Solar Panel Efficiency After 10 Years — What to Expect

The Quick Answer: 90-92% of Original Efficiency

Modern solar panels degrade at roughly 0.5% per year. After 10 years, your panels will still be producing 90-92% of their original output. On a typical 4kWp system generating 3,600 kWh per year when new, that means you are still getting around 3,310-3,350 kWh at the decade mark. The difference in your pocket? Roughly £20-30 less per year in savings. Not nothing, but not a reason to lose sleep either.

What Is Solar Panel Degradation?

Degradation is the gradual decline in a panel's power output over time. Every solar panel loses a tiny fraction of its efficiency each year. This is a normal, well-understood process — not a defect.

Three main factors drive it:

• UV exposure: Prolonged ultraviolet light slowly breaks down the encapsulant material that protects the silicon cells. This causes microscopic discolouration and reduces light absorption
• Thermal cycling: Daily and seasonal temperature swings cause the panel materials to expand and contract. Over thousands of cycles, this creates micro-cracks in the silicon cells and solder joints
• Moisture ingress: Even with robust sealing, tiny amounts of moisture can penetrate the panel laminate over the years. This accelerates corrosion of internal connections and reduces cell performance

The good news is that degradation rates have improved dramatically. Panels manufactured in the 2020s degrade far more slowly than those from a decade ago, thanks to better encapsulants, improved cell interconnection, and more rigorous quality control.

Degradation Rates by Panel Type

Not all panels degrade at the same speed. The type of solar cell technology matters:

• Monocrystalline (most common in UK installations): 0.3-0.5% per year. After 10 years, expect 95-97% efficiency. Premium brands like SunPower and LG have documented rates as low as 0.25% per year
• Polycrystalline: 0.5-0.7% per year. After 10 years, expect 93-95% efficiency. Still very respectable, and these panels were widely installed in the UK between 2010-2018
• Thin film (CdTe/CIGS): 0.5-1.0% per year. After 10 years, expect 90-95% efficiency. Less common on UK rooftops but used in some commercial installations

If you had panels installed in the last five years, they are almost certainly monocrystalline — and likely at the better end of these ranges.

What the Research Shows

This is not guesswork. Long-term studies back up these figures:

• The National Renewable Energy Laboratory (NREL) analysed nearly 2,000 degradation rates from panels worldwide and found a median degradation rate of 0.5% per year, with panels manufactured after 2000 performing significantly better
• A study published in Progress in Photovoltaics tracked UK installations over 10+ years and found that most systems were performing within 5-10% of their original rated output after a decade
• The European Commission's Joint Research Centre has consistently found that real-world degradation in northern European climates (including the UK) tends to be lower than in hotter regions like southern Spain or Australia, because cooler operating temperatures put less thermal stress on panels

In short, the UK climate is actually kinder to solar panels than many sunnier countries. Less heat stress means slower degradation.

Manufacturer Warranty Guarantees

Every reputable panel manufacturer backs their products with two types of warranty:

• Product warranty (10-25 years): Covers manufacturing defects, materials failure, and workmanship issues
• Performance warranty (25-30 years): Guarantees a minimum power output — typically 90% at year 10 and 80-85% at year 25

Here is what the major brands guarantee at the 25-year mark:

• SunPower / Maxeon: 92% output at year 25
• LG (legacy panels): 88.4% at year 25
• Jinko Solar: 84.8% at year 25
• JA Solar: 84.8% at year 25
• Canadian Solar: 83.1% at year 25

These are guaranteed minimums. In practice, most panels outperform their warranty figures comfortably.

What 10 Years of Degradation Actually Costs You

Let us put real numbers on this. Take a typical 4kWp system in Milton Keynes:

• Year 1 output: ~3,600 kWh
• Year 10 output (at 0.5%/year degradation): ~3,420 kWh
• Lost generation: ~180 kWh per year
• Lost savings (at 28p/kWh self-consumption): ~£50 per year at most
• Realistic lost savings (blended self-use + export): ~£20-30 per year

That is roughly £2 per month. Over the full 10 years, the cumulative loss from degradation adds up to about £100-150 total — spread across a decade. Your system will still be saving you £800-1,200 per year at the 10-year mark.

To put it another way: degradation reduces your savings by about 2-3% after a decade. It is the least of your worries.

How to Maintain Panel Efficiency

You cannot stop degradation entirely — it is a fundamental property of silicon solar cells. But you can make sure your system is not losing output to avoidable problems:

Keep Panels Reasonably Clean

In the UK, rain does most of the cleaning for you. But bird droppings, tree pollen, and lichen can build up, especially on low-pitch roofs. A visual check once or twice a year is sensible. If panels are visibly dirty, a gentle clean with water and a soft brush (or a professional panel cleaning service) can recover 2-5% of lost output.

Manage Shade

Trees grow. What was a clear roof 10 years ago might now have overhanging branches casting partial shade. Even small amounts of shading can disproportionately reduce output — a single shaded cell can drag down an entire string of panels. Trim back any new growth that is encroaching on your roof.

Monitor Your System

Most modern inverters come with monitoring apps that show daily, monthly, and annual generation. Check your output periodically and compare it to previous years (accounting for seasonal variation). A sudden drop is worth investigating. A gradual decline in line with expected degradation is perfectly normal.

Check Your Inverter

Your inverter is the hardest-working component in the system. If your output drops unexpectedly, the inverter is often the culprit — not the panels themselves. Look for error codes, listen for unusual buzzing, and make sure ventilation around the inverter is clear.

Inspect Wiring and Connections

Have a qualified installer check your DC isolators, cable connections, and roof fixings every few years. Corrosion or loose connections can cause output losses that look like panel degradation but are actually fixable.

When to Worry: Sudden Drops vs Gradual Decline

Gradual decline of 0.5% per year is completely normal. You probably will not even notice it in your generation figures because year-to-year weather variation causes bigger swings than degradation does.

A sudden drop of 10-20% or more is a different story. This usually points to a specific fault:

• Inverter failure: The most common cause. Inverters are electronic devices with a finite lifespan. If your system stops generating entirely or output halves overnight, check the inverter first
• Hot spots or cracked cells: Physical damage from storms, impacts, or manufacturing defects can cause individual cells to fail. This shows up as a disproportionate output drop
• Wiring fault: A loose MC4 connector or corroded junction box can take out part of a string
• New shading: A neighbour's extension, a new tree, or even a satellite dish installed on your roof can cause unexpected losses

If your monitoring shows a sudden, unexplained drop, contact your installer or an MCS-certified solar engineer for a diagnostic check.

Inverter Lifespan vs Panel Lifespan

This is where people often get confused. Panels and inverters have very different lifespans:

• Solar panels: 25-30+ year lifespan. Warranted for 25 years. Many will still be producing useful output at 35-40 years
• String inverters: 10-15 year lifespan. Budget for one replacement (£800-1,500) during the life of your panels
• Microinverters/optimisers: 20-25 year lifespan. More expensive upfront but typically last as long as the panels
• Hybrid/battery inverters: 10-15 years. Similar to string inverters but more complex

If your system is approaching 10 years old and you are on the original string inverter, start budgeting for a replacement. When the time comes, consider upgrading to a hybrid inverter with battery storage — you get a new inverter and the ability to add a battery in one go.

Is It Worth Replacing 10-Year-Old Panels?

Almost never. Here is why:

• Your panels are still producing 90-92% of their original output — they have another 15-20 years of useful life left
• Replacing working panels means writing off the remaining value of equipment you have already paid for
• The cost of new panels plus installation labour would take years to recoup from the marginal efficiency gain
• The environmental cost of manufacturing new panels to replace working ones defeats the purpose

The only scenarios where replacement makes sense:

• Physical damage: Cracked glass, severe delamination, or hot spots that cannot be repaired
• Major roof work: If you are re-roofing anyway, it might make sense to upgrade panels at the same time — but only if the existing ones are genuinely underperforming
• Significant underperformance: If panels are producing less than 75-80% of rated output and are outside warranty, replacement could be justified

For most homeowners at the 10-year mark, the smart move is to keep your existing panels, replace the inverter when needed, and consider adding a battery to boost self-consumption.

What to Do Now

If you are thinking about solar for the first time, panel degradation should not put you off. Modern panels are incredibly durable and the financial case is strong even accounting for gradual efficiency losses over 25+ years. Use our free solar calculator to see what a system could save you — the numbers factor in real-world degradation over the system's lifetime.

If you already have panels and want to check their health, get in touch for a system health check. We can assess output, inspect your inverter, and advise on whether a battery upgrade makes sense at this stage.

Frequently Asked Questions