Solar Panels Underperforming?

Solar Panels Underperforming? What to Do (UK 2026 Guide)

Few things sink the heart faster than opening your solar app and seeing a number that looks wrong. I’ve been diagnosing underperforming solar panels for a long time, and here’s the truth that surprises most people: the majority of “broken” systems I’m called to look at aren’t broken at all. It’s the season, a grown tree, a splat of bird mess, or a switch that tripped weeks ago.

But not always. So this is the exact process I use on my own customers’ systems — work out whether it’s normal, find the culprit if it isn’t, and know what the fix should cost before anyone quotes you.

First, the reality check: is it just the weather?

Before you suspect a fault, suspect the British sky. A south-facing UK system generates roughly four times more in May than in December. I’ve had genuinely anxious phone calls in January from people whose systems were working perfectly.

Weather matters within a season too. A grey, drizzly July week can produce half what a bright June week did, and nobody’s system is at fault. Heat is the surprise one: in a proper heatwave, panels actually lose a little efficiency as they get hot, which I’ve covered in my heatwave guide.

Read the numbers properly

Gut feel isn’t diagnosis. Here’s the two-minute method I use before touching a ladder.

• Find your kWh, not your “savings.” Apps love showing pounds and CO₂, but diagnosis needs raw kilowatt-hours. Note the daily and monthly totals.
• Divide by your system size. A healthy UK system makes roughly 850 to 1,000 kWh per kWp per year. The Centre for Alternative Technology puts it at 800 to 1,000. So a 4kWp array should land around 3,400 to 4,000 kWh over a normal year.
• Compare against the same week last year, not last month. Most apps show year-over-year charts for exactly this reason.
• Use the clear-day test. On a properly sunny day, a 4kWp system should be showing 2 to 3.5 kW around solar noon. If it’s bright and you’re seeing a fraction of that, something’s up right now.
• Apply the 5% rule. A sustained drop of 5% or more over two to three weeks of normal weather is my trigger to investigate — clean or check. A consistent 15%-plus shortfall on clear days means a fault, not a mood.

If your app has gone completely quiet, don’t panic about the panels — it’s very often the Wi-Fi dongle or a broadband change, not the hardware. I’ve rounded up what each brand’s app should and shouldn’t look like in my best solar monitoring apps guide, and the wider monitoring system guide explains what good data looks like. And if you want an independent benchmark, the EU’s free PVGIS tool will model your exact roof.

The usual suspects, ranked by how often I see them

Shading deserves a special word because it’s the most misunderstood one. Trees grow. A neighbour’s extension goes up. A chimney you never noticed starts biting the array in winter sun. On a plain string-inverter setup, panels are a chain — weaken one link and the whole string suffers. If shade is permanent and pruning isn’t an option, power optimisers or microinverters take the shaded panel out of the chain; my piece on neighbours’ trees blocking panels covers the pruning and legal side.

Dirt is the gentle one. Rain handles a lot in the UK, but not traffic film, pollen or salt. If you’re near a busy road, farmland or the coast, or your roof is low-pitch, cleaning moves from annual luxury to genuine maintenance — my step-by-step cleaning guide has the safe method, and the annual maintenance checklist puts it all in one place. Older systems get one more suspect: physical ageing like yellowing or delamination, which I’ve written about here.

Inverter and switch checks you can safely do

When output falls off a cliff rather than fading, I’d bet on the inverter or a switch before I’d bet on a panel. Panels fail slowly; electronics fail on a Tuesday. From ground level, in this order:

• Look at the inverter. Blank screen usually means no AC supply or a dead unit. Error codes and red lights mean it knows what’s wrong — photograph everything with timestamps.
• Check the isolators. The AC isolator beside the inverter fully on? Any tripped breaker or RCD in the consumer unit? (Only touch it if you’re dry, confident and it’s easily accessible.)
• Check the boring stuff. Router on? Wi-Fi dongle seated? Half the “faults” I’ve closed were a broadband change that orphaned the monitoring.
• One controlled reset. AC isolator off, five minutes, back on. Once. If the same code returns, stop resetting.
• Eyes-only roof scan. Binoculars or phone zoom from the ground: lifted panels, dangling cables, connector damage. Never go onto the roof, and never unplug DC connectors up there — a string is live whenever there’s daylight, even with the AC side off.

What a fault is actually costing you

Here’s the maths that should make you act. Every lost kilowatt-hour is worth somewhere between 15p and 26p depending on whether you’d have used it yourself or exported it — self-use saves you the full cap rate (24.67p at the time of writing, rising to an average 26.32p from 1 October 2026), while export earns your SEG rate, anything from 4p to 15p on the best SEG tariffs.

So put numbers on it. A typical 4kWp system makes around 3,800 kWh a year. A fault that knocks a third off that is losing you roughly 1,250 kWh — call it £200 to £300 a year in vanished savings and export income. Even one quiet panel, silently down 200–600 kWh a year, is worth up to about £150 of your money. That’s why “I’ll keep an eye on it” is an expensive hobby.

The fixes, and what they cost in 2026

One honest note on inverters: studies of field data have found around a third of string inverters fail by year 15, so if your system is early-2010s and misbehaving, budget for the swap rather than mourning it. If you’ve been eyeing a battery, the repair visit is also the moment to price a hybrid inverter instead and make the repair budget do double duty.

Warranty, MCS and your legal rights

Before you pay for anything, check who already owes you the fix. A UK solar system carries three warranties: the panel product warranty (typically 20–30 years), the performance warranty (25–30 years, usually guaranteeing 87–90% of original output), and the installer’s workmanship warranty. If your output has fallen below the guaranteed performance curve, that’s a claim, not a repair bill. My warranty explainer goes through the small print that decides whether a claim sails through or sinks.

The route when things go wrong runs like this: your installer first, since most claims go through them. If they’ve stopped trading, check for the insurance-backed guarantee or the new MCS-approved financial protection, and remember Section 75 if any of it went on a credit card. If your installer was MCS certified and won’t play ball, the MCS complaints process is your escalation route. And separately from all paperwork, the Consumer Rights Act 2015 gives you up to six years in England, Wales and Northern Ireland (five in Scotland) to claim against the seller for faulty goods — warranty or no warranty. If your installer has gone bust entirely, I’ve written the full playbook here.

My verdict

Nine times out of ten, an “underperforming” UK system is one of three things: winter, shade, or a switch. Run the reality check, read the raw kWh, do the ground-level checks in the order above, and you’ll either fix it yourself for nothing or walk into a call-out knowing exactly what’s wrong and what it should cost.

The expensive mistake isn’t the fault — it’s the quiet one you never notice. A monthly glance at the app, compared with the same week last year, is the cheapest insurance your system will ever have.

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