Calculator
Solar Panel Degradation and Lifetime Yield Calculator
Modules do not fail on a cliff edge; they fade. A first-year drop of one to two percent from light-induced degradation, then a slow annual decline, compounds into a difference of several thousand kilowatt-hours over the system's life.
This calculator projects the whole curve, totals the lifetime energy, and compares the result against the linear performance warranty the manufacturer offers.
Set the degradation profile
Production in year 1 after LID
8,373 kWh
Production in year 25
7,424 kWh
87.3% of nameplate year
Total energy over 25 years
197,222 kWh
Energy lost to degradation
15,278 kWh
7.2% of the no-degradation case
Value of that loss
€2,750
Undiscounted, at the price entered
Modelled output at year 30
85.2%
Below the 87.4% floor — a claim would be justified
Warranty slope
0.42% per year
The manufacturer's committed maximum
How the calculation works
Degradation has two distinct phases. Light-induced degradation happens in the first hours to weeks of exposure as boron-oxygen complexes form in p-type silicon, costing 1 to 3% for PERC and considerably less for n-type TOPCon and heterojunction cells. After that, the decline is a slow compounding process driven by encapsulant discolouration, solder-joint fatigue, cell cracking and moisture ingress.
The model applies the first-year drop once, then compounds the annual rate. Compounding matters: 0.5% a year over 25 years leaves 88.6% of the original output, not 87.5% as simple subtraction suggests.
Manufacturer warranties are stated as a linear floor — for example, 98% in year one and no less than 87.4% in year 30. The floor is deliberately below typical measured performance, so a system tracking its modelled curve will normally sit well clear of it.
The formula
E₁ = E_nameplate × (1 − LID) E_y = E₁ × (1 − d)^(y − 1) E_total= Σ E_y for y = 1..N Warranty floor(y) = 100% − LID_w − s × (y − 1) LID first-year light-induced degradation d annual degradation rate · s warranty slope per year
Assumptions behind the numbers
- Year-one production is entered before light-induced degradation; the model applies it.
- Degradation is treated as a constant annual rate, which matches field data reasonably well after the first year and up to about year 20.
- No inverter replacement, cleaning improvement or repowering event is modelled — those change the curve sharply and should be added separately.
- Weather is held constant, so the projection shows the technology trend rather than a year-by-year forecast.
- The warranty comparison uses the linear floor as published; some warranties use a stepped curve with a larger year-one allowance.
How to read the result
Measured degradation across large fleets clusters around 0.5 to 0.7% a year for crystalline silicon, with the median in recent studies closer to 0.5%. Using 0.8% in a financial model is conservative to the point of distorting comparisons; using 0.25% is optimistic beyond the published evidence.
The difference between 0.4% and 0.7% a year over 25 years is roughly 4% of lifetime energy — smaller than most people expect, and usually smaller than the uncertainty in the tariff assumption.
If measured output falls faster than the modelled curve, degradation is rarely the cause. Look for soiling, a failed optimiser, a partially disconnected string or a drifting irradiance sensor first.
Worked example
An 8,500 kWh system uses PERC modules with 1.5% first-year LID and 0.5% annual degradation over a 25-year analysis, valued at €0.18/kWh.
Year one delivers 8,373 kWh. Year 25 delivers 8,373 × 0.995²⁴ = 7,423 kWh, which is 87.3% of the nameplate year. Total lifetime energy is about 198,400 kWh, against 212,500 kWh for a hypothetical non-degrading array — a loss of 14,100 kWh, worth roughly €2,540 undiscounted.
At year 30 the modelled output is 90.1%, comfortably above a typical 87.4% warranty floor, which is the expected outcome: warranties are written so that normal modules never trigger them.
Mistakes that skew the answer
- Applying the annual rate to year one as well as the LID drop, double-counting the initial loss.
- Using linear subtraction rather than compounding, which quietly overstates late-life output.
- Assuming the warranty curve is a prediction. It is a floor with commercial margin built in, not an expected value.
- Ignoring that n-type modules have both lower LID and lower annual rates, which changes technology comparisons over 25 years.
- Blaming degradation for a sudden drop. Degradation is gradual by definition; a step change is a fault.
Questions readers ask
- How fast do solar panels actually degrade?
- Field studies of crystalline silicon report a median around 0.5% a year, with most modules between 0.3 and 0.8%. Modern n-type modules trend towards the lower end, and manufacturers now warrant 0.4% or better for TOPCon and heterojunction products.
- Will my panels still work after 25 years?
- Almost certainly, at around 85 to 90% of their original output. The 25-year figure is a warranty and accounting convention, not a lifetime. Arrays from the 1980s are still producing, generally with degraded encapsulant and lower output but no failure.
- What causes degradation?
- Encapsulant browning from ultraviolet exposure, solder-joint fatigue from daily thermal cycling, micro-cracks propagating from handling or hail, moisture ingress through the backsheet or edge seal, and potential-induced degradation in high-voltage strings that are not properly earthed or use susceptible cell designs.
- Does degradation invalidate my payback calculation?
- It shifts it by a few months on a typical residential system, not years. The dominant uncertainties are the electricity tariff, the export price and self-consumption share; degradation is a second-order effect by comparison.
- How do I prove a warranty claim?
- You need a flash test or an on-site IV curve trace at reported conditions, corrected to STC, from an accredited laboratory or a calibrated field tracer, plus the original purchase documentation. Manufacturers require module-level evidence, not system-level production data.