product liability gives me compensation for all lost electricity revenue.”

The claimant may instead need to rely on:

contract;

warranty;

hidden-defect law;

tort;

insurance;

national sale-of-goods law.

12. Case 6 — Gebr. Weber GmbH v Wittmer and Putz v Medianess Electronics

Joined Cases C-65/09 and C-87/09

This is not a solar-panel case, but it is an important EU conformity/remedy authority.

Facts

Defective goods had already been installed before the defect became apparent.

Legal Issue

Who pays when defective goods must be removed and replaced?

CJEU Principle

Where consumer goods are not in conformity and replacement is necessary, the seller must generally:

remove the defective goods; and

install the replacement goods,

or bear the necessary costs. (EUR-Lex)

Application to Solar Panels

Imagine:

A homeowner installs defective PV panels on the roof.

The panels later fail.

Replacing them requires:

roof access;

removal;

labour;

transport;

new mounting;

electrical reconnection.

Weber/Putz provides a strong analogy for treating reasonable removal and reinstallation expenses as part of the conformity remedy.

Exam Principle

Defective installed product → replacement → reasonable removal and installation costs can follow the seller's conformity obligation.

13. Case 7 — Faber v Autobedrijf Hazet Ochten BV

C-497/13

Facts

The dispute concerned the conformity of consumer goods and the burden of proof where a defect became apparent after delivery.

CJEU Principle

Where the consumer establishes that:

the goods are not in conformity; and

the lack of conformity became apparent within the relevant period,

the EU consumer-sale regime can provide a favourable evidentiary presumption concerning conformity at delivery. (EUR-Lex)

Application to Solar Panels

Suppose:

panels are installed in January;

expected performance is documented;

serious output problems appear shortly afterwards.

The purchaser may rely on technical evidence showing non-conformity, while the seller may need to explain why the problem resulted from:

improper installation;

external damage;

shading;

maintenance failure;

misuse.

Importance

This is particularly useful where the consumer cannot determine the exact technical cause of the underperformance.

Exam Keyword

Faber = conformity + early defect + burden of proof.

14. Case 8 — Boston Scientific Medizintechnik

Joined Cases C-503/13 and C-504/13

This is a classic EU product-liability authority.

Principle

The CJEU held that products belonging to the same group or production series could, under particular circumstances, be considered defective where a potential defect creates an abnormal safety risk, even without proving that every individual product contains the exact same defect. (EUR-Lex)

Application to Solar Panels

Imagine a manufacturer discovers:

A particular production batch of 100,000 panels contains a dangerous junction-box defect.

The owner of one panel may argue that the entire production series presents an established abnormal risk.

But caution is necessary:

Boston Scientific concerns safety defects, not merely lower electrical efficiency.

Exam Principle

Known systemic defect → potentially defective product series → safety-based product liability.

15. Case 9 — W and Others v Sanofi Pasteur

C-621/15

Principle

The CJEU addressed proof of:

defect;

causation;

scientific uncertainty.

The Court accepted that national evidentiary rules may allow courts to consider serious, specific and consistent evidence, but they cannot effectively eliminate the claimant's obligation to prove defect and causation. (EUR-Lex)

Application to Solar Panels

This is highly relevant to technical disputes.

Suppose an expert cannot say with absolute certainty:

“This precise microscopic defect caused the 35% loss in output.”

But the evidence shows:

all affected panels came from the same batch;

the same defect is repeatedly found;

degradation began unusually early;

output declined in the same pattern;

laboratory testing confirms the mechanism.

W and Others provides an analogy for evaluating complex scientific evidence.

Exam Principle

Technical uncertainty does not automatically destroy causation, but the claimant still needs sufficiently persuasive evidence.

16. Efficiency Defect vs Safety Defect

This is perhaps the most important distinction.

17. What Is a “Performance Defect”?

A performance defect exists where the solar installation does not achieve the contractual or legally required level of output.

For example:

Contractual guarantee:
1 MW annual output

Actual output:
700 kW

Potential shortfall:

300 kW / 30%

But the claimant must determine why.

Output can legitimately be reduced by:

weather;

shading;

dust;

orientation;

temperature;

inverter efficiency;

grid outages;

curtailment;

maintenance;

degradation;

installation quality.

Therefore:

Low output ≠ automatically defective panel.

18. Performance Ratio

Commercial solar contracts may use a Performance Ratio (PR) rather than simply annual electricity output.

A dispute may therefore concern:

expected irradiation;

actual irradiation;

system losses;

module degradation;

inverter losses;

availability;

temperature coefficients.

An expert may need to calculate:

Expected Energy – Actual Energy = Performance Loss

The claimant then has to establish which portion of the loss was caused by the defendant.

19. Hidden Defects

Solar-panel defects frequently become apparent only after installation.

Examples:

micro-cracks;

solder failure;

junction-box failure;

delamination;

corrosion;

hot spots.

This makes hidden-defect law especially important.

The Scheuten/Alrack litigation is a strong illustration: defects in connection components were not necessarily visible at delivery but manifested during operation. (Cour de Cassation)

20. Delamination

Delamination occurs when layers of a photovoltaic module separate.

It can lead to:

moisture penetration;

corrosion;

reduced electrical output;

shortened lifespan;

safety problems.

The CSNSP/Avancis litigation is particularly useful because early degradation and delamination formed part of the controversy concerning module conformity and performance. (Cour de Cassation)

21. Junction-Box Defects

The junction box is critical because it connects the electrical circuits of the panel.

A defect may cause:

Poor connection → resistance/heat → arcing → shutdown → loss of output → potentially fire

The Scheuten/Alrack cases demonstrate how a junction-box defect can move beyond an ordinary efficiency dispute into a safety/product-liability dispute. (Cour de Cassation)

22. Loss of Electricity Revenue

This is often the most valuable claim in a commercial solar dispute.

Suppose:

Expected annual generation = 10,000 MWh

Actual generation = 7,500 MWh

Shortfall = 2,500 MWh

If the contractual value of electricity is €100/MWh:

Potential gross production loss:

2,500 × €100 = €250,000

But the claimant must establish:

causation;

actual loss;

contractual entitlement;

mitigation;

electricity price;

grid availability;

whether the loss was foreseeable;

whether the warranty limits consequential losses.

23. Product Liability and Economic Loss

This requires special caution.

Traditional EU product liability is principally directed toward damage such as:

death;

personal injury;

damage to property other than the defective product.

It is not a universal mechanism for recovering every commercial loss caused by an underperforming product.

The new Product Liability Directive retains important limits: it specifies the categories of compensable damage and expressly excludes certain property used exclusively for professional purposes from the harmonised compensation regime. (EUR-Lex)

Therefore:

Commercial solar-farm loss will often be primarily a contractual claim.

24. New EU Product Liability Directive — 2024/2853

The new Directive becomes particularly important from 9 December 2026 for products placed on the market or put into service after that date.

It modernises EU product liability for technological products and clarifies issues such as:

components;

digital elements;

product modifications;

evidence;

causation;

foreign manufacturers;

economic operators;

updates/upgrades.

It also allows courts, in sufficiently difficult technical cases, to ease the claimant's evidential burden in appropriate circumstances. (Legislative Observatory)

Important limitation

The Directive is not a general European warranty law for solar-panel efficiency.

Contractual performance claims continue to depend heavily on:

the sale contract;

EPC contract;

warranty;

national contract law;

consumer conformity law.

25. Manufacturer vs Installer

This distinction is essential.

Manufacturer

Potentially responsible for:

defective cells;

defective junction boxes;

defective design;

manufacturing defects;

defective materials.

Installer

Potentially responsible for:

wrong orientation;

poor wiring;

inadequate mounting;

shading caused by installation;

incorrect configuration;

failure to commission correctly.

Operator

Potentially responsible for:

poor maintenance;

failure to clean;

improper operation;

failure to replace components.

Therefore:

The claimant must identify the technical cause before identifying the legally responsible party.

26. EPC Contract Claims

Large solar farms commonly use EPC contracts.

The EPC contractor may promise:

completion date;

installed capacity;

performance ratio;

annual energy production;

availability;

degradation limits.

Failure can lead to:

repair;

replacement;

liquidated damages;

price reduction;

damages;

termination/resolution.

The CSNSP/GenSun litigation is particularly valuable because it involved a photovoltaic power-plant EPC structure and disputes over module performance and conformity. (Cour de Cassation)

27. Direct Claim Against Manufacturer

A major problem occurs when:

Owner → EPC Contractor → Distributor → Manufacturer

The owner may want to sue the manufacturer directly.

The question becomes:

Is there contractual privity?

Possible legal routes include:

direct contractual warranty;

third-party beneficiary rights;

assignment;

subrogation;

national direct-action doctrine;

product liability;

hidden-defect action;

tort.

The CSNSP v Avancis litigation demonstrates the complexity of direct actions by downstream purchasers in a cross-border photovoltaic supply chain. (Cour de Cassation)

28. Cross-Border Solar Projects

Solar projects frequently involve:

owner in France;

EPC contractor in France;

manufacturer in Germany;

project in Portugal;

insurer in Luxembourg.

This creates questions about:

Jurisdiction

Which court hears the case?

Applicable law

Which country's law applies?

Contractual jurisdiction clause

Does the clause bind a sub-purchaser?

Product warranty

Can a later owner rely on it?

The CSNSP/Avancis case is especially important because it involved Portuguese, French and German companies and contractual jurisdiction/choice-of-law questions. (Cour de Cassation)

29. Evidence Required

A successful solar efficiency claim normally needs strong technical evidence.

Technical evidence

module testing;

IV-curve testing;

electroluminescence testing;

thermal imaging;

degradation analysis;

insulation testing;

laboratory reports.

Documentary evidence

purchase contract;

technical specification;

warranty;

EPC agreement;

datasheets;

installation records;

commissioning report.

Performance evidence

inverter records;

SCADA data;

irradiation data;

historical production;

weather data;

grid availability records.

30. Causation Analysis

The legal chain should be:

Defective Module

↓

Reduced Electrical Performance

↓

Lower Energy Production

↓

Lower Electricity Revenue

↓

Financial Loss

↓

Claim

But the defendant may argue:

“The loss was caused by insufficient sunlight.”

or:

“The inverter caused the problem.”

or:

“The claimant failed to maintain the panels.”

or:

“Grid curtailment caused the production loss.”

Therefore, technical causation is often the decisive issue.

31. Remedies

Possible remedies include:

1. Repair

Repair the defective panel/component.

2. Replacement

Replace defective modules.

3. Price reduction

Reduce the purchase price to reflect the defect.

4. Rescission/termination

End the contract where the defect is sufficiently serious.

5. Restitution

Return the price after contractual resolution.

6. Damages

Compensation for legally recoverable losses.

7. Lost revenue

Potentially recoverable under contract where sufficiently established and legally permitted.

8. Removal and reinstallation

Particularly important for rooftop installations.

Weber/Putz strongly supports reimbursement of necessary removal and installation costs in the consumer conformity context. (EUR-Lex)

32. Most Important Case-Law Table

33. Six Cases to Memorise

For a short examination answer, remember these six:

1. CSNSP v Avancis

Solar panels + underperformance + conformity

2. Scheuten/Alrack

Solar panels + junction-box defect + safety

3. AIG Europe v Scheuten/Alrack

Solar defect + product liability + economic loss

4. Weber/Putz

Defective installed goods + replacement costs

5. Faber

Conformity + burden of proof

6. Boston Scientific

Systemic product defect + safety risk

34. Key Legal Distinction

The easiest way to remember the whole subject is:

Efficiency Defect

Usually:

Contract → Warranty → Conformity → Performance → Damages

Safety Defect

Usually:

Product Liability → Defect → Safety Expectation → Damage → Causation

Installation Defect

Usually:

Installation Contract → Negligence/Breach → Causation → Damages

Commercial Solar-Farm Loss

Usually:

EPC Contract → Performance Guarantee → Shortfall → Causation → Lost Revenue

35. Exam-Ready Conclusion

Solar panel efficiency defect disputes in Europe sit at the intersection of contract law, consumer conformity law, product liability, hidden-defect law and cross-border private international law. The strongest recent European authorities are the French photovoltaic cases involving CSNSP/GenSun/Avancis and Scheuten/Alrack. They show that insufficient performance, technical non-conformity, premature degradation and defective components can generate contractual or warranty remedies, while dangerous defects such as defective junction boxes may additionally engage product-liability principles.

The crucial distinction is that underperformance is not automatically a product-safety defect. A claimant must identify the promised technical performance, establish the actual shortfall, prove that the shortfall resulted from the defendant's product or contractual breach, and establish the legally recoverable loss. The CJEU authorities Weber/Putz, Faber, Boston Scientific and W and Others provide the broader European principles concerning conformity, remedies, proof, defect and causation. (Cour de Cassation)

Ultra-Basic Revision Keywords

Solar Panel → Photovoltaic → PV Module → Efficiency → Power Output → Energy Yield → Performance Ratio → Performance Guarantee → Warranty → Contract → EPC → Manufacturing Defect → Design Defect → Junction Box → Soldering → Micro-Crack → Delamination → Corrosion → Degradation → Non-Conformity → Hidden Defect → Safety Defect → Product Liability → Consumer Sale → Causation → Expert Evidence → SCADA Data → Irradiation → Actual Output → Guaranteed Output → Performance Shortfall → Lost Revenue → Repair → Replacement → Price Reduction → Rescission → Restitution → Damages → Weber/Putz → Faber → Boston Scientific → Sanofi → CSNSP/Avancis → Scheuten/Alrack → Economic Loss → Cross-Border Contract → Applicable Law → Jurisdiction → EU Product Liability Directive → Compensation