What is the Degradation Rate of Monocrystalline Silicon PV Panels Per Year

High-quality monocrystalline PV panels degrade ~0.3-0.5% annually; standard ones 0.5-0.8%, retaining >80% output after 25 years—lower rates achieved via low-iron glass and tight encapsulation to block moisture/UV damage.

Degradation Rate Range

Currently, the general consensus in the industry for high-quality monocrystalline silicon panels is an annual degradation rate between 0.5% and 0.8%. This means that a brand new 400W panel might lose between 2W and 3.2W of its peak power after one year of operation.

The linear power warranty offered by the vast majority of tier-one brands is based on this range: typically guaranteeing no more than 1% to 2% degradation in the first year, and thereafter, from the second year to the 25th or 30th year, an annual degradation of no more than 0.45% to 0.55%, ultimately ensuring an output power of no less than about 84.8% to 87% of the initial value after 25 years.

Warranty Range

Currently, the warranty standards for monocrystalline silicon modules from tier-one brands are highly unified: first-year degradation is typically guaranteed not to exceed 2%, and thereafter, from year 2 to year 30, the annual degradation rate is guaranteed not to exceed 0.45% to 0.55%.

For a nominal 430W module, the power at the end of the first year might not be less than 421.4W, and by the end of the 25th year, the power is guaranteed to be no less than 365.5W (85% of the initial value).

What are the key numbers hidden in the warranty?

First-Year Degradation of 2%: A Conservative "Safety Cushion"

First is the initial light-induced degradation (LID), which is determined by the properties of the silicon material, usually occurring within the first few months, potentially causing a power drop of ~1% to ~1.5%. Second is the initial stress release of materials, such as minor deformations of the encapsulate film under sunlight and temperature variations. Manufacturers setting this value at 2% is actually a very generous "safety cushion".

In reality, for P-type PERC modules using low-degradation defect silicon ingots and advanced passivation technology, the first-year LID can be controlled within 1.2%; while the latest N-type TOPCon or HJT modules, being immune to LID, can have their first-year degradation commitment as low as 1%, with actual performance even better than 0.5%.

0.45% vs. 0.55%: What does an annual difference of 0.1% mean?

The linear degradation rate starting from the second year is the true measure of a manufacturer's technical capability and quality confidence. Although the difference is only 0.1% per year, over a cycle of 25 to 30 years, this leads to a huge difference in energy generation. Taking a 430W module with a 25-year lifespan as an example:

· Annual degradation 0.45%: Guaranteed power after 25 years is 430W * (1-0.02) * (1-0.0045)^24 ≈ 366.2W

· Annual degradation 0.55%: Guaranteed power after 25 years is 430W * (1-0.02) * (1-0.0055)^24 ≈ 359.4WThe difference is nearly 7W in end-of-life output power. For a 50kW residential power station, the cumulative energy generation gap over 25 years could amount to thousands of kilowatt-hours, directly affecting the return on investment.

Unstated nuances in the warranty that you must clarify

"Not less than" is the bottom line, not the average

The warranty states "power in year 25 is not less than 84.8% of the initial value" – this is the legal minimum guarantee. However, the actual degradation curve of high-quality modules is usually far better than this bottom line. Field studies by NREL show that many older modules that have been operating for 20 years still maintain actual power above 90% of their initial value. You should view the warranty value as the "worst-case scenario", and based on brand reputation and technology route (e.g., N-type usually outperforms P-type), you can expect better actual performance.

Prerequisites for warranty validity: Improper installation may void the warranty

Almost all manufacturer warranties have an important prerequisite: installation must be performed according to the manufacturer's provided "Installation Manual". For example, incorrect clamp placement and force during installation may cause micro-cracks in the glass; improper grounding may induce potential-induced degradation (PID).

Actual Performance

When you view the PV panel warranty as a promise of "passing the final exam with 60 points," its actual performance might often deliver report cards with scores above 85. The manufacturer's warranty terms are a legally conservative promise based on extensive testing and risk assessment, with the primary goal of ensuring the manufacturer itself does not face financial risk due to large-scale claims.

The annual degradation of 0.45%-0.55% written on the warranty sheet often proves to be a quite generous upper limit in the real world. Extensive tracking and monitoring of systems that have operated for 10, 20, or even 30 years by third-party research institutions (such as the US National Renewable Energy Laboratory, NREL) show that the average annual degradation rate for early high-quality monocrystalline silicon systems is generally stable in the range of 0.3% to 0.5%.

A set of 250W nominal power modules installed in 2010 might have an actual power today closer to 215W or even higher, not the approximately 200W calculated by the warranty. This extra 10-15W of power, and the thousands of kilowatt-hours of cumulative energy generation it represents, is the "hidden benefit" beyond the warranty.

Real-world data is more optimistic than the warranty sheet

A long-term study of over 1,000 PV systems globally, involving more than 40,000 modules in total, found that about 85% of the systems had an annual degradation rate lower than 0.5%, and 40% of the systems even achieved an excellent level below 0.3%. For example, in temperate European regions, some systems that have operated for 15 years have had their total power degradation controlled within 5%, translating to an average annual degradation of only 0.33%.

Even systems installed in high-temperature, high-humidity coastal areas of Southeast Asia, as long as they are installed properly and maintained correctly, mostly have their actual degradation rates controlled at 0.6%-0.7%. These data indicate that as long as the modules themselves are of good quality, their "lifespan" in most practical application scenarios surpasses the 30-year warranty period; many modules can continue to operate at over 70% of their initial power even after 35 or 40 years.

Why can actual performance exceed Warranty commitment?

Firstly, accelerated aging tests are far more stringent than reality. For example, the damp heat test requires modules to operate continuously for 1000 hours in an environment of 85°C temperature and 85% relative humidity to simulate decades of outdoor aging.

Secondly, advancements in manufacturing processes and materials are key to surpassing old standards. Ten years ago, the anti-yellowing ability of EVA encapsulate was an industry challenge, whereas current co-extruded high-performance encapsulate films have significantly improved anti-UV ability and hydrolysis stability, ensuring that the transmittance decrease does not exceed 2% over 25 years.

New technology routes are refreshing the lower limits of degradation

If you have recently been focusing on N-type TOPCon or HJT modules, the margin by which actual performance exceeds the warranty will be even greater. The reason these modules dare to promise first-year degradation ≤1% and annual degradation ≤0.4% is that their physical mechanisms are fundamentally superior. The "Light-Induced Degradation" (LID) that P-type PERC modules cannot completely avoid, rooted in boron-oxygen complexes, causes a power loss of 1%-2% in the first few months.

Whereas N-type silicon wafers use phosphorus as the dopant, basically eliminating the impact of boron-oxygen complexes on performance, their initial light-induced degradation is negligible. This means that an N-type module may have almost no measurable initial degradation after running for one year, directly entering a slow aging channel with an average annual rate of 0.3%-0.4%. Some plant owners report that for N-type TOPCon power stations installed for 3 years, the actual power generation degradation curve is almost a flat line, with an annualized degradation rate below 0.2%.

Your maintenance habits directly determine the excess return

The inherent potential of the modules requires good external conditions to be realized. Regular cleaning is the most direct and effective method. Dust covering the panel surface at 5 grams/square meter can lead to a 3%-5% decrease in power generation. In areas with little rain and much dust, cleaning once a month compared to never cleaning can result in an average annual energy generation gap of over 8%.

Furthermore, avoiding partial shading is extremely important. Shading caused by a small leaf or bird droppings not only results in the loss of power generation from the covered part but may also form hot spots, causing local temperatures to rise sharply to over 100°C, which in the long term accelerates permanent damage due to EVA yellowing and cell performance degradation. A well-ventilated installation structure can lower the module operating temperature by 5°C-10°C, and for every 1°C decrease in operating temperature, the output power increases by about 0.3%-0.5%, while the degradation pressure of high temperature on materials is also reduced accordingly.

Understanding the gap between actual performance and warranty promise allows you to more accurately assess the long-term true return of a PV investment. This reminds us that when choosing products, we should not only focus on the 0.1% numerical difference on the warranty sheet but also pay attention to its technology route, the reputation of the materials used, and the manufacturer's historical data.

Key Factors Affecting the Range

The same PV panel, installed on a rooftop in Qinghai for 10 years, might still have 92% of its power left, while installed by the seaside in Hainan, it might only have 88% left. Degradation rate is never a fixed value, but a variable strongly dependent on the external environment.

The core reasons for the differences in attenuation lie in three main stresses: thermal stress, moisture stress, and mechanical stress. For every 10°C that the ambient temperature continues to exceed the standard test condition of 25°C, the chemical aging rate of the module approximately doubles, and the annual degradation rate may increase from 0.5% to 0.6% or even higher.

In areas with strong ultraviolet radiation, the annual UV dose received by the back sheet may exceed 150 kWh/m², and inferior back sheets may show powdering or cracking within 5-8 years. A bolt torque error exceeding ±10% during installation, or an unreasonable bracket design causing excessive vibration amplitude at a wind speed of 35m/s, can trigger unexpected material fatigue and microcracks over 15 years.

Environment is the number one killer: High temperature, high humidity, high salt spray

PV panels most fear the "three highs" environment. Hot and humid weather is a 'pressure cooker' for PV panels. In places like Hainan or Southeast Asia, when the ambient temperature is 35°C in summer, the module surface temperature can easily exceed 70°C. The internal encapsulate material, EVA film, accelerates hydrolysis under continued high temperature, causing the film to yellow and its light transmittance to decrease. The aging speed of this process in an 85°C environment is 4-8 times that in a 65°C environment.

If the humidity is consistently over 80% at the same time, moisture can penetrate the back sheet edges and junction box seals, causing electrochemistry corrosion with the metal grid lines on the cell surface. Cases have shown that in coastal power plants, increased series resistance due to grid line corrosion can be observed within 3 years, causing an additional power loss of over 3% for the module.

UV is invisible wear. In plateau areas at an altitude of 2000 meters, UV intensity is more than 30% higher than by the seaside. UV photon energy can break the chemical bonds of the polymer material of the back sheet. A qualified back sheet needs to withstand at least 60 kWh/m² of cumulative UV irradiation without significant performance decline, Inferior back sheets may become brittle after bearing 15-25 kWh/m² of power, with tensile strength decreasing by more than 50%.

Installation operations plant long-term hidden dangers

Many problems are sown on the first day of installation. Non-standard handling and installation are the main sources of microcracks. For a 72-cell module, the glass thickness is usually only 3.2 mm. If there are fewer than 4 support points during handling, or the force is uneven, the internal cells will bearing mechanical stress exceeding 1 MPa. Microcracks invisible to the naked eye may initially cause only a 0.5% power loss, but after 1500 thermal cycles (equivalent to 4-5 years of day-night alternation outdoors), the microcracks can expand into macroscopic cracks, causing the failure of that cell and a sharp increase in power loss to over 5%.

Improper bracket design amplifies the effects of wind pressure and snow load. Assuming the local basic wind pressure is 0.5 kN/m², if the bracket stiffness is insufficient, vibration exceeding 5 Hz can occur on windy days. This high-frequency vibration accelerates the loosening of connectors and the expansion of cell microcracks. In areas with a winter snow depth of 50 cm, snow pressure may reach 1.2 kN/m².

The quality watershed of the product itself

The initial quality of the cell determines the starting point of degradation. Using ultra-high-purity silicon ingots with carbon content below 0.5 ppma, their light-induced degradation can be controlled within 1%; whereas for cells using ordinary silicon material (carbon content may exceed 2 ppma), LID may reach 2.5% or even higher.

The anti-aging performance of encapsulation materials can be worlds apart. High-performance co-extruded POE film can have a water vapor transmission rate below 2 g/m²·day, which is one-fifth that of ordinary EVA film, effectively protecting the cells from moisture corrosion. In the double 85 test (85°C/85% humidity), after 3000 hours of aging, the light transmittance retention rate of high-quality POE film remains above 95%, while ordinary EVA may have dropped below 90%.

Additional degradation caused by system design

Losses due to series mismatch are typically 1%-3%. Connecting modules of different power grades (e.g., 425W and 430W) in the same string limits the output current to the smallest one, causing power loss for the entire string.

DC line loss and inverter conversion efficiency are equally critical. Over a 100-meter DC cable distance, if the wire gauge is chosen improperly, line loss can reach 3%. This loss is often mistaken for module degradation. An inverter with 98.5% efficiency can output 1.5% more electricity annually than one with 97.0% efficiency. This gap over a 25-year operation cycle is equivalent to a difference in module degradation rate of 0.1%/year.

Key Factors Affecting Degradation Rate

The degradation rate of PV panels is by no means a fixed value. The difference between an annual degradation rate of 0.5% and 0.8% translates directly into a tangible gap in annual power generation revenue. For a 10kW power station, a difference of 0.3% in degradation speed could result in a total energy loss of 5000-8000 kWh over its 25-year life cycle, directly corresponding to a difference of thousands of RMB in electricity revenue.

Product Quality

The cumulative energy generation gap between modules with an annual degradation rate of 0.3% and 0.8% might be as high as 15% or more by the end of their life cycle, directly corresponding to a significant difference in the investment payback period.

Wafer: The Ultimate Contest of Purity

The purity of solar-grade silicon material is usually expressed in "number of nines." Mainstream products require a purity of at least 6 nines (99.9999%). But top manufacturers use silicon material with purity of 7 nines (99.99999%) or even higher. For every order of magnitude increase in purity, the metal impurity content can decrease by an order of magnitude. Trace impurities like iron, copper, chromium, etc., are recombination centers that accelerate the recombination of photogenerated carriers, not only reducing initial efficiency but also, under long-term light exposure and thermal stress, becoming "foci" for accelerated performance degradation. Modules using ultra-high-purity wafers can have a power retention rate 2-3 percentage points higher than ordinary products after 25 years.

PERC Technology: The Battle Against "Light-Induced Degradation"

The root cause lies in the boron-oxygen (B-O) complex in P-type silicon wafers. In the initial stages of light exposure, boron and oxygen combine to form recombination centers, leading to efficiency loss. This degradation occurs mainly within the first 72 hours of light exposure, contributing about 0.5%-0.8% to the first-year degradation. To combat this issue, the industry developed Light-Induced Degradation Regeneration (LIR) technology: by applying a certain temperature (typically 50-70°C) and current injection, these complexes are dissociated. After treatment, high-quality manufacturers' PERC modules can have their first-year degradation controlled within 1.5% and stabilize subsequent annual degradation at around 0.45%.

TOPCon: Inherently Robust Stability

Because it uses phosphorus doping instead of boron doping, it fundamentally avoids the formation of boron-oxygen complexes. Its initial light-induced degradation is almost negligible. Therefore, the first-year degradation can be directly set at 1%, or even lower. In accelerated aging tests, such as after DH2000 (Damp Heat test for 2000 hours), the power degradation of TOPCon is typically 0.5-1% lower than that of comparable PERC modules. This means that in real-world hot and humid environments, its performance curve is smoother. After the first year, TOPCon's annual degradation rate can easily stabilize below 0.4%, with tier-one brands often warranting 0.4%.

HJT: Top Performer in Both Bifaciality and Degradation

· HJT cells have a symmetrical structure and a low-temperature fabrication process (<200°C), avoiding the damage high-temperature processes inflict on the wafer. Its first-year degradation can be as low as 0.5%, and the subsequent average annual degradation rate has the potential to be controlled at an extremely low level of 0.25%-0.3%. Furthermore, HJT modules typically have a higher bifaciality factor (90%-95%), meaning higher rear-side power generation gain. The combination of lower degradation rate, higher initial generation capacity, and bifacial gain means that its energy generation per watt over a 25-year life cycle can be 5%-10% higher than that of PERC modules.

Encapsulation Materials: The Module's "Immune System"

· Anti-PID Capability: System voltages can be as high as 1000V-1500V. The high potential difference between the cells and the grounded frame can cause power degradation, known as Potential Induced Degradation. Modules with poor anti-PID performance can degrade by more than 30% during a 96-hour PID test. By using POE encapsulate with high volume resistivity (resistivity can reach above 5×10^15 Ω·cm) and cell anti-PID processes, it can be ensured that the module's power degradation is less than 2% when tested under 85°C, 85% humidity, -1000V bias for 96 hours.

· Back sheet Moisture Barrier Properties: The back sheet needs to block water vapor. The water vapor transmission rate of high-quality fluoridated back sheets can be below 1.5 g/m²/day, while some non-fluoridated or composite back sheets may exceed 5 g/m²/day. After the DH1000 test, modules encapsulated with back sheets having poor moisture barrier properties can have a degradation rate more than twice that of high-quality modules.

Environmental Factors

The standard test conditions (STC) for PV panels are a constant 25°C and ideal light in the laboratory, but their real working environment is the harsh outdoors. The same top-tier module with a nominal power of 550W, installed in a power station in Qinghai with an average annual temperature of 15°C, versus one in a desert in the Middle East where summer extreme temperatures often exceed 45°C, may have an actual output power difference of over 20W after the first year, and this gap will widen year by year over time.

High Temperature: Not Just Temporary Efficiency Drop, but an Accelerator of Material Aging

For every 1°C increase in the module's operating temperature, its output power decreases by approximately -0.3% to -0.4% according to its temperature coefficient. On summer afternoons, it is common for the module surface temperature to reach 65°C, meaning its instantaneous output power will be 12%-16% lower than under standard conditions. But this is only a short-term, reversible effect.

The EVA or POE encapsulate film that forms the core of the encapsulation ages according to the Arrhenius equation, roughly meaning that for every 10°C increase in temperature, the chemical reaction rate (aging speed) doubles. A module operating long-term at an average temperature of 70°C will have an encapsulate material aging speed that is 4 times that of a module operating at 50°C. This directly leads to a decrease in the encapsulation's cross-linking degree, intensified yellowing, and transmittance attenuation potentially exceeding 5% within 5 years, whereas in a normal environment, this value should be below 2%.

Humidity and Salt Spray: Quietly Building a "Corrosion Factory" Inside the Module

A module's moisture resistance is measured by its Water Vapor Transmission Rate, in units of g/m²/day. The WVTR of inferior back sheets may exceed 2.5 g/m²/day, while high-quality back sheets can control it below 1.0 g/m²/day.

In hot and humid regions (like southern coastal China), the average annual relative humidity exceeds 80%. Driven by the internal-external air pressure difference, water molecules slowly penetrate the back sheet or edge sealant and enter the module interior. When the temperature changes, this moisture condenses into liquid water inside. This creates conditions for two main problems:

1. Potential Induced Degradation (PID): Moisture reduces the volume resistivity of the encapsulate material. Under a system voltage of 1500V, it forms a leakage current path. The current generated by the cells leaks to the ground, causing severe power degradation. Cases of degradation exceeding 30% within 96 hours are not uncommon.

2. Internal Circuit Corrosion: Moisture and acetic acid (from EVA degradation) form a weakly acidic environment, corroding the silver bus-bars and ribbons of the cells, leading to increased series resistance and decreased fill factor. In coastal areas, chloride ions in the atmosphere accelerate this process; the corrosion rate can be 3-5 times that in dry inland areas.

Ultraviolet (UV) Light: The "Invisible Killer" Deteriorating the Protective Layer

Inferior EVA, when the cumulative UV radiation dose reaches 15 kWh/m² (roughly equivalent to 1-2 years of outdoor exposure), may see its transmittance decrease by over 3%. In contrast, high-quality POE or anti-PID EVA, even after an equivalent radiation dose of 60-90 kWh/m², can keep the transmittance loss controlled within 1%. For the back sheet, if the outer fluoroscope film is too thin (below 20μm) or has insufficient fluorine content, UV light will directly degrade the underlying PET substrate, causing the back sheet to chalk and crack, losing its insulation and moisture barrier functions within 3-5 years.

Wind, Sand, and Dust: Continuous Physical Abrasion on Generation Efficiency

First is the shading effect: when the dust density on the module surface reaches 10 g/m², the power generation loss is generally 5%-8%. In the sandy northwest regions of China, after a sandstorm, the daily energy loss due to dust shading can exceed 15%.

Second is the abrasion effect: strong winds (wind speed >10m/s) carrying sand grains (mainly composed of silicon dioxide, Mohs hardness 7) continuously abrade the anti-reflective coating on the surface of the ultra-clear patterned glass. The hardness of the glass itself is 5-6. Long-term friction increases micro-scratches on the glass surface, increasing surface diffuse reflection, and reducing transmittance. In areas with strong wind and sand, the additional average annual transmittance loss of the module glass due to abrasion can be as high as 0.2%-0.3%.

Ammonia and Acid Rain: Chemical Sniping Targeting Materials

In industrial areas or heavily polluted cities, acid rain (pH less than 5.6) slowly corrodes the glass surface and aluminum frame. The roughened glass surface is more prone to dust accumulation, forming a vicious cycle. Long-term exposure to acid rain with a pH around 4.0 can cause the glass transmittance to decrease more than 50% faster than in clean air areas.

Manufacturer's Linear Power Warranty

Currently, tier-one brands such as LONGi, Jinko, Trina Solar, etc., generally promise that first-year degradation will not exceed 2%, and from the second year to the 25th year (sometimes extended to 30 years), the annual degradation rate will not exceed 0.45%, guaranteeing that at the end of the 25th year, the module's output power will still be maintained at above 84.8% of the initial nominal power.

Some products using advanced technologies like N-type TOPCon even improve the end-of-life power guarantee at year 30 to above 87%. Understanding every number in this warranty is key to avoiding pitfalls and ensuring your power generation revenue for over 20 years in the future.

Warranty Structure

The industry-standard first-year degradation of 2% is not set arbitrarily. It mainly stems from the initial compressive stress of encapsulation materials, minor shrinkage during the cross-linking and curing process of the EVA encapsulate film, and light-induced degradation caused by boron-oxygen pair formation—this is particularly evident in P-type silicon wafers, causing an initial power loss of approximately 1-3%.

Starting from the second year, the linear degradation phase has an annual rate strictly controlled at 0.45%. Its physical mechanism shifts towards slower anti-attenuation aging of the cells, the yellowing rate of the encapsulate material (typically requiring transmittance decrease of less than 0.5% per year), and trace amounts deterioration of electrode contacts. Understanding this two-phase model of "initial jump, subsequent crawl" is the cornerstone for accurately predicting the full life cycle energy generation revenue of your power station. A misjudgment of one percentage point could mean a difference of tens of thousands of RMB in electricity revenue after 25 years.

Why does it 'lose weight' so much in the first year?

This first-year 2% degradation can be understood as the superposition of three main parts:

1. Initial stabilization of materials: During the module lamination process, internal stress exists in the EVA or POE encapsulate film under high temperature and pressure. Within the first 300-500 hours of use, influenced by heat and UV light, these stresses are released, causing micron-level shrinkage of the film, leading to tiny changes in the adhesion between the cells and the glass, affecting light transmittance. The loss caused by this process is about 0.3%-0.5%.

2. Light-Induced Degradation (LID): Especially for the mainstream P-type boron-doped monocrystalline silicon wafers, within the first few days of exposure to sunlight, boron and oxygen elements in the wafer combine to form "boron-oxygen complexes." These complexes act as charge traps, reducing the lifetime of minority charge carriers, directly leading to power decline. This phenomenon accelerates when the module operating temperature reaches 50-70 degrees Celsius, and the power loss it causes is typically between 0.5%-1.5%, constituting the bulk of first-year degradation.

3. Potential Induced Degradation (PID): Although the PID resistance of modern modules is already strong, when the system is initially energized, establishing a high voltage difference of 1000V-1500V between the module frame and the cell circuit, some ion migration may occur, causing weak power degradation. High-quality modules, by using anti-PID cell technology and POE encapsulation, can control this loss below 0.1%.

Why does the subsequent degradation become a "steady slow jog"?

After the module safely passes the first year, the internal materials and major structures have basically stabilized. The dominant factor for degradation becomes long-term, slow aging processes. That 0.45%/year linear degradation rate is the manufacturer's comprehensive promise for the rates of a series of microscopic changes:

· Yellowing of encapsulate materials: EVA encapsulate film, under 25 years of cumulative UV exposure exceeding 55,000 hours, will slowly oxidize and turn yellow. Its yellowing index (YI) annual increase is controlled to single digits. This causes transmittance to decrease at a rate of about 0.1%-0.2% per year, directly reducing the number of photons received by the cells.

· Abrasion of the glass surface: Although PV glass has high hardness, years of exposure to wind, sand, and rain cause micro-scratches on the surface, increasing light scattering and reducing effective transmittance. In areas with more wind and sand, this effect is more significant, contributing about 0.05% degradation annually.

· Slow increase in contact resistance of metal electrodes: The silver paste busbars and fingers on the cell surface, under long-term thermal cycling (daily fluctuations from -20℃ to +70℃), experience tiny atomic diffusion at the contact interface with silicon, causing a very slight increase in series resistance, affecting current output, with an annual impact of about 0.05%-0.1%.

· Bulk attenuation of the cell itself: The minority carrier lifetime of the silicon material decreases extremely slowly over time, but this rate can be controlled below 0.05% per year for high-quality silicon material.

Beware of High Degradation Rate Promises

When Brand A promises 0.45%/year and Brand B promises 0.55%/year, it seems like a tiny difference of only 0.1%. But when compounded over a 25-year cycle, the result is astonishing. For a residential power station with an initial power of 10kW, this 0.1% difference in annual degradation rate results in a cumulative power difference of about 85W by year 10. By year 25, this gap widens to nearly 300W—equivalent to losing an old-style 300W module for free.

Calculated based on an average annual generation of 1100 hours and an electricity price of 0.6 RMB/kWh, the cumulative difference in electricity revenue generated over 25 years will exceed 5500 RMB. The reason manufacturers dare to set the number higher is that the vast majority of users won't create a 25-year degradation calculation sheet in Excel.

Calculate this 25-year compound interest account: How the 0.1% gap snowballs

Many people understand degradation rates as simple multiplication, thinking that 0.55% is 0.1% worse than 0.45% per year, so over 25 years it's 2.5% worse in total. This is completely wrong. PV degradation is compound degradation; each year's degradation is based on the already reduced power from the previous year.

Let's take a 410W module and compare the 0.45% and 0.55% annual degradation standards, assuming the first-year degradation is 2% for both.

· Calculation model: Remaining power in year N = Initial power × (1 - First-year degradation rate) × (1 - Linear degradation rate)^(N-1)

Let's calculate the account at a few key points:

· Year 10:

o 0.45% standard: Power = 410W × 98% × (99.55%)^9 ≈ 410W × 0.98 × 0.960 ≈ 377.5W

o 0.55% standard: Power = 410W × 98% × (99.45%)^9 ≈ 410W × 0.98 × 0.953 ≈ 382.7W

o By year 10, the power gap has reached 5.2W. For a 10kW system (about 24 modules), the total power difference is about 125W.

· Year 25 (end of warranty period):

o 0.45% standard: Power = 410W × 98% × (99.55%)^24 ≈ 410W × 0.98 × 0.898 ≈ 360.7W

o 0.55% standard: Power = 410W × 98% × (99.45%)^24 ≈ 410W × 0.98 × 0.874 ≈ 351.1W

o The power gap widens to 9.6W. The total power difference for the 10kW system reaches 230W.

This 230W power difference means that during the peak generation hours at noon on the sunniest days, the Brand B system's maximum output capacity will always be 230 watts less than the Brand A system.

Energy generation loss: Invisible "electricity bills" are being stolen

The power difference must ultimately be converted into energy generation and economic loss. Let's continue the calculation with the above 10kW system.

· Average annual generation hours: Assume 1100 hours (average level for most regions in China).

· Average electricity price: Assume 0.6 RMB/kWh (including subsidies and tiered pricing effects).

We calculate the cumulative energy generation loss from year 2 to year 25, totaling 24 years. To simplify the model, we take the power difference at the mid-term year 13 (approximately 7.5W) as an approximation of the average power difference over the 24 years.

· Average annual energy loss ≈ 7.5W × 1100 hours / 1000 = 8.25 kWh

· Total energy loss over 24 years ≈ 8.25 kWh/year × 24 years = 198 kWh (This is the loss per module)

For a 10kW system with 24 modules:

· Total energy loss ≈ 198 kWh/module × 24 modules = 4752 kWh

· Direct electricity revenue loss ≈ 4752 kWh × 0.6 RMB/kWh = 2851 RMB

Considering that the power difference is small in the early years and larger in the later years, the actual loss might be close to or even exceed 5000 RMB. This is equivalent to an invisible increase of more than 0.5 RMB per watt in the cost you paid for the Brand B modules.

Manufacturer's pricing strategy: Why dare to use a high degradation rate?

You get what you pay for. Manufacturers daring to promise 0.45% have real cost investments behind them:

1. Silicon quality: Using higher purity electronic-grade silicon material, with lower internal impurities and metal content, slows bulk attenuation from the source. The cost per kilogram may be 10-20 RMB higher.

2. Cell technology: Advanced N-type technologies like TOPCon, HJT, etc., have inherently superior degradation mechanisms compared to P-type. The initial light-induced degradation of N-type cells is almost 0, and first-year degradation can be controlled within 1%.

3. Encapsulation materials: Using POE film with stronger UV resistance instead of EVA can more effectively prevent hydrolysis and yellowing, but the material cost is 15%-30% higher. High-quality back sheets have stronger moisture barrier properties and weather resistance.

4. Warranty cost: Promising a lower degradation rate means the manufacturer must reserve a larger financial budget for potential future claims; this risk cost is also included in the selling price.

Manufacturers promising 0.55% or higher often lower standards in one or more of the above links. Through a seemingly "almost the same" warranty clause, they save themselves potentially 0.1-0.2 RMB per watt in material costs, thereby gaining competitiveness in pricing.

Action guide: How to see through the warranty threshold at a glance

1. Go straight to the numbers: Get the warranty document, and look directly for the item "Annual Linear Degradation Rate" or "Annual linear decay rate". Only recognize numbers of 0.45% or lower (e.g., 0.40%). Any number higher than 0.45%, regardless of the salesperson's explanation, should be considered a low-quality or outdated technology product.

2. Calculate the end-of-life power: Quickly estimate the guaranteed power at the end of year 25. Formula: Initial power × (1 - First-year degradation) × (1 - Annual degradation)^24.

3. Question the gimmick of "extra-long warranty": Some manufacturers promote 30-year or even 35-year warranties, but the annual degradation rate is as high as 0.6%. This is meaningless. Calculate the power at year 30; it might be lower than the power at year 25 for another brand with 0.45% degradation. The quality of the degradation rate is far more important than the length of the warranty period.

Warranty Scope

A warranty promising 85% power retention after 25 years absolutely does not mean your energy generation will only drop by 15%. The reality is, for a well-designed and installed 10kW system, the actual AC-side energy generation in the first year might only be 85%-90% of the module's nominal power (i.e., system efficiency), and the power warranty only covers the DC power degradation part.

The main enemies affecting energy generation – such as daily shading for 2 hours covering about 10% of the array area caused by trees or chimneys, may directly cause a loss of 25%-30% of daily average energy; uniform dust 0.5 mm thick on the panel surface can lead to a transmittance decrease of 8%-10%, resulting in a comparable energy generation loss of up to 6%; when the module temperature rises to 65 degrees Celsius in summer, its power output can decrease by up to 15%-18% compared to standard conditions (25°C).

Shadows and soiling: Local shading, global damage

· Quantified impact: Even if just one leaf's shadow covers about 5% of the area of one module (about 2 square meters), i.e., 0.1 square meters, it affects far more than 5% of the energy generation. Because the shaded cell will heat up and pull down the voltage of the entire module, potentially causing the output power of that module to drop by 50% or more. If this module is one of 20 in a string, then the power generation efficiency of the entire string of 20 modules will be dragged down by it, potentially causing the total system output to drop by more than 10%.

· Dust effect: Taking North China in spring as an example, modules not cleaned for 30 days can have surface dust density reaching 10 grams/square meter, causing an average energy generation loss of 4%-6%. If adhered to by more sticky pollutants (like bird droppings, pollen), forming stubborn stains in specific locations, the resulting local hot spot effect may cause microcracks in the cells.

The system's own "internal consumption": The loss chain from DC to AC

· Inverter efficiency: An inverter with a nominal efficiency of 98.5% does not mean it maintains this level at all power points. Under low light conditions in the early morning and evening, its efficiency may drop below 95%. The module power guaranteed by the manufacturer is DC-side data and completely ignores losses during inverter conversion. This fixed loss is 1.5%-3%.

· Line loss and voltage drop: DC cables have resistance, and current flow generates heat loss. If the wire gauge is chosen improperly or the cable run is too long, causing a DC voltage drop exceeding 2%, then 2% of the electrical energy is directly converted into heat and lost along the way. The AC side also has about 1%-1.5% line loss. These losses combined can account for 3%-4%.

· Temperature loss: The module's power temperature coefficient is about -0.35%/℃. When the module temperature reaches 60℃ at summer noon, which is 35℃ higher than the standard test temperature of 25℃, its instantaneous power decreases by 35℃ × 0.35%/℃ ≈ 12.25%.

Ongoing impact of environment and climate

· Environmental degradation is irreversible: Modules exposed for a long time to the air will have their encapsulate material's transmittance slowly decrease due to UV exposure. Even though the manufacturer guarantees an annual power degradation of 0.45%, which includes the expectation of normal material aging, if local air quality is poor, with high concentrations of sulfides and nitrogen oxides accelerating the aging of the encapsulate material, causing transmittance loss exceeding expectations, this extra loss is difficult to attribute to the manufacturer as it belongs to special environments.

· Wind erosion and temperature difference fatigue: In windy areas with an average annual wind speed exceeding 5 meters/second, sand and dust carried by the wind cause persists abrasion on the glass surface, increasing scattering. Simultaneously, in areas with a daily temperature difference exceeding 20 degrees Celsius, the thermal expansion and contraction of module materials (glass, silicon wafer, metal frame) are greater, potentially leading to minor fatigue stress in internal connectors over the long term, introducing additional degradation factors. These regional differences cannot be individually covered by standard warranty terms.