Annual Degradation Rate

What Is Annual Degradation Rate?

Answer: Annual degradation rate describes a change in PV performance per year on a stated reference basis. It may be a measured trend, a forecasting assumption or a warranty-limit slope; those are different quantities. Comparing rates requires the same power or energy boundary, time origin, normalization method and linear or compounded convention, with uncertainty stated where measured.

The useful question is not just “what percentage?” but “a percentage of what, measured or assumed over which years?” Module maximum power at standard test conditions is different from annual plant AC energy. Weather, downtime, soiling and changes in equipment can alter energy without establishing module aging.

Measured Rate, Forecast and Warranty

PVsyst’s aging documentation explicitly distinguishes the warranted minimum for individual modules from the average degradation assumed for an array. It also identifies increasing mismatch as a separate aging effect. Do not equate a warranty curve with a measured fleet average or guarantee identical plant energy decline.

A warranty can support an equipment-selection discussion, but its verification process and remedies matter. It does not automatically promise replacement, compensation or a particular financial return.

Calculation Conventions

Use a declared starting reference. For hypothetical 400 W reference power, a 0.5% annual compounded decline and 25 full years elapsed, with no separate initial loss:

P(t) = P0 × (1 − d)^t

P(25) = 400 × 0.995^25 = 352.89 W, or 88.22% of the starting reference.

Under a linear decline of 0.5 percentage points of the starting reference per year:

P(t) = P0 × (1 − d × t)

P(25) = 400 × (1 − 0.005 × 25) = 350 W, or 87.5%.

Neither is a vendor warranty or an observation. The conventions produce different answers. If a curve has a separate first-year step, identify its baseline and the number of subsequent annual steps; do not add that loss again when already included in the reference.

This also matters in software. SAM’s degradation documentation distinguishes linear single-rate treatment in Detailed PV/PVWatts from compounding in other models, and states how annual-table inputs are applied. Check the selected model and version rather than assuming every tool uses the formula above.

Lifetime Energy Without an Extra Year

Assume, only for illustration, 10,000 kWh in Year 1, identical weather and other operating conditions each year, and a 0.5% compounded reduction between annual outputs. Sum 25 annual outputs, starting with an undegraded Year-1 value:

Lifetime energy = 10,000 × Σ(0.995^n), for n = 0 through 24.

The result is 235,559.51 kWh, about 5.78% below the zero-decline comparison of 250,000 kWh. Using n = 0 through 25 would count 26 outputs. A linear convention gives 235,000 kWh for these same declared Year-1 and 25-output boundaries.

These are arithmetic scenarios, not site yield or savings predictions. Electricity prices, export compensation, costs, finance and discounting require separate assumptions; increasing tariffs do not automatically offset declining production.

Mechanisms and Measurement

Module aging can involve light-induced changes, environmental stress and material/interconnection changes. Initial effects and later trends need not follow one constant slope. The 2025 NLR PV Lifetime project report describes early-life differences across technologies and repeated indoor IV measurements at controlled irradiance and temperature to characterize maximum power.

That methodology explains why raw year-to-year kWh decline is insufficient. Retain baseline measurements and event records; assess soiling, shading changes, outages, curtailment and sensor problems before attributing a trend to aging. Loss analysis supports that diagnostic distinction.

Do not assume an n-type label eliminates every degradation mechanism, or change grounding to a generic polarity rule to “prevent PID.” Use the exact module/inverter instructions and qualified project review.

Practical Handoff

Designers should record the source, reference, start year, convention and separate initial/mismatch assumptions. Show sensitivity rather than silently using the same rate for every module and climate.

Installers should follow handling instructions and retain commissioning records. Monitoring teams should preserve a comparable dataset and document replacements and missing-data treatment. Sales teams should explain the modeled year-by-year energy and warranty limits without fictional projects, universal retained-power ranges or guaranteed payback.

When evaluating SurgePV, ask how degradation inputs and annual-output conventions are represented in the proposed workflow. See panel degradation for the related physical topic.

Common Questions

How much do solar panels degrade each year?

There is no single rate for every module, site or measurement method. Use documented evidence appropriate to the exact equipment and distinguish measured trends from forecast assumptions and warranty limits.

Does a 25-year performance warranty give the measured degradation rate?

No. It defines a contractual threshold and conditions; it is not automatically the expected average output trend.

Is annual energy decline always degradation?

No. Weather, soiling, outages, curtailment and measurement changes can also affect annual energy. Investigate those boundaries before diagnosing aging.

How does degradation affect ROI?

It changes the modeled energy profile. Financial results additionally depend on tariffs, costs, financing and other stated assumptions; no fixed payback change follows from the rate alone.