A solar panel installed today will still be generating power in 2050 — but not as much. Module degradation is the slow, steady erosion of output that determines whether your financial model holds up over the project's lifetime. Understanding LID, PID, and annual degradation rates helps you choose modules that protect long-term yield.
Types of solar panel degradation
Light-Induced Degradation (LID)
LID occurs in the first hours to days of sunlight exposure for standard P-type silicon cells. Boron-oxygen (BO) complexes form in the silicon, creating defects that reduce charge carrier lifetime and cut output by 1–3%. LID is a one-time event — it stabilises quickly and does not continue throughout the module's life.
N-type silicon (TOPCon, HJT, IBC) does not contain boron in the base layer and is essentially immune to LID — one of the key performance advantages of N-type technology over P-type PERC. LONGi, Jinko, and Trina now offer N-type TOPCon modules with LID <0.5% in their datasheets.
Potential-Induced Degradation (PID)
PID occurs when a high electrical potential difference between the module frame (ground) and the cell causes ion migration through the encapsulant and glass. Sodium ions from the glass accumulate at the cell surface and create shunting paths that reduce output — sometimes by 20–50% in severely affected modules.
PID risk is highest in:
- Hot, humid climates (relative humidity consistently above 60%)
- Large string voltage systems (1000V or 1500V DC bus)
- Negative-grounded or floating inverter configurations
- Systems where the string's negative rail is at significantly negative potential to earth
PID can be mitigated by specifying PID-resistant modules (tested per IEC 62804), using transformer-based inverters or inverters with PID protection features, and ensuring correct system grounding. Some Sungrow inverters include active PID compensation.
Long-term annual degradation
Beyond LID and PID, modules experience gradual performance loss from UV exposure, thermal cycling, and cell metallisation changes. This is the "annual degradation rate" specified in the power warranty.
| Module type | Typical first-year loss | Annual degradation (years 2–30) | Output at year 25 |
|---|---|---|---|
| Standard P-type PERC (mono) | 2–3% (LID) | 0.55%/year | ~83–87% |
| Premium P-type PERC (LONGi, Jinko) | 1–2% | 0.45%/year | ~87–90% |
| N-type TOPCon | <0.5% (no LID) | 0.40%/year | ~90–92% |
| HJT (Heterojunction) | <0.5% | 0.25–0.35%/year | ~93–95% |
How degradation affects lifetime yield
The difference between a 0.40%/year and 0.55%/year degradation rate sounds small but compounds significantly. For a 1 MWp system generating 1,400 MWh/year at commissioning:
- 0.55%/year module at year 25: Output factor ~86.6% → 1,213 MWh/year
- 0.40%/year module at year 25: Output factor ~90.6% → 1,268 MWh/year
- Difference: 55 MWh/year → $5,500/year at $0.10/kWh
- Cumulative 25-year difference: ~450 MWh → $45,000 in lost revenue
The premium for N-type TOPCon modules over P-type PERC is typically $0.01–0.03/W — for a 1 MWp system, $10,000–30,000. The lifetime yield benefit is $45,000+, making the N-type premium easily justified on an NPV basis in most markets.
What to check in the power warranty
- First-year guarantee: Should be ≥97% of rated power (P-type) or ≥98% (N-type)
- Annual degradation rate: ≤0.55% for P-type, ≤0.40% for N-type. Some premium modules now warrant 0.25–0.30%/year.
- Year 25 guarantee: Compare directly — 80.7% is the old standard; ≥84.8% (LONGi Hi-MO X), ≥87.4% (Jinko Tiger Neo) and ≥89% (some HJT) are now achievable.
- Linear vs stepped: Always choose a linear warranty. A stepped warranty (80% guaranteed only at year 25) provides no protection against excessive degradation in intermediate years.
- PID warranty: Confirm PID resistance testing per IEC 62804 is included, especially for 1500V DC systems in humid climates.
Field degradation rates: real-world data
NREL's analysis of 11,000+ modules from 230+ real-world systems found median degradation of 0.5%/year for crystalline silicon — consistent with datasheet values. However, the distribution is wide: bottom-quartile modules degrade at 1.0–1.5%/year, driven primarily by encapsulant browning, delamination, and cell cracking from inadequate mechanical protection during handling and installation. Module quality and installation quality both matter.