Solar panel degradation in the UK: what you really lose over 25 years

Solar panels do degrade. They just do it slowly.

If you are researching solar in the UK, you will see scary-sounding phrases like “only 80% after 25 years”. That is usually a warranty minimum, not a realistic forecast.

This guide explains degradation in a practical, UK-relevant way:

• What degradation is, and what it is not
• What a normal yearly decline looks like
• Why year 1 can behave differently
• What performance warranties actually guarantee
• How much lifetime kWh and savings are typically affected (see Glossary for kWh, kW and kWp)

If you want the baseline for your own roof first, Find your postcode.

We will use live postcode examples across the UK, from coastal Kent to upland Derbyshire, and we will link to a few regional hubs where it helps the intuition.

Postcode examples used in this guide:

• FY3 (Blackpool)
• TQ7 (South Hams)
• DD11 (Angus)
• HU12 (East Riding of Yorkshire)
• TN28 (Folkestone and Hythe)
• CR8 (Croydon)
• PE20 (Boston)
• S32 (Derbyshire Dales)

Quick Answer: how much output do solar panels lose over time?

If you want to benchmark your own long-term expectation, start with your area’s baseline estimate:

Check your postcode: SolarByPostcode

Assumptions and variability

• The percentages here describe typical real-world degradation, not what every panel model will do in every home.
• Your outcome varies with panel model and tier, installation quality, roof ventilation and heat, and long-run exposure (coastal vs inland).
• Short-term “drops” in the UK are usually weather and seasonality, not true degradation.
• Performance warranties are minimum guarantees (a floor), and are often more conservative than typical outcomes.
• SolarByPostcode estimates use consistent modelling assumptions so you can compare areas and scenarios like-for-like. See: Data sources and methodology.

What “panel degradation” actually means

Degradation is a gradual reduction in how efficiently a solar panel converts light into electricity.

It is not:

• “solar doesn’t work in the UK”
• your inverter getting tired
• one cloudy month
• a monitoring app glitch
• dirt on the glass (soiling is separate and often fixable)

In practice, people often mislabel ordinary UK variability as “degradation”. For example, a grey week in FY3 (Blackpool) can look like “something has faded”, while the same system pattern in CR8 (Croydon) might look fine simply because the weather baseline is different.

If you are seeing disappointing daily graphs, do not reach for “degradation” first. Weather drives most short-term variation in the UK.

If you have not read them yet, these two guides help prevent the most common misdiagnoses:

• Cloud cover vs solar output: what actually happens on overcast UK days
• Why your solar panels will never hit their rated output (and why that’s completely normal)

The pattern most people miss: small early drop, then slow decline

Real-world degradation is often not perfectly linear from day one. Many systems follow a simple two-phase story:

1) Early stabilisation (often within year 1)
2) Slow decline over the rest of the system’s life

Why year 1 can look different

If you want a grounded year-on-year comparison, pick a location anchor and stick to it: compare summer to summer in the same place, like PE20 (Boston), rather than comparing your first summer to your first winter.

In the first year, you are also learning your system. Many owners unintentionally compare:

• peak summer weeks vs winter
• a sunny new-installation phase vs a cloudier year
• different ways of using electricity at home

That is why it is risky to “diagnose” degradation from a few months of app graphs.

Typical degradation rates: what is “normal”?

You will see a lot of numbers online. The simplest way to think about it is:

• Good modern panels in a good install: often near the low end
• Hotter roofs, poor ventilation, harsh coastal exposure, or weaker build quality: drift towards the high end

A practical range for many modern systems is roughly:

• ~0.3% to 0.8% per year (after any early stabilisation)

If you want a rule-of-thumb outcome without overthinking it:

• Year 25 output is often around 85% to 90% of year 1 output for a typical system

This is exactly why “my neighbour gets more” is a dead end. A system in DD11 (Angus) and one in TN28 (Folkestone and Hythe) can have very different seasonal ceilings even with identical panels and inverter.

That is why it is so important not to interpret “80% at 25 years” as the expectation. That number is usually the lowest level the manufacturer is willing to guarantee.

Warranties: what “80% after 25 years” really means

Manufacturers often provide a performance warranty that looks like:

• a guaranteed minimum output at year 25 (commonly around 80% to 85%)
• sometimes a separate first-year guarantee plus a yearly decline thereafter

That warranty is best read as a floor.

It is not the manufacturer saying: “this is what your panel will do”.

It is the manufacturer saying: “if your panel is worse than this, we will consider a claim”.

A practical way to read a performance warranty

When you see “80% at year 25”, translate it into plain English:

“If the panel is worse than this minimum, you might have a warranty claim.”

It does not mean:

“You should expect to be at 80%.”

This difference matters because many UK households incorrectly “bake in” the warranty floor as their expected long-term generation, which makes solar look worse than it is.

How much does degradation reduce lifetime kWh?

Here is the important part for savings.

Even if your year-25 output is, say, around 88% of year-1 output, the lifetime hit is smaller than people assume, because you are not at 88% for the whole period. You drift down gradually.

A simple approximation is:

• If output slides from 100% to 88% over 25 years, the average over the period is about 94%.

So the total lifetime energy is typically only about 6% lower than “no degradation”.

Why this matters for payback and “is solar worth it?”

Most UK savings come from:

• how much solar you can use in the home (self-consumption)
• your electricity tariff (unit rate and standing charge)
• export payments (Smart Export Guarantee or other export tariff)
• system cost and financing terms

Degradation affects the long-run kWh, but the typical lifetime impact is usually smaller than:

• a small change in system size
• a change in how much you self-use
• a change in tariff or export rate
• shading or sub-optimal roof orientation

If you want to size your system properly (often the bigger financial lever), this guide is designed to be paired with:

• Solar system sizing in the UK: choosing the right kWp without wasting money

What causes degradation to be faster?

Degradation is a mix of material ageing and environmental stress. The most common accelerators are:

Heat and poor ventilation

Higher operating temperatures tend to increase stress on materials. A roof with poor airflow behind panels can run hotter.

This is part of why roof design, mounting method, and installer quality can matter long-term.

If you want to understand the temperature side properly, this companion guide helps:
- Temperature losses in UK solar panels: how heat quietly eats your kWh

Harsh exposure (coastal air, wind, salt)

Coastal exposure can be tougher on materials. The UK is not extreme compared to desert climates, but coastal environments can still be harsher than inland.

A system near the coast in TN28 (Folkestone and Hythe) may face different long-run exposure than an inland system in S32 (Derbyshire Dales).

Microcracks and mechanical stress

Panels are robust, but they are not indestructible. Handling, transport, and installation quality can affect the risk of microcracks.

This is one reason choosing a competent installer matters more than many people expect.

Moisture ingress and poor sealing

Modern panels are well sealed, but manufacturing quality still varies. Water ingress and seal failure are rare, but when they happen they can accelerate decline.

How to tell “normal degradation” from a problem

If you suspect your system is underperforming, the key is to separate:

• weather variation (normal)
• monitoring variation (common)
• real faults (less common)

Step 1: stop looking at one-day graphs

Cloud and seasonality dominate daily charts in the UK. Use longer windows:

• last 30 days (minimum)
• last 12 months (better)
• year-on-year comparisons (best)

If you want a grounded explanation of why UK graphs look the way they do, start here:

• Cloud cover vs solar output: what actually happens on overcast UK days

Step 2: focus on kWh, not peak power

Peak kW is easily distorted by weather, temperature, and cloud edges (see Glossary for kW and kWh).

kWh totals are what drive savings.

The “rated output” confusion is a common reason people think their panels have degraded when they have not:

• Why your solar panels will never hit their rated output (and why that’s completely normal)

Step 3: benchmark with your postcode instead of your neighbour

Comparing a system in the South East to one in the North West is usually meaningless, even at the same kWp (see Glossary for kWp).

Use local baselines (regional hubs) instead of cross-country guesswork:

• South East
• North West
• Yorkshire
• Southern Western
• Northern Scotland

If you want a quick intuition check before you run the calculator: the South East generally has a higher year-to-year ceiling than the North West or Northern Scotland, so always compare within your region first.

Or go direct to the most useful tool:

Check your area: SolarByPostcode

FAQs

How long do solar panels last in the UK?

Most panels are designed to operate for decades. Degradation is typically slow, so the panels can remain productive well beyond 25 years, even if output is lower than new.

Do solar panels degrade faster in the UK because of clouds?

Cloud affects short-term output, not long-term degradation. UK cloudiness changes day-to-day kWh far more than it changes the long-run ageing rate.

Is “80% at 25 years” bad?

Not necessarily. That figure is usually a warranty minimum. Many systems remain above it. What matters is the total lifetime kWh, which is often only modestly reduced by degradation.

Does degradation ruin the payback?

Usually no. For most UK homes, savings are more sensitive to system size, tariffs, and self-consumption than to small differences in yearly degradation.

Bottom line

• Degradation is real, but usually slow
• Many systems have a small early drop, then a steadier decline
• Warranties are minimum floors, not predictions
• Lifetime kWh impact is often single-digit percent
• For savings, system sizing and self-use usually matter more

Next reads

• Solar warranties and insurance in the UK: what is covered, what is not, and who to call first
• Solar monitoring: how to spot underperformance early (without obsessing)
• Why solar panels never hit their rated output (and why that is normal)
• Cleaning, bird droppings, and pigeon protection: what is worth doing (and what is not)
• Solar panel cost in the UK: what you actually pay and why quotes vary

If you want a realistic expectation for your own roof and postcode, start here:

Check your area: SolarByPostcode

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