Every solar panel loses a small amount of power output each year — that’s simply how silicon photovoltaic cells age. The question homeowners actually need answered isn’t “do panels degrade” (they do) but “how much, how fast, and does it matter for my payback sums.” This guide walks through the real numbers, the difference between older PERC panels and the newer N-type cells now dominating UK installs, what a performance warranty actually promises versus what it doesn’t, and what 25 years of gentle decline looks like on a typical British roof.
Why panels degrade at all
Solar cells convert sunlight to electricity through the photovoltaic effect, and that process very slowly wears on the materials involved. Three mechanisms do most of the damage over a panel’s life:
- Light-induced degradation (LID) — a small, one-off drop (typically 1-3%) in the first weeks of exposure as the cell structure settles. Reputable manufacturers usually quote power ratings after this initial dip, not before it.
- UV and thermal cycling — repeated heating and cooling (and UK weather delivers plenty of both) micro-stresses solder joints, encapsulant (the EVA or POE layer sealing the cells) and the anti-reflective coating.
- Potential-induced degradation (PID) and moisture ingress — slow electrochemical effects at cell edges and connections, more common in cheaper panels with weaker encapsulation and framing.
None of this means a system suddenly fails. Degradation is a gentle, near-linear decline in output — a panel doesn’t drop off a cliff, it just produces slightly less each year than the year before.
PERC vs N-type: the number that changed
For most of the 2010s, the UK residential market ran on PERC (Passivated Emitter and Rear Cell) panels, and the industry-standard degradation assumption was around 0.5-0.55% per year after the first-year dip. A 25-year-old PERC panel installed at, say, 340W nameplate could reasonably be expected to be producing in the region of 290-300W by year 25 — call it 85-88% of original output.
Since roughly 2023-24, the panels going onto UK roofs have shifted decisively to N-type cell architectures — TOPCon, heterojunction (HJT) and back-contact (ABC) designs. These use a different silicon wafer doping process that’s inherently more resistant to the light and boron-oxygen defects that drive PERC’s faster decline. Manufacturer datasheets for these panels now commonly specify degradation of around 0.4% per year, sometimes as low as 0.25-0.3% for premium HJT/ABC lines, against 0.5%+ for older PERC stock.
Run that out over a 25-year performance warranty and the difference compounds:
| Panel type | Year-1 output (after LID) | Year 25 estimated output | Cumulative loss |
|---|---|---|---|
| Older PERC (~0.55%/yr) | ~98% | ~85% | ~15% |
| Modern N-type (~0.4%/yr) | ~98% | ~88.5% | ~11.5% |
| Premium HJT/ABC (~0.3%/yr) | ~98.5% | ~91% | ~9% |
Those few percentage points sound marginal, but on a 4kW system generating perhaps 3,400 kWh a year in the south of England, an extra 3-4% retained output in year 20 onward is worth real money over the remaining warranty term — and it’s one of the genuine reasons panel technology has actually improved rather than just got cheaper.
What a performance warranty actually guarantees
This is where a lot of confusion creeps in, so it’s worth being precise. A solar panel typically carries two separate warranties:
- A product warranty (usually 12-25 years depending on manufacturer and tier) covering manufacturing defects — delamination, frame failure, junction box faults. This is a straightforward “it broke, we’ll replace it” guarantee.
- A performance (or power output) warranty, almost always running 25-30 years now, which guarantees the panel will still produce at least a stated percentage of its original rated output by a given year — commonly something like “97% by year 1, declining to 87.4% by year 25” on a linear scale.
The performance warranty is not a promise that your roof will generate a specific number of kWh — it’s a warranty against the panel’s rated output falling faster than the stated curve, tested under standard lab conditions (STC), not your actual weather, shading, or inverter losses. If your installer used MCS-certified equipment and standards (a prerequisite for Smart Export Guarantee eligibility), the warranty documentation should be in your handover pack — keep it, because claims are made against the manufacturer, not the installer, and some manufacturers have gone bust over 25 years.
Worth noting too: the inverter is not covered by the same clock. String inverters typically last 10-15 years and cost roughly £500-£1,000 to replace — budget for at least one replacement across a 25-30 year panel life. This is a maintenance-schedule point that specialists like Solar Maintenance Solutions build servicing plans around, because a tired inverter, not degrading cells, is usually the first thing that actually dents your generation numbers.
What 25 years of output really looks like
Putting the degradation curve into context against typical UK yields helps make it concrete. A well-sited 4kW system in the south of England, with roughly 1,000-1,050 kWh per kWp per year of solar resource, might generate around 4,000-4,200 kWh in year one. Applying a realistic 0.4%/yr N-type curve:
- Year 1: ~4,100 kWh
- Year 10: ~3,950 kWh (roughly 96% of year-1 output)
- Year 20: ~3,800 kWh (roughly 93%)
- Year 25: ~3,750 kWh (roughly 91-92%)
Compare that to a lower-yield location further north — say 850 kWh/kWp/yr, closer to what you’d expect across much of Scotland or northern England — and the absolute kWh numbers shift down proportionally, but the percentage degradation curve is identical; panel chemistry doesn’t care about latitude, only irradiance and heat cycling do (and cooler climates are, if anything, slightly gentler on thermal stress). Installers such as ecoaim working across Central Scotland routinely model this lower-yield baseline into customer payback figures rather than borrowing southern England assumptions, which is exactly the right approach — a degradation curve applied to an inflated yield estimate produces a misleadingly rosy 25-year total.
The upshot for payback maths: even at the more conservative end, a system is still producing close to 90% of its first-year output as it exits a 25-year warranty term. With panel costs for a typical 4kW residential system now sitting around £6,000-£8,000 installed, and 0% VAT on residential solar and battery storage in Great Britain confirmed until 31 March 2027, the economics were never resting on panels staying at 100% forever — they were always priced against a gentle decline, and that decline has got measurably gentler with the move to N-type cells.
Does degradation rate actually change your buying decision?
In practice, degradation is rarely the deciding factor between panel brands — inverter reliability, warranty administration (will the manufacturer honour a claim in year 18?), and installer workmanship matter more day-to-day. But it’s a legitimate tie-breaker when two quotes are otherwise similar, and it’s a useful sanity check on any quote that leans heavily on 25-year “total savings” projections: ask to see the degradation assumption used in the calculation, not just the headline output figure.
It’s also a good reason to ask about panel provenance specifically rather than trusting a generic “premium panel” label. Installers with genuine technical depth — FLD Electrical in Swansea and South Wales, or Premier Electrical Renewables covering solar, battery and EV installs — should be able to tell you the cell technology (PERC vs TOPCon vs HJT), not just the wattage, and show you the manufacturer’s actual degradation datasheet rather than a marketing summary. If a quote can’t answer that question, treat it as a small red flag on the rest of the paperwork too.
For system sizing and cost context alongside degradation, our sister site thecostofsolar.co.uk has a dedicated payback-period breakdown, and if you’re still weighing up whether panels are worth it on a typically overcast British roof, we’ve covered that separately in do solar panels work in the UK.
Maintenance’s role in protecting your real-world output
Degradation is the slow, predictable decline built into the panel itself — but it’s often not the biggest threat to your actual annual generation. Soiling (dust, pollen, bird mess), moss creep at panel edges, a failing MC4 connector, or a string inverter quietly derating itself are all faster and more fixable causes of underperformance, and they’re exactly what a mid-warranty inspection catches. If your system is coming up to 8-10 years old, it’s worth having someone check actual output against expected output for your degradation curve rather than assuming any shortfall is “just the panels ageing” — installers offering ongoing servicing, from regional firms like Hazell Electrical in West Kent to dedicated aftercare specialists, can usually tell within one visit whether you’re looking at normal degradation or a fixable fault.
The same logic scales up to commercial roofs, where the stakes are proportionally larger. A warehouse solar array or a factory installation generating hundreds of thousands of kWh a year turns even a 1-2% unexpected shortfall into a meaningful sum, which is why commercial O&M contracts increasingly specify performance-ratio monitoring against the manufacturer’s degradation curve as a contractual baseline, not just a nice-to-have.
The bottom line
Modern N-type panels degrading at around 0.4% a year (against roughly 0.5%+ for the PERC generation they’re replacing) means a well-installed UK residential system should still be producing somewhere around 88-91% of its original output after 25 years — comfortably within the terms of a standard performance warranty, and a genuinely better number than the panels installed five or six years ago. Degradation isn’t a reason to delay going solar, and it isn’t something a specific brand can eliminate; it’s a known, gentle, and now well-documented curve that installers and their manufacturers already price into every quote and warranty. The more useful question to ask any installer isn’t “will these degrade” — they will, gently — but “show me the datasheet, the warranty terms, and how you’d tell degradation apart from a fault.”