Fixing underperforming solar panels starts with ruling out the ordinary causes – weather, season, dirt and shading – before assuming there’s a fault. Work through those first, then check your inverter for error codes, and only call a professional once you’ve confirmed the drop isn’t explained by something completely normal.
Step 1: Rule Out Weather and Season First
Compare today’s generation against a similarly cloudy or sunny day from the same time last year, not against your best-ever reading. UK solar output naturally varies hugely with the seasons – a normal winter day can produce a fraction of a summer day’s generation – so a single low reading rarely means anything on its own. It’s only a persistent, unexplained gap against comparable past conditions that’s worth investigating further.
Step 2: Check for New Shading or Dirt
Shading and dirt are the two most common causes of a genuine, sustained decline, and both are fixable without calling anyone out. Look for a tree or hedge that’s grown since installation, new scaffolding or a building next door, or simply a long dry spell that’s let dust and pollen build up. If it’s been more than a year since a clean, that alone is worth ruling out before moving on to anything more technical – see our guide to cleaning solar panels safely for how to do it without climbing on the roof.
Step 3: Check Your Inverter for Error Codes
Your inverter’s display or app is usually the fastest way to narrow down a real fault. A specific error code will often point directly at the cause – a grid connection issue, an overheating warning, or a fault on a specific string – and many manufacturers publish a code list online. A routine restart (following the manufacturer’s instructions, not just switching things off and on arbitrarily) resolves a surprising number of minor glitches; if the same code comes straight back, that’s your signal to stop troubleshooting yourself and call for support.
Step 4: Narrow It Down to a Specific Panel or String
If you have optimisers or microinverters, your app should show generation panel by panel, which makes this step straightforward – look for one panel sitting noticeably below its neighbours. Without panel-level monitoring, you’re working with string-level data at best, which only tells you a problem exists somewhere in that string, not exactly where. Either way, isolating the affected area (rather than assuming the whole system has failed) helps whoever you call next diagnose it faster.
Common Causes Matched to Fixes
| What you’re seeing | Likely cause | What to do |
|---|---|---|
| Gradual decline over weeks/months | Dirt build-up or growing shade from vegetation | Clean the panels; trim nearby trees or hedges |
| Sudden drop to zero, error code shown | Inverter fault | Restart per manufacturer guidance; call installer if it recurs |
| One panel consistently low, no shading explanation | Possible microcrack, connector fault, or manufacturing defect | Request a professional inspection, ideally with thermal imaging |
| Whole string affected equally | Wiring or connector issue somewhere in that string | Professional inspection – not a DIY job given the voltages involved |
| Output lower than expected but stable | Roof orientation, pitch or shading was always suboptimal | Compare against realistic expectations for your specific roof, not a generic average |
Step 5: Know When to Call a Professional
Call your installer, or another MCS-certified professional, if you’ve ruled out weather, dirt and shading and the drop persists; if an error code recurs after a proper restart; if one panel or string is consistently underperforming with no obvious cause; or if you notice any physical damage, unusual noise, or a burning smell. Don’t attempt to open the inverter casing or investigate wiring and connectors yourself – solar systems carry DC voltages that remain live even when your home’s power is off, and this genuinely isn’t a safe DIY job.
Making a Warranty Claim for Underperformance
Most panels carry a 20–25 year performance warranty guaranteeing a minimum output level (often around 80–87% of original capacity at the 25-year mark), separate from the shorter product warranty covering manufacturing defects. If you believe your system is underperforming beyond what’s reasonable, documented monitoring data covering a meaningful period, ideally with a professional inspection report, gives you the strongest basis for a claim. Contact your original installer first, since they’re usually the point of contact for both the panel and inverter warranty.
Frequently Asked Questions About Fixing Underperforming Solar Panels
What percentage drop in solar output should worry me?
There’s no single number, since normal seasonal variation alone can account for an 80% swing between summer and winter. The useful comparison is against a similar-weather day from a previous month or year for your specific system – a persistent 20% or more shortfall against that comparable baseline, with no obvious explanation, is worth investigating.
Can I fix an underperforming inverter myself?
You can usually perform a basic restart following the manufacturer’s own instructions, but anything beyond that – opening the casing, checking internal components, or working on wiring – should be left to a qualified professional because of the voltages involved.
How long does a professional repair usually take?
A straightforward fix, such as reseating a connector or replacing a single panel, is often a same-day job once someone’s on site. Diagnosis and parts availability (particularly for an older or less common panel model) can add more time if a replacement component needs to be sourced.
Will fixing underperformance be covered under warranty?
If the cause is a genuine manufacturing defect or the panels are producing below their guaranteed performance level within the warranty period, yes, it should be covered. Issues caused by external factors – storm damage, or a tree that’s grown to cause shading, for example – typically fall outside warranty and would be your own responsibility to resolve.
