Technical Due Diligence Solarpark: Thermografie als Bewertungsinstrument
Acquiring a solar farm is a long-term investment decision. Yield models, financing structures and grid connection contracts can be read from documents – the actual technical condition of the system cannot. This is precisely where thermography comes in: as the only method it provides a complete picture of the current operating condition of all modules before the purchase contract is signed.
What is technical due diligence for solar farms?
Die Technical Due Diligence (TDD) is the systematic technical inspection of a PV system as part of a transaction process. It is typically commissioned by the buyer – often through an independent expert – and delivers a structured assessment of the system condition, remaining service life and technical risks.
A complete TDD for solar farms covers several inspection components: the document review (permits, grid connection contracts, maintenance logs, insurance), the technical on-site inspection and a yield analysis based on historical monitoring data. Thermography is the most important part of the Vor-Ort-Inspektion – it delivers information that no other inspection method provides.
The market for solar farm transactions has grown considerably in Germany. According to Bundesnetzagentur thousands of existing systems from the EEG support period 2010–2015 are on the market or nearing the end of their feed-in tariff. The technical condition of these systems – many of them 12–15 years old – varies considerably and justifies careful inspection before every transaction.
Why thermography is indispensable for TDD
The energy monitoring of a PV system shows the total yield – but it conceals the causes of deviations. A system that is 5% below expected values could have a single large defect, hundreds of small individual damages or a combination of both. The monitoring does not distinguish between these.
Die Thermografie löst dieses Problem direkt: it makes every individually active defect visible and localisable. The TDD expert thereby receives not just a defect list but also a basis for estimating the remediation costs – a decisive input for purchase-price negotiations.
Furthermore: many defects develop undetected over years. PID degradation, gradual delamination and progressively failing bypass diodes are barely detectable in the monitoring but clear in the thermogram. A system that performs well can still have considerable latent defects, which will lead to failures in the coming years.
Thermography process in the TDD workflow
Document review before the survey
System documentation, monitoring data from the last 3 years, maintenance logs and any previous damage are reviewed. Anomalies in the historical yield trend provide indications of possible problem zones – the drone surveys these areas with particular attention.
100 % Modulabdeckung, georeferenziert
The drone thermography captures all modules in a single survey pass. Simultaneous IR and RGB images are stored geo-referenced. The result is a gap-free thermogram of the entire site – independent of the size and geometry of the farm.
Wechselrichter, Stringboxen, AC-Verteiler
All electrical components outside the modules are inspected with the IR hand camera. In older systems overheated junction boxes and corroded connectors are common findings – and often cheaper to remedy than module damage when detected early.
Defects by criticality and remediation costs
All anomalies are classified to IEC TS 62446-3 and assigned a priority level. For each defect type an estimate of the remediation costs is produced. The result: a structured defect list with monetised risk – the direct basis for purchase-price negotiations.
IEC-compliant report with investment risk assessment
The complete thermography report is integrated into the overall TDD report. It contains geo-referenced thermograms, a defect list with cost estimates, yield-loss calculation and a final risk assessment. This report can be used directly in purchase contract negotiations.
Thermographically relevant defects in TDD
Not every defect has the same relevance for an investment decision. The following overview shows the most important finding categories and their typical impact on purchase price and risk assessment:
When more than 5–10% of modules show PID patterns this indicates a systemic problem – possibly faulty earthing or incorrect inverter settings. Reversible only with considerable effort.
Hotspots with ΔT above 30 K represent an acute fire risk. Insurers frequently require immediate remediation as a prerequisite for insurance cover in such cases.
An increased proportion of defective bypass diodes indicates quality issues with a specific module type or production year and can point to further, not yet visible failures.
Corroded connectors and faulty seals in junction boxes are common in older systems. Cheap to remedy but an indicator of inadequate maintenance – which requires further investigation.
Isolated hotspots and substring failures are normal in systems from 5 years of operation. What matters is the rate: below 1% of modules affected is industry-standard, above 3% is noteworthy.
Increasingly common in systems from 8–10 years of age. Partially visible thermographically; EL measurement recommended as a supplement for a complete diagnosis. Relevant for assessing remaining service life.
Thermography findings and purchase-price negotiation
A detailed thermography report is not a purely informational document – it is a Verhandlungsinstrument. The defects found can be directly translated into monetary values:
Defects found can be directly translated into monetary remediation costs that serve as a basis for purchase-price negotiations. Depending on the type and extent of the defects these amounts can be considerable – our thermography report contains a qualified cost estimate for every finding that can be brought directly into the negotiation.
The capitalised yield loss is the strongest negotiating lever: when a system loses considerable yield annually due to defects and these defects can be remedied at a cost, this justifies a purchase price reduction of at least a considerable amount – covering both the remediation costs and the lost yield until remediation.
Checkliste: Was ein guter TDD-Thermografiebericht enthalten muss
Not every thermographic expert report is suitable for TDD purposes. The following points must be included in the report for it to be reliably used in purchase contract negotiations:
Kosten einer TDD-Thermografie
The costs for thermography as part of a technical due diligence are based on the general inspection prices but are typically 20–40% higher than for a regular maintenance inspection due to the increased documentation effort. For the complete IEC-compliant TDD report with yield-loss calculation and remediation cost estimate:
Preisanfrage: The costs of a TDD thermographic expert report depend on farm size, location and level of detail. Contact us – we will prepare an individual quote that fits your transaction framework.
Relative to the transaction volume these costs are minimal: relative to the transaction volume of a solar farm the costs of a thermographic expert report are minimal. The risk they hedge against is many times greater.
TDD-Thermografie anfragen
For solar farms from 750 kWp: no-obligation enquiry as a basis for your investment decision. Nationwide, IEC-compliant, with complete documentation.
Request now →Frequently asked questions
A TDD is the technical condition assessment of a PV system before an investment decision. It covers document review, on-site inspection including thermography and an assessment of remaining service life and yield potential.
Thermography is the only method that documents the current operating condition of all modules. Defects that are invisible in the monitoring become thermally visible and can significantly influence the purchase price and risk assessment.
Yes. Significant defects justify purchase price reductions or the seller's obligation to remedy. An IEC-compliant thermographic report with yield-loss calculation forms the basis for such negotiations.
Usually the buyer as part of the technical inspection. In some cases the seller proactively provides a current expert report to accelerate the sales process and build trust.
Conclusion
Thermography is not an optional add-on in the technical due diligence of solar farms – it is the most important component of the on-site inspection. No other method delivers as complete a picture of the system condition in comparable time. The costs are minimal relative to the transaction volume; the risk of a superficial inspection, by contrast, is considerable.
Anyone who buys a solar farm without first carrying out thermography is buying a black box. Anyone who does carry it out buys with knowledge – and negotiates accordingly. More information on the technology can be found in the vollständigen Leitfaden, and on drone inspection in the article about Drone thermography for solar farms.