Solar Panel Degradation: How Much Output Do You Lose?

Year-by-year output for a 4kW system (1,000 kWh/kWp starting):

Key insight: The year 1 to year 25 drop is only 456 kWh — worth approximately £112 per year at current rates. Your panels lose roughly £4.50 per year in output value. This is negligible compared to the £500–£900/year you save.

NREL study data (2,000+ systems worldwide): - Median degradation rate: 0.5% per year - Best panels: 0.2–0.3% per year - Worst panels: 0.7–0.8% per year - Panels from reputable manufacturers consistently fall in the 0.3–0.5% range

Source: National Renewable Energy Laboratory (NREL) PV degradation meta-study; IEC 61215 testing standards.

Solar panel degradation has several causes:

1. Light-Induced Degradation (LID) — Year 1 only: - Occurs in the first 24–72 hours of sun exposure - Causes the initial 2–3% drop - A one-time event — does not recur - Normal and expected for all silicon solar cells

2. UV exposure: - Prolonged UV light causes microscopic changes in the silicon crystal structure - The anti-reflective coating gradually degrades - Responsible for approximately half of the ongoing 0.3–0.5%/year degradation

3. Thermal cycling: - Daily temperature changes cause microscopic expansion and contraction - Over thousands of cycles, this creates hairline micro-cracks in cells - UK's moderate climate causes less thermal cycling than desert environments

4. Moisture ingress: - Trace amounts of moisture can penetrate panel lamination over decades - Causes cell corrosion and reduces electrical conductivity - Quality lamination and edge sealing minimise this

5. Mechanical stress: - Wind loading, snow loads, and vibration cause gradual fatigue - UK wind speeds are well within panel design limits - Not a significant degradation factor for properly installed panels

Source: NREL PV module reliability workshop; IEC 61215/61730 durability testing.

Not all panels degrade at the same rate:

Monocrystalline (PERC/TOPCon): - Degradation: 0.3–0.5% per year - Year 25 output: 85–88% of original - Performance warranty: typically 84–86% at 25 years

Monocrystalline (HJT — heterojunction): - Degradation: 0.25–0.35% per year - Year 25 output: 87–91% of original - Performance warranty: typically 86–90% at 25 years - Lower LID (initial drop is smaller)

Polycrystalline: - Degradation: 0.5–0.7% per year - Year 25 output: 80–83% of original - Performance warranty: typically 80% at 25 years

Thin-film (CdTe/CIGS): - Degradation: 0.5–0.8% per year - Year 25 output: 75–80% of original - Shorter lifespan overall (15–20 years typical)

Premium panels (SunPower Maxeon): - Degradation: 0.2–0.25% per year - Year 25 output: 92%+ of original (industry-leading) - Performance warranty: 92% at 25 years

Source: Manufacturer datasheets; NREL degradation data by technology.

Barely. Here is why:

4kW system, 25-year financial summary accounting for degradation:

Without degradation: 25-year total would be £14,500 With degradation: 25-year total is £13,855 Difference: £645 over 25 years = £26/year

Degradation costs you approximately £26 per year in reduced savings. This is far less than the £500–£600 per year you save. The financial case is overwhelmingly positive despite degradation.

All reputable savings estimates already account for degradation. If our calculator says you will save £14,000 over 25 years, degradation is already included.

Source: Calculations using 0.4% annual degradation; Ofgem price cap July-September 2026 rates.