Solar Panel Degradation Calculator (UK)
Estimate UK solar panel output loss with first-year LID and annual degradation. Free calculator for 25-year cumulative kWh, MCS-aligned warranty checks.
Solar Panel Degradation Calculator
| Year | % | kWh |
|---|---|---|
| 1 | 98% | 2,902 |
| 5 | 96.1% | 2,844 |
| 10 | 93.7% | 2,774 |
| 15 | 91.4% | 2,705 |
| 20 | 89.1% | 2,638 |
| 25 | 86.9% | 2,573 |
How to use this calculator
Enter seven values and the calculator returns current output, output at year 25, 25-year cumulative kWh, and the lifetime kWh and revenue you’ll lose to natural decay:
- System size (kW) — total nameplate. The average UK domestic installation is 4 kW (10 panels at 400 W).
- Peak sun hours per day — UK average is 2.6 (south of England) to 2.2 (Scottish Highlands). Solar Energy UK publishes regional values.
- System efficiency (%) — the derate factor. 78% is standard for UK rooftop installations with string inverters.
- Panel age today (years) — 0 for a new install; 7 for a 7-year-old SEG-era array.
- First-year LID drop (%) — 1.5–2.5% for typical p-type mono-PERC panels, 0.3–0.5% for n-type modules.
- Annual degradation (%/yr) — 0.5% UK median, 0.3–0.4% for Tier-1 panels (REC, Q CELLS, SunPower).
- Electricity rate (£/kWh) — your current Ofgem cap rate or fixed tariff. UK average late-2026 is around £0.27/kWh.
How solar panel degradation works
Every solar panel loses output over time. The decay has three distinct phases:
Phase 1 — LID (light-induced degradation). In p-type crystalline silicon (the dominant residential technology), the first 30–100 hours of sun exposure trigger boron-oxygen defects that drop output by 1–3%. The reaction completes within two weeks and never reverses. Tier-1 manufacturers bin panels at +1.5% over rated power so the panel stabilises at nameplate. N-type silicon (SunPower Maxeon, LG NeON R) and gallium-doped p-type (modern Trina, LONGi, Jinko) are essentially LID-free.
Phase 2 — linear degradation, years 1–25. After LID stabilises, the panel decays at a steady 0.3–0.7% per year. Causes include slow EVA (encapsulant) yellowing, microcracks from thermal cycling, soldering fatigue, and PID where high-voltage stress drives sodium ions into the cell. NREL’s Jordan & Kurtz review of 11,000 systems puts the median at 0.5%/yr for crystalline silicon. UK field data from Solar Energy UK aligns closely.
Phase 3 — accelerated failure, post-warranty. Beyond the 25-year warranty endpoint, failures accelerate: junction box delamination, glass fracture, backsheet cracking. UK panels installed in 2010 under the original Feed-in Tariff are now mostly past 15 years and still producing 90–93% of original output.
The degradation math
For year n of system life, output relative to STC nameplate is:
year_factor(n) = (1 - LID) × (1 - degradation_rate)^(n - 1)For year 0 (before any sun exposure), the factor is 1.0. Year 1 the LID drop applies. From year 2, annual degradation compounds.
A worked example for a 4 kW UK system, 2% LID, 0.5% annual degradation:
- Year 1: 4 kW × (1 − 0.02) = 3.92 kW = 98.0% of STC
- Year 5: 3.92 × (1 − 0.005)^4 = 3.842 kW = 96.0% of STC
- Year 10: 3.92 × (1 − 0.005)^9 = 3.747 kW = 93.7% of STC
- Year 15: 3.92 × (1 − 0.005)^14 = 3.654 kW = 91.4% of STC
- Year 20: 3.92 × (1 − 0.005)^19 = 3.564 kW = 89.1% of STC
- Year 25: 3.92 × (1 − 0.005)^24 = 3.476 kW = 86.9% of STC
That matches the warranty curves published by MCS-listed Tier-1 manufacturers: 85–87% at year 25 for standard mono-PERC.
Degradation rates by panel type
NREL median rates by technology, validated against Solar Energy UK and Energy Saving Trust field studies:
| Technology | First-year LID | Annual rate | Year-25 output |
|---|---|---|---|
| n-type mono (SunPower Maxeon, LG NeON R) | 0.3% | 0.25%/yr | 93.7% |
| Premium p-type mono (REC Alpha, Q CELLS Q.PEAK) | 1.0% | 0.30%/yr | 91.9% |
| Standard p-type mono-PERC | 2.0% | 0.50%/yr | 86.9% |
| Polycrystalline silicon | 2.5% | 0.55%/yr | 85.5% |
| CdTe thin-film (First Solar) | 0.5% | 0.40%/yr | 90.0% |
Tier-3 imports — often sold through unaccredited installers — frequently show 0.8–1.2% annual degradation in MCS field audits. UK building regs and SEG eligibility require MCS certification, so most homeowners are protected from the worst panels by default.
What accelerates degradation in the UK
Temperature
The UK’s cool, cloudy climate is one of the kindest in the world for solar panels. Rooftop cell temperatures peak around 45–55°C, versus 65–75°C in Mediterranean Europe. This roughly halves the EVA yellowing rate. UK panels routinely beat their warranty curves.
Damp and condensation
The flip side of cool climate: high humidity. Backsheet degradation from prolonged moisture exposure is a documented UK failure mode. IEC 61215 humidity-freeze testing covers it, but installers should specify panels with backsheets rated for damp-heat (DH1000+ hours).
Hail
Rare but devastating in the UK. The April 2018 hailstorm in Hampshire caused widespread microcrack damage to thousands of residential panels. MCS-installed systems with insurance reported claims under both product warranty and household contents — the latter usually wins because microcracks are excluded by most product warranties as “external damage”.
PID (potential-induced degradation)
Modern UK installations use transformerless inverters (SolarEdge, Fronius, Solis), which can induce PID in non-resistant panels. UK distributors now stock almost exclusively PID-resistant modules — verify the datasheet lists IEC TS 62804-1 testing.
Reading your UK warranty
Two warranties cover panel output:
- Product warranty — 10–25 years, covers manufacturing defects (cell cracks, junction box failure, hot spots). REC, SunPower, Q CELLS, and Project Solar UK offer 25-year product warranties.
- Performance warranty — 25–30 years, guarantees a minimum output curve. Linear is friendlier than stepped two-tier warranties.
Under MCS, the installer is the first contact for warranty claims for the first 24 months. After that, the manufacturer takes over. The MCS Consumer Code requires the installer to provide both warranties at handover alongside the MCS certificate (essential for SEG registration with Octopus, OVO, or another licensed supplier).
Common mistakes
- Confusing nameplate with year-1 output. Most p-type panels lose 1–2% within the first 100 hours.
- Forgetting tariff escalation when valuing lost kWh. Ofgem cap rates rose 50% between 2021 and 2025. A kWh saved in 2050 is worth materially more than one saved in 2026.
- Buying non-MCS panels to save money. They don’t qualify for SEG, can’t be insured against degradation, and often aren’t supported by UK distributors when warranty claims arise.
- Ignoring degradation in payback calculations. UK paybacks under current SEG rates run 9–12 years. Adding 0.5%/yr degradation extends that by 6–9 months — handled correctly by the solar panel payback calculator.
Sources
- MCS — Microgeneration Certification Scheme — product and installer certification standards
- Solar Energy UK — Member Reliability Survey — annual field data on installed system performance
- Energy Saving Trust — Solar Panel Performance Guide — official UK consumer guidance
- Ofgem — Smart Export Guarantee — export tariff and registration rules
- NREL — Photovoltaic Degradation Rates: An Analytical Review — global meta-analysis foundation