Solar PV Not Generating Power: Fault-Finding Guide

Summary

Domestic solar PV is now common enough on UK roofs that "why has my system stopped generating" is a routine call-out, not a rarity. For the tradesperson attending, the fault sits in one of three places: the point of connection to the property's electrics (isolators, RCDs, consumer unit), the inverter itself (which converts DC panel output to AC and manages grid synchronisation), or the panels/strings on the roof. Getting the diagnosis order right matters — most call-outs are resolved without going on the roof at all.

A common misconception among homeowners is that "no generation" means a dead panel. In practice, panel failure is one of the least likely causes. Far more common are a tripped RCD/MCB in the consumer unit, an inverter that has gone into fault mode after a grid disturbance, or someone (often an electrician working on an unrelated job) leaving the DC or AC isolator switched off after other work. Genuine panel or bypass-diode failure usually shows as partial output loss (one string down) rather than total shutdown.

Anyone fault-finding beyond visual inspection and isolator checks on the AC side needs to understand that DC-side work on an MCS-certified system can affect warranty and insurance validity if done by someone who isn't competent to do it — and DC arc faults are a genuine fire risk. If the fault is on the DC side (string wiring, connectors, panel-level), and you're not an MCS-certified solar installer, the correct action is safe isolation and referral, not opening junction boxes on the roof.

Key Facts

• DC side is always live in daylight — isolating the AC side does NOT make the DC cabling or panels safe. A dedicated DC isolator (usually located at or near the inverter) is required to isolate the array.
• BS 7671:2018+A4:2026 Section 712 covers solar photovoltaic power supply systems — DC isolation, earthing arrangements, and cable protection requirements.
• MIS 3002 is the Microgeneration Certification Scheme's installation standard for solar PV installation and commissioning — required for the property to remain eligible for the Smart Export Guarantee (SEG). (MCS 012 is a separate standard, covering the mounting/fixing system and wind-uplift resistance for panel racking, not general installation and commissioning.)
• G98 applies to small-scale generation (typically up to 16A per phase, ~3.68kW single-phase) — installer self-certifies, no witnessed test required.
• G99 applies to larger installations — requires DNO (Distribution Network Operator) application and, in most cases, witnessed commissioning before connection.
• Inverter fault lights: red/amber status LEDs or an on-screen fault code almost always mean grid fault, insulation resistance fault, or DC undervoltage — check before assuming panel failure.
• RCD/RCBO nuisance tripping: a common cause of intermittent "no generation" reports — inverters have their own internal residual current monitoring (RCMU) as well as relying on the consumer unit's protective devices.
• Bypass diodes: each panel has 2-3 bypass diodes; a failed diode causes partial shading-like output loss on that panel without stopping the whole string.
• String architecture matters for diagnosis: a single failed panel in a string (not using microinverters/optimisers) drags down the whole string's output, not just its own; microinverter or optimiser systems isolate the fault to one panel.
• Shading and soiling account for a large proportion of "underperforming" (not zero-output) complaints — even small shadows from aerials, chimneys, or overhanging branches disproportionately cut output on string inverters.
• Export/generation meter faults can make a working system appear to be "not generating" when it's actually a monitoring/display fault, not a generation fault — check the physical inverter display, not just the app.
• Part P (Building Regulations) applies to the AC-side electrical work in England and Wales — new consumer unit connections must be notified or carried out by a registered competent person.
• Only MCS-certified installers should carry out DC-side fault-finding and repair if the system needs to remain SEG-eligible and under warranty.
• Inverter lifespan: typically 10-15 years — string inverters commonly fail (not just fault) around this age; panels themselves typically carry 25-year performance warranties.
• Isolator switch degradation: DC isolators exposed to UV and weather can fail mechanically (contacts welding shut or failing to make contact) — a known failure mode after 5-10 years outdoors.
• Night-time zero readings are normal — check the timestamp on any "not generating" report before assuming a fault.

Quick Reference Table

Detailed Guidance

Isolation and safety before any inspection

Before touching any part of the system, confirm what you are and aren't qualified to isolate. The AC isolator (usually near the consumer unit) de-energises the inverter's grid-side connection. The DC isolator (at or near the inverter, sometimes also at the roof level on larger systems) isolates the panel string from the inverter — but the cabling between the panels and that isolator remains live in daylight. Never assume a system is "off" because the AC side has been isolated.

SOLAR PV NOT GENERATING — DECISION TREE
|
├─ Is it daytime with reasonable daylight?
│   ├─ NO → not a fault, check again in daylight
│   └─ YES
│       │
│       ├─ Does the inverter display/LED show anything?
│       │   ├─ NO (completely dead) →
│       │   │     Check AC isolator position
│       │   │     Check DC isolator position
│       │   │     Check consumer unit MCB/RCD for the PV circuit
│       │   │     → if all closed/on and still dead: inverter failure, call installer
│       │   │
│       │   └─ YES (display/LED on) →
│       │         ├─ Fault code or red/amber warning light?
│       │         │     → grid fault / insulation fault / DC undervoltage
│       │         │     → note exact code, call MCS-certified installer
│       │         │     → DO NOT open DC junction boxes yourself
│       │         │
│       │         └─ No fault code, but output is low or zero →
│       │               Check for shading on any part of array (even partial)
│       │               Check panel surface for soiling, bird mess, debris
│       │               Compare output to a known-good day (same season/weather)
│       │               → if genuinely underperforming with no shading/soiling:
│       │                     failed bypass diode, failed panel, or optimiser fault
│       │                     → call MCS-certified installer
│
└─ Is the complaint "monitoring app shows nothing" but bills/meter look normal?
      → check physical inverter display first
      → if inverter is generating normally: monitoring/CT clamp/Wi-Fi fault only
      → not a generation fault, lower priority

Isolator and protective device checks

This is the first and most productive step on any call-out, and it's within scope for a general electrician even without MCS certification. Confirm:

• AC isolator switch is in the ON/closed position
• DC isolator switch is in the ON/closed position
• The dedicated MCB/RCBO for the PV circuit in the consumer unit hasn't tripped
• No recent electrical work (extension, new circuit, EV charger installation) that could have involved switching off or disturbing the PV isolators

A significant proportion of "system down" call-outs are resolved at this stage with no further work needed.

Reading inverter fault codes

Inverter fault codes vary by manufacturer and this guide does not recommend specific brands or model-specific codes — check the installer's handover documentation or the inverter manufacturer's technical support line for the exact code meaning. Broadly, fault codes fall into three families:

• Grid fault (over/under voltage, over/under frequency) — the inverter has detected the grid supply is outside acceptable parameters and has disconnected as a protective measure. Often resolves itself once the DNO's supply stabilises; persistent grid faults may indicate a wider network issue worth reporting to the DNO.
• Insulation resistance fault — indicates a potential fault to earth on the DC side, often from moisture ingress at a connector or cable damage. This is a DC-side electrical safety issue and should be referred to an MCS-certified installer.
• DC undervoltage / string voltage fault — can indicate a failed panel, disconnected connector, or (in cold, low-light conditions) can be a normal transient that clears once irradiance increases.

Shading, soiling and gradual output decline

Genuine panel degradation is real but slow — typically around 0.5% output loss per year, built into the manufacturer's 25-year performance warranty. A sudden or seasonal drop in output is far more likely to be:

• New shading from tree growth, a new extension, or a satellite dish/aerial installed nearby
• Soiling — dust, pollen, bird droppings, or (in some areas) salt spray — a visual inspection and, if needed, a professional panel clean resolves this
• Snow or debris sitting on the array after weather events

When to escalate to an MCS-certified installer

Any fault that requires opening a DC junction box, disconnecting MC4 connectors, testing string voltages, or working at the panel level should go to the original installer or another MCS-certified solar PV installer. This protects the homeowner's SEG eligibility, keeps any remaining product/workmanship warranty valid, and avoids the DC arc-fault risk that comes with untrained work on live or potentially live DC cabling.

Frequently Asked Questions

Can I just reset the inverter myself?

Turning the AC isolator off and back on (after waiting the manufacturer's specified time, often around 30 seconds to a few minutes) is generally safe and can clear a transient grid-fault trip. It will not fix a genuine hardware fault, insulation fault, or panel-level issue — if the fault code returns immediately, it needs a competent installer.

Is it dangerous to inspect the panels myself?

Visual inspection from the ground (checking for shading, debris, obvious physical damage) is safe. Going onto the roof, opening junction boxes, or disconnecting MC4 connectors without training is not — the DC side is live whenever there's daylight, and unlike mains AC, DC arcs don't self-extinguish, which is a recognised fire risk investigated by fire services on rooftop PV installations.

Why does my system show generation on the app but my bill hasn't gone down?

This is very rarely a generation fault — it's more likely a tariff, export metering, or usage-timing issue (generating during the day while the property's main consumption is in the evening, for a system without battery storage). Worth checking, but it's a different diagnosis path from "not generating."

The system stopped after a power cut — is that expected?

Yes. Grid-tied inverters are required to disconnect during a mains outage as an anti-islanding safety measure (protecting anyone working on the network) and should reconnect automatically once stable grid supply resumes, typically within a few minutes.

Does a failed inverter always need full replacement?

Not always — some faults are repairable (fan failure, capacitor failure, firmware issues) but string inverters around 10-15 years old failing outright are commonly replaced rather than repaired, as the manufacturer's warranty period has usually expired and repair costs approach replacement cost.

Regulations & Standards

• BS 7671:2018+A4:2026 — Section 712, Requirements for electrical installations: solar photovoltaic (PV) power supply systems.

• MIS 3002 — Microgeneration Certification Scheme standard for the installation, commissioning and handover of solar PV systems. (MCS 012 covers the mounting/fixing system only.)

• Engineering Recommendation G98 — DNO connection standard for small-scale embedded generation (typically ≤16A/phase).

• Engineering Recommendation G99 — DNO connection standard for larger embedded generation requiring formal application and often witnessed commissioning.

• Building Regulations Part P (England and Wales) — electrical safety, applies to AC-side connection work.

• Smart Export Guarantee (SEG) — Ofgem-administered scheme requiring MCS certification for eligibility; not a safety regulation but affects whether unauthorised DC-side repair work can jeopardise payments.

• MCS – Microgeneration Certification Scheme — installation standards including MIS 3002 for solar PV

• Ofgem – Smart Export Guarantee — SEG eligibility requirements

• HSE – Electrical safety and fire risks of PV systems — general electrical safety guidance relevant to rooftop DC systems

• BSI – BS 7671 Wiring Regulations — Section 712 solar PV requirements

• solar pv — Solar PV fundamentals: panel types, string vs microinverters, DNO notification, MCS certification

• solar pv installation pricing guide — Installation pricing by system size, inverter type and DNO notification route

• solar battery systems — Battery storage integration, G98/G99, DC vs AC coupling

• solar pv cleaning and maintenance — Panel soiling and output-loss data, maintenance inspection points