Is low power generation due to panel degradation? How to view a 20-year lifespan

Table of Contents

• If you feel that the power output is low, don't immediately assume it's a malfunction.

• The correct way to view a 20-year lifespan: panels gradually decline rather than suddenly stopping

• 7 main causes of low electricity generation

• Three criteria to determine whether a panel has degraded

• Points to note when viewing 1-day, 1-month, and 1-year power generation

• Signs That Should Make You Suspect Power Conditioner Degradation

• Checklist you can verify on your own

• When to Request a Professional Inspection

• Options after the 20th year: continue using, repair, or replace

• Frequently Asked Questions

• Summary

If you feel the power output is low, don't immediately assume it's a "fault"

When people look at the monitor of their solar power system and think "the generation is lower than last year" or "it's sunny but it's not generating as much as I expected," many immediately suspect panel degradation or end of life. However, low generation is not necessarily caused by the panels themselves. There are multiple factors that can reduce output, such as weather, seasons, solar irradiance, shading, dirt, power conditioners, wiring, settings, and output control.

What you should pay particular attention to is viewing performance as "low for just one day" or "low for just this month." Solar power generation is strongly affected by solar radiation, so months with many cloudy or rainy days see large drops. In winter, shorter sunshine hours and a lower solar altitude make it a period when generation often appears low. On the other hand, although summer brings greater solar irradiance, panel efficiency falls as panel temperature rises, so the highest generation month is not necessarily midsummer.

In short, when power generation is low, it is important to check three categories: "panel degradation", "equipment failure", and "temporary external factors". The service life of solar panels is generally considered to be about 20-30 years, and the longer they are used the output gradually decreases, but they do not suddenly become unusable the moment they pass 20 years. JPEA also explains that solar panels are equipment whose performance changes gradually rather than items that suddenly break like home appliances.

The right way to look at a 20-year lifespan: panels decline gradually rather than abruptly stopping

When people hear that "the lifespan of solar panels is 20 years," they often misunderstand and assume the panels will stop generating electricity after 20 years. In fact, it's more realistic to view 20 years not as an absolute deadline for replacement but as a guideline for the period during which they can be used while maintaining a certain level of performance. JPEA explains that the service life of solar panels is generally about 20–30 years, and depending on installation conditions and maintenance, there are cases where they continue to operate for more than 30 years.

What matters in a decline in power generation is how the degradation progresses. Solar panels do not suddenly stop generating all power one day; rather, their output gradually falls over the years. According to JPEA materials, degradation accumulates at around 0.5% per year in generation efficiency, and after 25–30 years output may drop to 80% or less.

For example, if the decline is about 0.5% per year, a simple calculation over 20 years is roughly a 10% decrease. In other words, a slight reduction in power generation compared to when it was installed is natural, but a sudden halving is often difficult to explain by mere aging alone. In such cases, you need to suspect causes other than the panels, such as shading, soiling, or a malfunctioning power conditioner.

The phrase "20-year lifespan" can serve as a guideline for considering when to replace equipment. However, some systems continue generating electricity beyond 20 years, while conversely generation can drop sharply in less than 10 years if there are installation defects, heavy shading, or equipment failures. What matters is not to judge solely by the number of years, but to assess actual generation data and the condition of the equipment.

7 Main Causes of Low Power Generation

To correctly identify the causes of low power generation, you need to consider separately “since when it has been low,” “how much lower it is,” and “during which time(s) of day it is low.” Here, we summarize seven common causes seen in residential solar power generation.

1. Effects of Weather, Seasons, and Solar Radiation

The most common cause is simply a reduced amount of solar radiation. When cloudy skies, rain, snow, yellow dust, or unsettled weather before and after typhoons persist, power generation drops significantly. Even when it looks sunny, thin clouds or haze can weaken solar radiation, causing the power output shown on the monitor to be lower than expected.

Also, differences in solar radiation by region cannot be ignored. NEDO provides an environment as a solar radiation database for Japan where monthly average data and hourly data can be accessed. If you want to assess power generation accurately, comparing regional solar radiation and past monthly data is more effective than relying on the subjective "it should have been sunny."

2. Effects of Shadows

Shadows are an easily overlooked cause of reduced power generation. Buildings, trees, utility poles, antennas, renovations to neighboring houses, and rooftop equipment can cast shadows on panels that did not exist before. JPEA explains that it is important to avoid shading on photovoltaic modules as much as possible, and that even thin shadows from mountains, buildings, trees, utility poles, or TV antennas can reduce power output.

What should be especially noted is that this is not limited to cases where the entire panel becomes shaded. Even if only part of a panel or only some cells are shaded, it can affect the power generation of the whole string. If opaque materials such as fallen leaves adhere for extended periods, in addition to reduced power output, a hot-spot phenomenon may also occur.

3. Dirt, fallen leaves, bird droppings, and yellow sand

Contamination on the panel surface is also a cause of reduced power generation. JPEA explains that when trash, dust, and similar matter adhere to the surface of photovoltaic modules, power generation can drop by a few percent, and that in a typical urban area the output reduction due to dirt is roughly 5% or less. In Japan, regular rain and wind generally wash them away and the panels are said to recover to nearly their original performance, but along roads with heavy traffic or in areas where oily airborne deposits are likely to adhere, rain alone may not be enough to remove them.

However, you should avoid climbing onto the roof to clean the panels yourself just because the power output is low. There is a risk of falling, electric shock, and panel damage. From the ground, check within the visible range for obvious leaves, bird droppings, snow, or mud, and consult a professional if necessary.

4. Degradation and Failure of Power Conditioners

When power generation is low, the power conditioner is often the thing to suspect before the panels. A power conditioner is a device that converts the direct current generated by the panels into alternating current that can be used in the home. If there is a fault in it, even if the panels themselves are generating power, the amount of electricity actually usable and the amount sold will decrease.

JPEA explains that solar cell modules are said to last more than 20 years, and power conditioners are said to last 10–15 years. It also states that power conditioners require inspections once every four years for residential use and once every six months for commercial use, which, as needed, can lead to component replacement or exchange.

If more than 10 years have passed since installation and you experience symptoms such as a sudden drop in power generation, error messages, a change in operating noise, or repeated shutdowns and restarts, you should prioritize an inspection of the power conditioner.

5. Wiring, connection points, and breaker malfunctions

Solar power generation consists not only of panels but of a combination of junction boxes, cables, breakers, distribution boards, mounting structures, grounding, communication equipment, and other components. Loose wiring, connector deterioration, breaker trips, junction box faults, or insulation degradation can reduce power output or cause only certain circuits to stop generating power.

This type of fault is characterized by being difficult to judge from appearance alone. Even if the monitor only shows "low power generation," in reality a single system may be down, a specific string may be producing low output, or the power conditioner may be suppressing output due to a safety function. If power generation suddenly drops, or if the generation curve is clearly abnormal even on sunny days, a professional inspection is necessary.

6. Output control, voltage rise suppression, and effects of settings

Even if power generation appears low, it does not necessarily mean that the equipment is broken. For example, depending on the region and contract conditions, output control may be applied. Also, in residential areas the grid-side voltage can become high, and the power conditioner may perform voltage-rise suppression. In such cases, it can appear that, despite good weather, the amount of electricity sold does not increase, and that output plateaus around midday.

Also, in households that have installed battery storage or HEMS, there can be a mistaken belief that a "low amount of electricity sold" means "low power generation." If the electricity generated is diverted to self-consumption or to charging the battery storage, the amount of electricity sold will decrease. However, this is not a decrease in generation — it simply means the way the electricity is used has changed. It is important to separate and examine the amounts of generation, consumption, charging, and electricity sold.

7. Monitor, Communication, and Measurement Equipment Malfunctions

A surprisingly common issue is problems on the display side rather than the actual power generation. Communication errors with the monitor, failure of the measurement unit, incorrect orientation of the CT sensor, poor Internet connectivity, or delays in app updates can cause the generated power to be displayed as lower than it actually is.

In this case, by cross-checking the electricity sales statement, the power company's measurement data, the display on the power conditioner itself, and the HEMS screen, the cause becomes apparent.

Rather than looking only at the monitor and concluding that the system is "not generating", it's important to compare multiple data sources.

Three criteria for telling whether a panel has degraded

When power generation is low, the following three criteria are useful for determining whether the cause is panel degradation or something else.

Criterion 1: Is the decline 'slow' or 'rapid'?

Panel aging typically progresses slowly. A degradation rate of about 0.5% per year is generally used as a guideline, so a gradual decline over several years may fall within the range of normal degradation.

On the other hand, if output suddenly fell by 30% starting in a certain month, the peak output on sunny days has dropped to half of what it was before, or it suddenly stopped generating power despite operating normally until yesterday, it is difficult to explain this by age-related degradation alone. Prioritize checking the power conditioner, breakers, wiring, shading, snow accumulation, communication issues, etc.

Criterion 2: Is the overall level low, or is only a part low?

If the decline is gradual across the entire system, age-related degradation or variations in solar irradiance are likely. Conversely, if only some panels, some strings, or only specific time periods show lower output, shading, wiring, connection issues, or panel-level faults should be suspected.

For example, if power generation is low only in the morning, it may be due to shading on the east side; if it is low only in the afternoon, due to shading on the west side; and if it levels off around noon, voltage-rise suppression or output limits of the power conditioner may be involved. Viewing power generation not only as the "total amount" but also as the "time-of-day curve" makes it easier to narrow down the cause.

Criterion 3: Are you comparing against past data for the same month?

When comparing power generation, the basic rule is to compare the same months. Comparing January and May, or rainy June and a month with many sunny days, will not allow you to make a correct judgment. What you should look at are items such as "April this year and April last year," "the same month over the past three years," and "peak output on sunny days."

If you want to further increase accuracy, also check regional solar irradiance data. Referring to public data such as NEDO’s solar irradiance database makes it easier to distinguish whether reduced power generation is caused by problems on the equipment side or by differences in solar irradiance conditions.

Points to note when looking at daily, monthly, and yearly power generation

Whether power generation is low depends on the period you examine. On a daily basis, the weather has too great an impact, so it is not suitable for determining faults. On a monthly basis you can see trends, but prolonged rain, typhoons, or snowfall can cause large fluctuations. The most reliable approach is to compare the same season across multiple years.

Let's divide the points to review by period as follows.

As a guideline, if power generation is low for only one day, it is often acceptable to wait and see. If, on a monthly basis, generation is significantly lower than the same month of the previous year and the difference cannot be explained by weather alone, consider arranging an inspection. If peak output is clearly lower than before even on sunny days, or if the generation curve is cut off partway, an early check is necessary.

Signs You Should Suspect a Power Conditioner Is Deteriorating

The power conditioner is like the heart of a solar power generation system. Even if the panels are generating electricity, if the power conditioner is not functioning properly, it cannot convert it into electricity usable in the home. In systems that have been installed for 10–15 years, if power output is low, the deterioration of the power conditioner is sometimes suspected before panel degradation. JPEA also explains that photovoltaic modules are said to last 20 years or more, while power conditioners are said to last 10–15 years.

If you have any of the following symptoms, please consider having the power conditioner inspected.

In particular, if power output drops "suddenly" rather than "gradually", it is more likely to be an equipment failure than the panels reaching the end of their lifespan. Power conditioners (inverters) can sometimes be repaired, but if parts are no longer available, replacement may be necessary. Consider the warranty period, the manufacturer's repair support, replacement costs, and the system's expected remaining service life together when making a decision.

Checklist You Can Verify Yourself

If you feel the power output is low, before immediately calling a contractor there are things you can safely check yourself. The key is to avoid climbing onto the roof, avoid disassembling electrical equipment, and limit checks to visual inspection and data verification.

Data Confirmation

First, check the following items on the monitor or in the app.

• Was the decline only today, the entire month, or has it been ongoing for several months?

• How much lower is it compared with the same month of the previous year?

• Is the power generation curve on clear days mountain-shaped?

• Has the output suddenly dropped around noon?

• Are you looking at the actual power generation itself rather than the amount of electricity sold?

• Is it being used to charge the storage battery or for self-consumption?

• Whether any error messages or communication errors are appearing

Be careful if you only look at the amount of electricity sold. If you use a lot of electricity at home, the amount of electricity sold will be lower. If you have a storage battery installed, generated electricity will be used for charging, so the amount of electricity sold will decrease. What you should check is the relationship between "generation", "consumption", "electricity sold", "electricity purchased", and "battery charge/discharge".

Visual inspection

Next, check within the visible range from a safe place, such as on the ground or from a balcony.

• Are there fallen leaves, bird droppings, snow, or mud on the panels?

• Are there shadows cast by trees or buildings that weren't present before?

• Are the shadows of antennas, chimneys, and rooftop equipment not extending?

• Are the ventilation openings of the power conditioner unobstructed?

• Whether the circuit breaker has tripped

• Is an error code displayed on the power conditioner?

Panel soiling often results in only a few percent decrease, but if fallen leaves or bird droppings are strongly adhered to a portion they can act like a shadow and affect power generation. JPEA also mentions the reduction in power generation due to soiling and that much of it is washed away by rain and wind, but along busy roads some soiling may remain that is not easily removed by rain alone.

What Not to Do

Even if the power generation is low, please avoid the following actions.

• Climb onto the roof and clean the panels.

• Disassemble power conditioners and junction boxes

• Touch electrical equipment while wet

• It ignores the error code and repeatedly restarts.

• Make modifications at your own discretion while under the manufacturer's warranty.

• Wash the panels with a pressure washer.

Solar power generation equipment is electrical equipment. For safety reasons, it is generally best to leave inspections that require expertise to specialist contractors.

When to Request a Professional Inspection

If you cannot isolate the cause of low power generation yourself, or if there is an obvious abnormality, request an inspection by a professional contractor. JPEA states that photovoltaic power generation systems require inspection as electrical equipment, and indicates inspection frequencies such as at least once every four years for general systems under 50 kW, and for self-use systems of 50 kW or more, inspection of the panels and power conditioners once every six months.

In particular, early inspection is recommended in the following cases.

When requesting work from a contractor, preparing as much of the power generation data as possible will make the diagnosis smoother. Specifically, organize the installation year, manufacturer name, panel capacity, power conditioner model number, warranty documents, past generation output, error codes, when you noticed the decline, any relationship with the weather, and whether a storage battery is present.

Specialized inspections may include not only visual checks but also IV-curve measurements, insulation resistance testing, per-string voltage and current verification, thermographic inspection, and power conditioner diagnostics. This enables more accurate isolation of panel degradation, wiring faults, connection faults, hotspots, and equipment failures.

Options After the 20th Year: Continue Using, Repair, or Replace

When nearly 20 years have passed since installation, worries arise such as "Can I keep using it as is?" and "Should I replace it?". The factors for making that decision are not limited to power output. You need to consider safety, warranties, the power conditioner, the condition of the roof, power sales contracts, and your self-consumption policy together.

Option 1: Continue using it if the power output is stable

Even after more than 20 years, if power output has not dropped significantly and there are no abnormalities in the power conditioner or wiring, you have the option to continue using them. Solar panels typically have a service life of around 20–30 years, and depending on the installation environment and maintenance, there are cases where they operate for more than 30 years.

However, if the warranty has expired, you may be responsible for repair costs in the event of a malfunction. It is important to continue recording power generation, carry out regular inspections, and promptly address any abnormalities.

Option 2: Replace only the power conditioner

The panels may still be usable, but if the power conditioner is nearing the end of its life, it is possible to replace only the power conditioner. The service life of a power conditioner is generally considered to be 10–15 years, so in installations reaching their 20th year, it may already have been replaced once or be past its replacement time.

When replacing a power conditioner, check compatibility of voltage and current with existing panels, coordination with storage batteries, autonomous operation during power outages, manufacturer warranty, and your future self-consumption policy. Rather than simply choosing the cheapest model, compatibility with existing equipment is important.

Option 3: Replace some of the panels

If only some panels are faulty, partial replacement may be possible. However, for older installations the same model panels may not be available. Mixing different panels can affect the overall system’s power generation efficiency due to differences in voltage, current, and output characteristics, so a specialist’s assessment is required.

Also, work on the roof may require scaffolding, which can add to the cost. Even replacing just one shingle can become expensive once scaffolding, labor, and inspection fees are added. Considering the timing together with other roof repairs or repainting can help reduce unnecessary work.

Option 4: Consider Full Replacement or Repowering

If power generation has declined significantly, there are multiple panel failures, the power conditioner is also old, and roof work is required, it may be worth considering a full replacement or repowering. New panels can sometimes deliver higher output on the same roof area, and combining them with self-consumption and battery storage can help reduce electricity bills.

However, because a full replacement is costly, you need to carefully calculate whether the recovery of power generation alone can recoup the expense. Compare the feed-in tariff, electricity rates, self-consumption rate, presence or absence of a storage battery, expected remaining years of residence, and the roof's lifespan.

Troubleshooting Quick Reference for Low Power Generation

If you feel the power output is low, please use the table below as a guide to isolate the cause.

The important point is not to jump to the conclusion that "low power output = panel lifespan." Even after 20 years, if the cause is the power conditioner or shading, measures other than replacing the panels may be more effective.

Frequently Asked Questions

Should you immediately suspect a malfunction on days with low power generation?

If it's low for just one day, first check the weather and solar irradiance. Cloud cover, thin clouds, rain, snow, yellow sand, or extreme heat can change power generation. You should suspect a fault if output is clearly low even on sunny days, the decline persists for several weeks, error messages appear, or the power generation curve looks abnormal.

Should I replace solar panels that have been in use for 20 years?

They do not necessarily need to be replaced. The service life of solar panels is generally considered to be about 20–30 years, and depending on the installation environment and maintenance, there are cases where they operate for more than 30 years. When making a decision, consider the power generation output, inspection results, warranties, the power conditioner, and the condition of the roof comprehensively.

Is the 10% lower power output due to panel degradation?

If the system has been installed for many years and a comparison of the same month across multiple years shows a gradual decrease of about 10%, age-related degradation is possible. However, differences in weather, shading, soiling, the power conditioner, or output control can also cause differences of about 10%. Don’t judge based on a single month; check past data and solar irradiance conditions.

Is it at the end of its life when power output is halved?

If the output suddenly falls to half, you should suspect not only natural panel degradation but also a power conditioner shutdown, a tripped breaker, faulty wiring, a string failure, severe shading, a monitor malfunction, etc. Since age-related degradation usually progresses slowly, if there is a sudden drop, have it inspected promptly.

If I clean the panels, will the power output return?

If the cause is dirt, performance may improve. However, in an average urban area the output reduction due to dirt is said to be about 5% or less, and it can recover with rain or wind. In cases of heavy soiling, bird droppings, fallen leaves, or oily contamination, cleaning may have an effect, but for safety reasons you should avoid climbing onto the roof to clean it yourself.

Will replacing the power conditioner increase power generation?

If low power generation is caused by degradation or failure of the power conditioner, replacement may improve performance. On the other hand, if the cause is solar irradiance, shading, or panel degradation, replacing only the power conditioner may not result in significant improvement. Before replacement, it is important to check the error history and output data to isolate the cause.

Does a low amount of electricity sold mean that the amount of power generated is low?

They are not necessarily the same. If household electricity usage increases or the storage battery is charging, the amount of electricity sold will decrease even if generation remains the same. To determine whether generation is low, check not only the amount sold but also the generation itself, self-consumption, and the battery’s charge/discharge amounts separately.

Summary

When you feel the power output is low, the first thing to question is "has the system's actual generation capacity really decreased?" If you are looking only at the amount sold to the grid, it can appear low due to self-consumption or the impact of battery storage. If the drop lasts only one day, weather and solar irradiance have a large impact, and it's too soon to conclude that the panels have reached the end of their life.

Solar panels generally have a lifespan of about 20-30 years, and their power generation efficiency gradually declines over time. JPEA explains that cumulative degradation of around 0.5% per year can result in output falling to 80% or less after 25-30 years. In other words, a slight drop in power output after 20 years is natural, but if it suddenly falls significantly, you should suspect causes other than panel degradation.

In particular, if more than 10–15 years have passed since installation, deterioration or failure of the power conditioner can cause a decrease in power output. Since photovoltaic modules are expected to last more than 20 years and power conditioners about 10–15 years, it is important during the 20th-year inspection to check not only the panels but also the ancillary equipment.

When power generation is low, check in the following order: first, compare with the same month in the previous year or the same month in past years. Next, look at the generation curve on sunny days. Then check for shading, dirt, error messages, circuit breakers, and the condition of the power conditioner. If you still cannot determine the cause or there is a sudden drop, request an inspection by a professional.

"A 20-year lifespan" is not a phrase meant to make you rush into replacing equipment. It is a milestone for deciding whether to continue using it, to perform partial repairs, or to replace it, based on a comprehensive assessment of power generation, safety, warranty, the power conditioner, and the condition of the roof. When power generation is low, the first step to avoid losses is to isolate the cause rather than hastily buying a replacement.