Summary
Solar PV inverters convert the DC output from the panel array into AC power synchronised with the UK grid, and like any electronic power conversion equipment, they display fault/error codes on an integrated screen, via a smartphone app, or via a web-based monitoring portal when something prevents normal operation. The code numbering and exact terminology differ significantly between manufacturers — SolarEdge, Fronius, GivEnergy, SolaX, and others each use their own schemes — but the underlying fault categories map onto a consistent set of root causes across the industry, which is the more useful level to diagnose at for a general electrician or roofer who isn't a solar PV specialist but needs to triage a fault before deciding whether to escalate.
The most safety-critical category is an isolation (sometimes called "insulation resistance") fault — this indicates the normal electrical separation between the DC side of the system and earth has degraded, commonly from moisture ingress at a rooftop DC isolator, a damaged MC4 connector, cable insulation damage, or a degraded panel junction box. This is not a cosmetic or nuisance fault: a genuine isolation fault means a real risk of electric shock exists somewhere in the DC wiring, and the inverter's automatic shutdown in response to detecting it is a safety feature working correctly, not an overcautious fault trip to simply reset past.
Grid-related faults are, by contrast, very often not a fault with the solar installation at all — they reflect the local electricity network's voltage or frequency moving outside the permitted operating window the inverter is required to disconnect for, which can be caused by factors entirely outside the property (a neighbour's solar export pushing local voltage up, general grid quality issues) rather than anything wrong with the inverter or array.
Key Facts
- Grid voltage/frequency faults — most inverters will disconnect and display a grid-related fault if local mains voltage or frequency moves outside the permitted range defined by the relevant grid code (G98/G99 in the UK) — this is the inverter behaving correctly to protect the grid and itself, and is often caused by local grid conditions (voltage rise from multiple nearby solar installations exporting simultaneously, general grid quality) rather than a fault within the installation
- DC isolation/insulation resistance faults — indicate the insulation resistance between the PV DC conductors and earth has fallen below a safe threshold; common causes include moisture ingress into a rooftop DC isolator or MC4 connector, physically damaged cable insulation (rodent damage, abrasion against a roof surface), or a degraded panel junction box — this is a safety-critical fault requiring investigation by a competent solar PV engineer before the system is restarted, since it can indicate a genuine shock risk
- SolarEdge-specific — isolation faults commonly shown as codes in the "2x1" and various "8x" ranges; grid voltage limit faults shown in the "2x1F"/"2x20"/"2x21"/"2x29" range — ****
- GivEnergy-specific — F01 (grid over-voltage, commonly triggered above 253V, often linked to local grid quality or high levels of nearby solar export rather than a GivEnergy-specific fault), F06 (general grid fault), E24 (DC isolation fault, requiring engineer inspection before restart)
- Fronius-specific — State codes rather than "Error" numbering on many models; State 201 indicates grid voltage beyond permitted limits, States 522/567 are grid-related, States 105–109 point to internal power-stage issues requiring manufacturer-level diagnosis
- Internal power-stage/PCB faults — codes indicating an internal inverter electronics fault (varies by manufacturer's specific code range) generally require inverter repair or replacement by the manufacturer or an authorised service partner, not field repair
- Export limitation and smart export guarantee (SEG) monitoring faults — distinct from a genuine inverter fault, some "faults" reported by a customer are actually export limitation devices or SEG monitoring equipment malfunctioning, which can look like reduced generation without an actual inverter error code being present — check whether an error code is actually displayed before assuming an inverter fault
- G98/G99 grid code compliance — UK inverters must comply with Engineering Recommendation G98 (installations up to 16A per phase, the large majority of domestic systems) or G99 (larger installations) — grid-related fault behaviour (disconnection thresholds, reconnection delay) is governed by these standards, not arbitrary manufacturer settings
- Battery storage system interaction — on systems with integrated battery storage (increasingly common alongside PV), some fault codes relate specifically to the battery/hybrid inverter function rather than the PV generation side — check whether the fault is PV-specific or battery-specific before diagnosing, as these can have different root causes even on the same hybrid unit
- MCS and warranty implications — most UK domestic solar PV installations are MCS-certified for Smart Export Guarantee eligibility; non-MCS-certified repair work can affect both the manufacturer's warranty and, in some cases, continued SEG payment eligibility — ****
- Monitoring portal vs on-unit display — many modern inverters report faults primarily via a cloud monitoring app/portal rather than (or in addition to) an on-unit display, so checking the manufacturer's monitoring platform is often the fastest way to retrieve the exact current fault code and its history, rather than relying solely on physically accessing the inverter
Quick Reference Table
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Try squote free →| Symptom | Likely fault category | First check |
|---|---|---|
| Inverter shows a grid voltage/frequency related code, system otherwise healthy | Grid quality issue (often external to the installation) | Check mains voltage with a meter; compare with neighbours/DNO if persistent |
| Isolation/insulation resistance fault code | DC-side insulation degraded — safety-critical | Do not restart — refer to MCS-certified solar PV engineer for investigation |
| Reduced output, no fault code displayed | Shading, soiling, panel degradation, or export limitation device issue — not necessarily an inverter fault | Check monitoring portal generation history, inspect panels for shading/soiling |
| Internal power-stage/PCB fault code | Inverter electronics fault | Refer to manufacturer or authorised service partner — not field-repairable |
| Fault code only visible on monitoring app, not on unit display | Normal for many modern inverters — not itself a separate fault | Check manufacturer monitoring portal for exact current code |
| Intermittent grid fault, clears and re-trips repeatedly | Borderline grid voltage, especially at peak solar export times locally | Monitor pattern against time of day/local export activity before escalating to DNO |
| Hybrid inverter shows battery-specific fault | Battery system issue, separate from PV generation fault | Distinguish PV-side vs battery-side fault before diagnosing |
| Fault appears after storm/heavy rain | Possible new moisture ingress causing isolation fault | Check DC isolator and connector condition for water ingress |
Detailed Guidance
Triage: is this fault safety-critical or a nuisance/external condition?
The first and most important triage question is whether the fault is a DC isolation/insulation fault (safety-critical, stop and investigate before restart) or a grid-related fault (often external to the installation and not indicating anything wrong on site). Checking which category the specific manufacturer's code falls into — using the manufacturer's own documentation or monitoring portal, since exact code numbers vary by brand — determines whether this is an urgent on-site investigation or something that may simply resolve as local grid conditions change.
Diagnosing a suspected isolation/insulation fault
Treat this as safety-critical from the outset. Do not simply reset the inverter and hope the fault clears — if it recurs, that confirms a genuine, ongoing insulation resistance problem somewhere in the DC wiring. Visual inspection should focus on the rooftop DC isolator (a common point of moisture ingress, particularly on older or poorly sealed units), MC4 connector condition at accessible points, and any visible cable damage. A fault that cannot be resolved by visual inspection and straightforward connector/isolator remediation requires a full insulation resistance test of the DC circuit by a competent solar PV engineer with appropriate test equipment — this is specialist diagnostic work, not a general electrical fault-finding task, given the DC voltages involved (commonly 300–600V+ DC across a string) and the specific safety procedures required to isolate a live PV array safely.
Diagnosing a grid-related fault
Before assuming an installation fault, check the actual mains voltage at the property with a standard multimeter during the fault condition — if it's genuinely outside the G98/G99 permitted range, this points to a local grid quality issue rather than an inverter or installation defect. This is increasingly common in areas with high levels of residential solar export, where cumulative export from multiple nearby properties can push local voltage up during peak generation periods (typically midday on sunny days). A persistent, voltage-confirmed grid fault should be reported to the Distribution Network Operator (DNO) for the area, since resolving it may require DNO-side action (voltage regulation, network reinforcement) rather than anything fixable at the individual installation.
Distinguishing reduced output from an actual fault
A customer reporting "my solar isn't working as well as it used to" without describing an actual error code displayed is very often describing gradual output reduction from shading (tree growth, new nearby construction), panel soiling, or seasonal variation — not a fault requiring the diagnostic approach in this guide at all. Check the manufacturer's monitoring portal for actual generation history and whether any genuine fault code has been logged before treating this as a fault-finding job rather than a performance/maintenance conversation.
Frequently Asked Questions
Can I just reset the inverter and see if the fault clears?
For a grid-related fault, many inverters will automatically reconnect once grid conditions return within normal limits, so a persistent recheck after some time is reasonable. For an isolation/insulation fault, do not simply reset and restart without investigation — if it recurs (which it usually will, since the underlying insulation defect hasn't been fixed by a reset), this confirms a genuine fault requiring proper diagnosis, and repeatedly resetting past a genuine safety-critical fault is not appropriate practice.
Do I need any specific qualification to diagnose solar inverter faults?
General fault triage (checking the monitoring portal, visual inspection of accessible components, checking mains voltage) doesn't require specialist solar PV certification. However, any work involving the DC side of the system — isolating a live PV array, testing insulation resistance, replacing DC-side components — requires the competency and safe isolation procedures specific to solar PV work, which goes beyond standard domestic electrical qualification. MCS-certified solar PV installers have this specific competency.
Does a solar inverter fault affect my customer's Smart Export Guarantee payments?
It can, indirectly — reduced or stopped generation during a fault period means reduced export and therefore reduced SEG payments for that period, but this is a consequence of the generation loss rather than a separate compliance issue, provided the system remains MCS-registered and the fault is resolved through appropriate channels. Using a non-MCS-certified repairer for solar PV work can, in some cases, have separate implications for continued scheme eligibility — ****.
Why does the fault code on the inverter's own screen not match what I see in the manufacturer's app?
Some inverters' on-unit displays show a simplified or legacy code set compared to the more detailed codes available through the manufacturer's cloud monitoring platform, which can also show fault history and additional diagnostic context not available from the unit alone. Where available, the monitoring portal/app is generally the more useful and complete diagnostic source.
Regulations & Standards
Engineering Recommendation G98 — grid connection standard for inverter-connected generating equipment up to 16A per phase (the large majority of domestic solar PV installations)
Engineering Recommendation G99 — grid connection standard for larger generating installations
BS EN 62109 series — safety standards for power converters used in photovoltaic power systems
IET Code of Practice for Grid Connected Solar Photovoltaic Systems — industry guidance on design, installation, and verification of solar PV systems
MCS (Microgeneration Certification Scheme) — installer certification scheme relevant to Smart Export Guarantee eligibility and warranty compliance
MCS — Microgeneration Certification Scheme — installer certification standards for solar PV
Ofgem — Smart Export Guarantee — official SEG scheme rules
Manufacturer technical documentation — SolarEdge, Fronius, GivEnergy, SolaX — model-specific fault code references should always be checked against the individual manufacturer's current documentation, as code numbering evolves across product generations
battery storage safety standards guide — related safety standards relevant to hybrid inverter/battery systems
solar battery systems — general solar PV and battery storage system overview
rcd types explained — RCD protection context relevant to solar PV AC-side connection
air source heat pump installation — comparable MCS-certified renewable technology installation requirements