Problems with Solar Panels: A 2026 UK Homeowner and Business Guide
Modern UK solar systems are reliable. When problems do happen, the most common are: (1) under-generation from new shading (trees grew, neighbour built an extension, satellite dish appeared), (2) MC4 connector failure (a loose or corroded DC plug behind a panel), (3) inverter fault (almost always recoverable, sometimes warranty replacement). Actual panel failures are rare on Tier 1 monocrystalline. Most problems are caught by comparing real generation against the PV*SOL forecast on your install paperwork — if you're within ~10% of forecast you're fine. Anything else: book a diagnostic with an MCS-accredited installer.
Why this guide exists
Spectrum Energy Systems has installed 10+ MW of solar across Nottinghamshire, Derbyshire, Leicestershire, Lincolnshire, Staffordshire and South Yorkshire since 2011 — and we've fault-found on systems installed by dozens of other companies. This guide tells you what actually goes wrong in real installs, how to spot it early, what's worth fixing yourself, and what needs a professional. Written for UK domestic and small-commercial customers; the commercial-fleet stuff is covered separately.
In This Guide
The most common solar problems we see
The remaining ~5% is split between battery faults, DC isolator failures, monitoring/comms issues, and the rare lightning-strike or pest damage. Pure panel-glass-and-cells failures are uncommon on Tier 1 hardware in its first decade. The boring truth is most "solar problem" enquiries we take are diagnosable in under 30 minutes and fixable for <£300.
Under-generation — the #1 problem by far
Generation has dropped or is below what the install paperwork suggested. This is the issue that drives the majority of fault-finding visits. The first thing to do is compare your actual annual generation against the PV*SOL forecast figure on your original install paperwork.
| Actual vs PV*SOL forecast | What it means | What to do |
|---|---|---|
| Within ±10% | Normal weather variance | Nothing — system is fine |
| 10–15% below | Likely soiling, mild shading, or one panel underperforming | Visual check, possibly clean — see our cleaning guide |
| 15–25% below | Material problem — usually new shading, MC4 issue, or panel fault | Book a diagnostic visit |
| >25% below | Serious — inverter, string, or multi-panel fault | Book a diagnostic urgently |
| Step-change drop on a specific date | Discrete event — lightning, MC4 failure, isolator trip | Diagnostic urgent |
Common under-generation causes — in order of how often we find them
1. New shading from trees, neighbour extensions, chimney additions, or satellite dishes. 2. MC4 connector resistance behind a single panel (corrosion or loose fit). 3. Inverter fault that hasn't cleared. 4. One panel in a string with cell damage or bypass-diode failure. 5. Soiling that rain didn't shift (bird droppings, agricultural dust).
Panel-level faults
Genuine panel failure is uncommon on Tier 1 hardware. When it does happen, the signs:
- Visible cracking — spider-web pattern under the front glass; sometimes from impact, sometimes from thermal stress
- Discoloured backsheet — yellow, brown or blistered backsheet, especially around junction box
- Hot-spotting — thermal imaging shows one cell 5–15°C hotter than neighbours, indicating cell damage or shading
- Bypass diode failure — one third of the panel produces nothing when any cell in that sub-string is shaded
- Snail trails — thin dark lines on the cell surface; rare but indicates micro-cracking
- Junction box scorching — serious; indicates internal overheating, replace immediately
For testing methodology see our how to test a solar panel guide.
Inverter problems
Usually recoverable
- Fault codes that clear themselves after an AC restart
- Grid disconnect events after a brief power cut
- Insulation alarms after heavy rain (usually clear once dry)
- Over-temperature trips on a hot summer afternoon
- Communications errors with the app
Warranty-replacement territory
- Repeated faults that won't clear
- Internal fan failure (audible noise change)
- Display going blank or showing corrupted text
- Burning smell — isolate AC immediately
- String over-voltage error after panels added without resize
Modern Solis hybrid inverters have 10–12 year warranties as standard, extendable to 20. SolarEdge similar. A failed inverter under warranty is parts-free, labour typically £250–£500.
Battery problems (LFP-specific)
LFP batteries (Fogstar for LV Solis, Pylontech Force H3 for HV Solis) are very reliable in the first 5 years. When problems do come up:
- BMS comms errors — inverter loses contact with battery management system. Usually a firmware update or CAN/RS485 cable check.
- Cell imbalance — single cell drifting out of balance, BMS reports. Some self-balancing over time; persistent imbalance is a warranty case.
- Capacity drift — the apparent "100% SoC" point shifts. Usually a calibration cycle resolves it.
- Reduced cycle range — usable capacity has dropped >5% from new in the first 2 years. Warranty case.
- Forced discharge events — battery discharging to grid unexpectedly. Typically a Predbat / inverter configuration issue, not a battery fault.
MC4 connector and DC wiring faults
This is the second most common problem after under-generation, and most homeowners have never heard of it. MC4 connectors are the standard DC plug-and-socket used between every panel and the DC isolator/inverter. A loose or corroded MC4 adds electrical resistance — sometimes only a fraction of an ohm, but enough to:
- Drop output 5–15% on the affected string
- Heat the connector (sometimes badly enough to melt the plastic housing)
- Cause arc faults in the worst cases (rare but serious)
Signs: a single panel in a string consistently underperforming on monitoring data, connector visibly discoloured, or a smell of plastic from behind a panel. Fix: replace the MC4 connector with a fresh crimp. About £30 of parts, plus the diagnostic visit.
DIY vs professional — what's safe to investigate yourself
Safe to do yourself
- Compare actual annual generation against PV*SOL forecast
- Check the inverter app for recent fault codes
- Visual inspection from ground level: cracked glass, sagging panels, discoloured backsheets
- Check for new shading (trees, extensions, dishes)
- Reset the inverter via its app (turn off, wait 60s, on)
Always call a professional for
- Any rooftop electrical work — DC strings run at 400–600V
- MC4 connector inspection or replacement
- String testing or IV-curve tracing
- Battery faults or BMS errors
- Any burning smell or warm-to-touch DC equipment
- Suspected lightning damage
Prevention and monitoring
1 Check the inverter app weekly
SolisCloud or your inverter's app shows daily generation and any active fault codes. A 30-second weekly check catches issues months before they show up in your annual figures.
2 Compare against PV*SOL forecast annually
Your install paperwork contains a predicted annual kWh figure from the PV*SOL design. Compare your actual generation against it each January. Within ±10% is fine; persistent under-performance flags a real issue.
3 Visual annual check from the ground
Binoculars from the garden are enough. Look for cracks, discolouration, lifted panels, or vegetation/satellite-dish encroachment. Five-minute job once a year.
4 Book a professional electrical test every 5 years
String voltages, DC isolator condition, MC4 connector resistance, inverter firmware, battery BMS health. Spectrum's domestic warranty includes this in years 1 and 5; commercial maintenance contracts include scheduled testing.
FAQs
What's the most common problem with solar panels?
By a wide margin: poor generation caused by shading the installer didn't account for. Trees grow, neighbouring extensions get built, satellite dishes get added. The panels themselves are usually fine. The second most common is MC4 connector failure — a loose or corroded DC connector behind the panel that gradually adds resistance until output drops noticeably. The third is inverter fault, almost always recoverable.
How do I know if my solar panels are faulty?
Three checks. First, compare your annual generation against the PV*SOL forecast on your install paperwork — within 10% of forecast is normal, more than 15% below is suspicious. Second, check the inverter app or SolisCloud for fault codes that haven't cleared themselves. Third, look at the panels visually for cracked glass, discoloured backsheet, or scorched MC4 connectors. If any of these flag up, get an MCS-accredited installer to diagnose.
How long should solar panels last in the UK?
Tier-1 panels installed in 2026 carry 25–30 year performance warranties guaranteeing 87–90% output at end of warranty. Real-world degradation on the panels we've monitored is around 0.3–0.5% per year. Inverters typically last 10–15 years (one replacement in panel lifetime is normal). Batteries 15–20 years on LFP chemistry. A well-installed Spectrum system today should run cleanly into the 2050s.
What does it cost to fix a solar panel problem?
Diagnostic visit and minor fixes (MC4 connector, isolator switch, monitoring reset): typically £150–£300. Inverter replacement under warranty: parts free, labour £250–£500. Panel replacement under warranty: parts free, labour £100–£250 per panel including site access. Full system diagnostic with IV-curve tracing on a problem 4–12 panel array: £300–£600. We give fixed-price diagnostic quotes after a 15-minute call.
Will Spectrum diagnose problems on a system we didn't install?
Yes — we offer fault-finding diagnostics on any brand, any installer's system. String testing, IV-curve tracing, thermal imaging, MC4 connector inspection, inverter health check. Common after a previous installer has gone out of business or stopped responding. We give an honest written report of what we find, fixed-price remediation quote, and we'll work with you to chase any remaining warranty claims against the original installer.
My panels generated less in 2025 than 2024 — is that a problem?
Not necessarily. Year-on-year generation varies with weather — a cloudier summer can drop output 5–10% on its own. Compare against the PV*SOL forecast (which accounts for typical UK weather) rather than against last year. Only if you're consistently under PV*SOL across multiple years is it likely a system issue.
When to call Spectrum
Got a suspected solar problem? We'll diagnose it
MCS NIC200223. We fault-find on any brand, any installer's system across the East Midlands. String testing, IV-curve tracing, thermal imaging. Fixed-price diagnostic quotes after a 15-minute call.
Request a feasibility assessmentRelated reading
- How to test a solar panel
- Solar panel maintenance guide
- How to clean solar panels
- How long do solar panels last in the UK?
- How efficient are solar panels in 2026?
Speak to Spectrum Energy Systems
MCS NIC200223. We install, maintain and diagnose solar PV across the East Midlands. Fault-finding on any brand, any installer.
Request a feasibility assessment