The 10 Most Common Faults in Photovoltaic Systems

1. Incompatible Connectors

The most common and most dangerous fault in practice: MC4-compatible connectors from different manufacturers are connected together. Even though they fit mechanically, the contact quality often does not meet the standard. The result: increased transition resistance, heat generation and, in the worst case, fire. The only safe connection is the approved combination from the same manufacturer using the correct crimping tool.

2. Incorrect Crimping of Contacts

Even with the correct connector brand, incorrect crimping leads to the same problems. Poorly crimped contacts create micro-arcing under load, accelerated ageing and heat. This fault is practically invisible without specialist measurement equipment and often only manifests after months or years.

3. Poor Cable Routing on the Roof

UV-exposed cables without UV protection, cable loops collecting water, bending radii that are too tight, or cables touching sharp metal edges are among the most common installation deficiencies. Over the years these lead to insulation damage, current leakage and earth faults.

4. Faulty String Planning

String planning errors have a direct and permanent impact on yield. Common errors: strings with different module orientations without optimisers, too long or too short strings for the inverter's MPP range, or mixed module types within a string. These errors are often not reflected in monitoring data and can only be identified with a targeted review of the as-built documentation.

5. Unsuitable Inverter Configuration

An inverter that is too small clips yield peaks; one that is too large operates inefficiently at partial load. Particularly critical: an incorrectly set MPP range, wrong feed-in limitation or incorrect string assignment to the MPP trackers. These setting errors often go unchecked during commissioning.

6. Installation Errors on Substructure and Roof Attachment

Mounting systems that are too light, inadequate spacing between modules and roof edge, inadequate wind load calculation or missing/incorrect end clamps lead to mechanical instability. In the worst case, modules can detach. Leaks at roof penetrations that are inadequately sealed are also a recurring finding.

7. Missing or Deficient Potential Equalisation

The potential equalisation of the PV system is prescribed by standard – but is frequently missing or incorrectly implemented in practice. This creates safety risks (touch voltage in the event of a fault) and can lead to increased corrosion of metallic components.

8. Inadequate Documentation

Missing string plans, no commissioning protocol, no measurement record – this is not just a formal problem. Without complete documentation, the condition of the system cannot be reconstructed objectively. This makes warranty enforcement significantly more difficult and complicates insurance settlement.

9. Missing Labelling of Strings and Inverters

Unlabelled cable routes, unclear string assignments and missing warning labels are standard violations that not only create confusion during maintenance but also pose safety risks. In an expert report they regularly appear as a formal deficiency.

10. Monitoring Data Misinterpreted

Monitoring data can only ever be as good as the configuration. Incorrectly entered system parameters (orientation, tilt, installed capacity) produce systematically distorted reference values. Operators then believe they see a deviation where in fact there is none – or vice versa. An expert can assess this through a plausibility check.

What Operators Should Take Away from These Faults

Most of these faults are avoidable – but only if quality control is not left entirely to the installer. An independent commissioning inspection or a review during the warranty period provides clarity on whether the system has actually been installed as contracted.

When an Expert Becomes Appropriate

If you suspect one of the above faults, or if your system is persistently underperforming, an independent technical assessment is the sensible next step. This is especially true before the warranty expires, before an insurance claim or before a purchase/sale.

Conclusion

The most common faults in PV systems are not exotic – they are systematic. They can occur at any installer and on any system. The question is not whether problems will arise, but whether they are identified and corrected early enough.

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