6 criteria to distinguish reduced power output from age-related degradation

When operating solar PV systems, it's not uncommon to encounter situations where the power output seems low. However, a drop in generation does not immediately mean age-related degradation. There are multiple factors that can reduce output—weather, shading, soiling, equipment settings, how monitoring data is interpreted, grid-side controls, and insufficient inspections. If you conclude, without isolating the cause, that "it's old so nothing can be done," you may overlook declines that could actually be remedied. Conversely, ignoring signs that should raise suspicion of age-related degradation can result in generation losses and delayed maintenance responses. This article explains six diagnostic criteria to help practitioners searching for "low power output" distinguish between general output decline and age-related degradation.

Table of Contents

• Consideration 1 Assess whether the decline in power generation is temporary or persistent

• Decision factor 2: Consider the effects of weather and solar radiation separately

• Judgment factor 3: Check for changes in panel soiling and shading

• Consideration 4: Inspect the condition of power conditioners and electrical equipment

• Decision factor 5: Compare differences between strings and areas

• Basis for judgment 6 Determine the possibility of age-related deterioration from inspection history and years since installation

• Summary: Isolate the causes of low power generation step by step.

Assessment Item 1: Determine whether the decline in power generation is temporary or persistent

In distinguishing between reduced power output and age-related degradation, the first thing to check is whether the decrease is temporary or persistent. Solar power output fluctuates greatly with daily weather, temperature, seasons, and solar irradiance conditions. Therefore, if output was low on a single day, it is premature to judge the equipment as degraded based on that alone. There are many factors that can reduce output over a short period, such as cloudy or rainy weather, snowfall, yellow dust, soiling after a typhoon, and temporary shading from nearby construction.

On the other hand, if power generation remains lower than before even when compared under the same season, similar weather, and identical operating conditions, this becomes a basis for suspecting age-related degradation or equipment malfunction. In particular, when a decline is observed as a trend over weeks to months rather than over a few days, it is difficult to explain by mere weather variability. In practice, it is important to look not only at daily generation but also to combine weekly and monthly analyses, year‑over‑year comparisons for the same month, and comparisons of days with the same conditions.

Be careful not to judge a decline in power generation solely by a simple year‑on‑year comparison. If last year had sustained clear skies and this year has been persistently cloudy, power output will be lower even if the equipment has no faults. Also, if the environment around the power plant has changed, you cannot assume that last year and this year were under the same conditions. Jumping to the conclusion that “it has degraded because it is lower than the previous year” when the premises for comparison are not aligned will lead to a mistaken judgment.

Power generation decline from aging generally isn't abrupt; it tends to progress slowly over the long term. Of course, sudden drops can occur if there are equipment failures, poor connections, or panel damage, but output reductions caused by the solar panels themselves aging do not necessarily appear dramatically over a short period. Therefore, it is important to look not only at the difference between yesterday and today but at trends over months to years.

When reviewing monitoring data, it's more helpful to look for when the decline began rather than extracting only the days with low power output. Check whether it dropped suddenly on a particular day, whether it has been decreasing gradually, whether it is noticeable only in certain seasons, whether it recovers only after rain, or whether it is low even on sunny days. If it dropped suddenly, possible causes include equipment shutdown, operation of circuit breakers, communication faults, grid-side control, broken cables, poor connections, or the occurrence of shading. If it has been declining gradually, possible causes include accumulation of dirt, vegetation growth, equipment degradation, and aging of the panels.

Also, it is important to determine whether the reduction in power generation is occurring across the entire plant or only in parts. If the whole plant is decreasing by the same proportion, consider influences such as weather, solar irradiance, output control, and measurement conditions. If only certain circuits or areas are declining, localized shading, soiling, wiring, junction boxes, power conditioners, or abnormalities at the string level are more likely suspects. Even with age-related degradation, the entire facility does not necessarily deteriorate uniformly; when there are large local differences, it is more realistic to first check for partial faults or environmental factors.

When power generation is low, the starting point is to sort out "since when", "to what extent", "over what range", and "under what conditions" the decline has occurred. Simply doing this will make it easier to see whether age-related degradation is likely, or whether other causes should be prioritized for investigation.

Decision Factor 2: Consider Separating the Effects of Weather and Solar Radiation

When judging a decline in power output, looking only at the power output can easily lead to misidentifying the cause. Because solar power generation is heavily influenced by solar irradiance, output is lower on days with less irradiance. A drop in output on cloudy or rainy days is a natural phenomenon, and it is not appropriate to regard such a decrease as long-term degradation. If you feel the power output is low, you should first check external conditions that affect generation, such as solar irradiance, weather, temperature, snowfall, fog, yellow dust, smoke, and surrounding dust.

In practice, it is useful not to look at power generation alone but to assess how much power is being generated relative to solar irradiance. If irradiance is low, a low power output is natural. Conversely, if irradiance is sufficient but power output is not increasing, that can be a reason to suspect equipment-side problems. If a pyranometer is installed on site, check the relationship between power generation and irradiance. Even without a pyranometer, nearby weather data, weather records in the monitoring system, and on-site photographic records can be used as references. However, because external weather data does not necessarily match the actual irradiance conditions at the plant exactly, it is important to treat them only as one piece of information for judgment.

When considering the effects of solar irradiance separately, it is also necessary to pay attention to seasonal differences. Solar elevation and sunshine duration change with the seasons, so you cannot simply compare power generation in summer and winter. In winter, shorter sunshine hours and lower solar elevation make power generation more likely to decrease even with the same equipment. Also, during periods of high ambient temperature the temperature of solar panels rises, and under certain conditions their output can be suppressed. Summer brings higher solar irradiance but panels can be affected by temperature, so multiple factors are involved in assessing power generation.

To suspect aging-related degradation, you need comparisons that remove the effects of weather and solar irradiance to some extent. For example, you can compare clear-sky days, look at multi-year data for the same month, examine changes in generation efficiency relative to irradiance, or compare multiple areas within the plant under the same conditions. If generation efficiency is lower than before despite similar irradiance conditions, the likelihood of aging-related degradation or equipment malfunction increases.

However, even when solar irradiance conditions appear similar, they can actually vary greatly due to cloud movement and short-term effects. Daily generation totals alone may make it difficult to see information such as it being cloudy only in the morning, shadows appearing only in the afternoon, or brief stoppages. Therefore, if possible, checking generation trends by time of day makes judgment easier. On clear days, the generation curve approaches a smooth, bell-shaped profile. If there is an unnatural dip along the way, it can be a cue to check for shading, equipment shutdowns, output curtailment, connection faults, or communication loss.

Before linking low power generation to age-related degradation, it is important to check its relationship with solar irradiance. If solar irradiance is simply low, an equipment inspection may not find any abnormalities. Conversely, if low generation repeatedly occurs despite sufficient solar irradiance, priority should be given to investigating the equipment. By considering weather and solar irradiance separately, you can reduce unnecessary inspections while making it less likely to overlook necessary actions.

Evaluation criterion 3: Check for changes in panel soiling and shadows

When power generation is low, two representative causes that are easily mistaken for degradation over time are dirt on the panel surface and shading. Because solar panels are installed outdoors, they are affected by sand and dust, pollen, yellow sand, bird droppings, fallen leaves, mud splashes, drainage marks, and dust from nearby construction. When dirt accumulates, the amount of light reaching the panel surface is reduced, which can decrease power output. If this is mistakenly judged as degradation over time, you may overlook generation losses that could be recovered through cleaning or by improving the surrounding environment.

Power generation losses caused by soiling can be influenced by the season and weather. Some dirt is washed off to some extent by rain, but other types of dirt are not easily removed by rain alone. In particular, installations where mud or dust tends to accumulate at the lower edge, sites near unpaved roads or agricultural land, places where birds congregate, and locations with a shallow slope where dirt does not wash away easily are more likely to experience soiling that affects power generation. If power generation gradually declines and partially recovers after cleaning or after heavy rain, you should suspect soiling as well as aging degradation.

Shadows are also an important factor in decision-making. Even if there were no problems at the time of installation, over the years surrounding trees may grow, new buildings or equipment may be installed, or fences, signs, utility poles, or vegetation around mounting racks may cast shadows. Because the sun’s angle changes with the season and time of day, phenomena such as shadows lengthening only in winter, shadows appearing only in the mornings and evenings, or reduced power output in particular rows can occur. When determining the cause of low power generation, you need to look not only at the current shadow situation but also at how shadows change by time of day and by season.

The effect of shadows can be difficult to judge based solely on the area that is shaded. Depending on the solar panels and circuit configuration, some shadows can have a large impact on power output. Therefore, it's risky to conclude by visual inspection that "it's only a little, so it's not a problem." If monitoring data shows the power-generation curve is distorted at certain times, or if only a specific string has low output, it is useful to check on-site whether shadows are present during those times.

When distinguishing from age-related degradation, it is also important to determine whether dirt or shading is localized or widespread. If the decline is similar across the entire plant, factors such as weather, solar irradiance, output control, and the overall condition of the equipment need to be considered. On the other hand, if power generation is low only in a particular row, a specific group of panels, or during certain times of day, the likelihood of dirt or shading increases. Age-related degradation often manifests as a long-term decline in output, whereas dirt and shading tend to be concentrated in specific locations or times.

At site inspections, taking photos during the time periods when power generation is low makes it easier to determine the cause. If morning shadows are the cause, photos taken only at midday will not reveal them. If winter shadows are the cause, inspections conducted only in summer will miss them. Dirt can also be difficult to see from a distance. Check the lower edge of the panel surface, drainage streaks, areas prone to bird damage, places where weeds are growing, and areas around the racking where dust tends to be kicked up, and it is important to link these observations with monitoring data when making a judgment.

Before considering whether a decline in power output is due to age-related degradation, it is practically useful to first check whether dirt and shading can be ruled out. These are relatively easy to detect and may be improved through countermeasures. When low power output persists, it is important to pay attention not only to the equipment’s age but also to changes in the site environment.

Evaluation Criterion 4: Inspect the Condition of the Power Conditioner and Electrical Equipment

The cause of low power generation is not necessarily limited to the solar panels. The electricity generated by the panels is managed through junction boxes, collection equipment, power conditioners, protective devices, measuring instruments, and communication equipment. If there is an abnormality, incorrect setting, stoppage, or deterioration somewhere, the power output can appear low even if the panels themselves are functioning normally. Even when using the term "degradation over time," you need to consider not only a reduction in panel output but also aging-related changes and failures of peripheral equipment.

What you should particularly check is the operating status of the power conditioner. If the power conditioner is partially stopped, frequently cycling between stop and restart, generating errors or alarms, experiencing output reduction due to rising temperature, or having its output limited by set values or protective actions, the amount of power generated will decrease. In such cases, the issue may be with the conversion equipment or operating conditions rather than panel aging.

When the power generation on the monitoring screen is low, check not only the total power generation but also the power generation of each power conditioner, operating hours, shutdown history, and alarm history. If only one unit among multiple units has low power generation, prioritize checking the circuit connected to that unit and the condition of the unit itself. If all units are similarly low, consider solar irradiance conditions, grid-side control, overall soiling, and issues with common equipment. By examining differences on a per-device basis, it becomes easier to distinguish between age-related degradation and individual faults.

Degradation of electrical equipment and poor connections should not be overlooked. Loose terminals, corrosion, damaged cables, insulation degradation, abnormalities inside junction boxes, and the operation of protective devices can lead to reduced power generation and safety issues. These problems are often difficult to detect from the outside and may require professional inspections. If low power generation persists and monitoring data suggests an abnormality in a specific circuit, it is important not to rely solely on on-site personnel for a judgment; consider verification by qualified personnel or specialist contractors as necessary.

Also, there are cases where the reported generation appears low not because output is actually low but because of measurement or communication issues. If communication is missing, the data acquisition interval has changed, measurement device settings have been altered, aggregation conditions in the monitoring system have changed, or there are inconsistencies in meter readings, the displayed generation may not accurately reflect reality. When generation suddenly becomes zero or extremely low, you should check not only for equipment shutdowns but also for communication and measurement anomalies.

To assess aging-related degradation, it is important to separately identify where the factors causing performance decline are—whether in the panels, power conditioners, cables, junction boxes, or measurement instruments. Even if the outcome is the same—reduced power generation—the response differs greatly depending on whether the cause is panel degradation, equipment shutdown, an electrical equipment fault, or a measurement anomaly. By checking operating data and alarm histories for each piece of equipment, it becomes easier to set priorities for investigation rather than vaguely suspecting the entire facility.

Decision Factor 5: Compare differences for each string and area

Comparisons within a power plant are effective for distinguishing between declines in power output and aging degradation. Groups of panels within the same plant have similar installation conditions and are affected by the same weather. Therefore, comparing power output by string, by circuit, by power conditioner, and by area makes it easier to separate the effects of the external environment from equipment-side problems. It is easier to identify where the decline is occurring than by looking only at the total power output of the entire plant.

For example, even if the plant's overall power generation appears low, a closer look may reveal that only certain areas have declined significantly. In that case, it is more likely that the issue is specific to those areas rather than aging of the entire plant. Candidates include shading, vegetation, soiling, cables, junction boxes, string disconnections, panel damage, and partial shutdowns of power conditioners. Conversely, if multiple areas have declined by similar proportions, consider irradiance conditions, overall soiling, grid-side impacts, and aging of the entire facility.

When comparing strings, it is important to compare those with the same orientation, the same tilt, the same number of panels, and similar installation conditions. If you simply compare items with different conditions, you may mistake a normal difference for an abnormality. Comparing south-facing and east-west-facing strings, mounting structures with different tilts, rows that are prone to shading, or circuits with different numbers of panels by the same standard will lead to inaccurate assessments. Before comparing, you need to confirm which circuit corresponds to which area and whether the installation conditions are consistent.

When examining differences in power generation, not only instantaneous values but also trends by time of day are important. If only a particular string is low in the morning, it suggests morning shading; if low in the evening, evening shading; if low throughout the daytime, dirt or circuit abnormalities; if the difference is large only on sunny days, temperature or output limiting may be the cause—possible causes vary by time of day. By looking at time-of-day curves, environmental factors or local faults can sometimes be identified rather than age-related degradation.

Also, if you have data covering multiple years, it is useful to check whether the differences between the same areas are widening year by year. If a consistent difference has existed since installation, it may be due to differing installation conditions. On the other hand, if areas that were previously similar have recently declined, that can indicate accumulated dirt, increased shading, equipment degradation, or connection faults. Because degradation progresses over time, it is important to monitor changes in those differences.

Comparisons within the power plant also help prioritize inspections. Since examining everything in detail at once is time-consuming, it is efficient to first identify areas with noticeable declines and proceed with on-site checks from there. Narrow down candidates for abnormalities using monitoring data, and then follow a process of checking for shading, dirt, damage, wiring, junction boxes, and operating conditions on site; this makes it easier to determine the cause. Especially when power generation is low, it is important to look not only at the overall value but also at the breakdown to identify where the problem lies.

Assessment Item 6: Determining the likelihood of age-related deterioration from inspection history and years since installation

To assess age-related degradation, you need to look not only at the years since installation but also at past inspection records, replacement history, cleaning history, failure history, and changes in the surrounding environment. Solar power generation systems operate outdoors for long periods, so components and equipment change over time. However, just because a system has been installed for many years does not mean that all reductions in power output are due to aging. Conversely, relatively new systems can also exhibit reduced power output because of poor installation, incorrect settings, localized shading, dirt, or equipment malfunctions.

When reviewing inspection history, check whether scheduled inspections were carried out as planned, whether there have been past alarms or shutdowns, whether the same locations have experienced repeated faults, and whether the timing of cleaning or weeding corresponds with changes in power generation. If power generation recovers after inspection, soiling, electrical connections, or equipment condition may have been affecting output. If there is no improvement despite inspection and a long-term decline continues even after accounting for solar irradiance conditions, the possibility of age-related degradation should be examined more carefully.

For installations that have been in place for many years, not only the panels but also the condition of power conditioners, cables, junction boxes, mounting structures, terminals, protective devices, and measuring instruments is important. Photovoltaic panels are designed for long-term use, but their output can change slightly over time. In addition, peripheral equipment is affected by component lifetimes and operating environments, and the rate of degradation varies depending on conditions such as high temperatures, high humidity, salt damage, dust, snowfall, strong winds, and vibration. Assessment of deterioration due to aging should be made based not only on the number of years but also on the installation environment and maintenance status.

When considering whether a decline in power generation is due to age-related degradation, it is also important to have past baseline values. If generation data from the time of installation or from past healthy periods is available, you can compare it with the current condition. Conversely, if past data is lacking, it becomes difficult to determine when the decline began and how far it has progressed. Keeping daily monitoring data, monthly reports, inspection photos, alarm histories, cleaning records, and repair records directly informs future deterioration assessments.

Even when aging degradation is suspected, it is important not to immediately decide on large-scale refurbishment or replacement, but to rule out other causes first. Check solar irradiance, weather, shading, soiling, output control, equipment shutdowns, measurement anomalies, poor connections, and so on; only if long-term declines that cannot be explained remain should you seriously consider panel or equipment degradation. If necessary, combining specialized inspections—on-site measurements, insulation checks, current and voltage verification, and thermographic inspection to identify abnormal areas—will make it easier to reach a decision.

A certain degree of aging is unavoidable, but if you detect signs early, it becomes easier to consider countermeasure options. Depending on the cause, you can choose responses such as partial equipment replacement, revising cleaning and weeding, adjusting inspection intervals, repairing abnormal areas, and improving monitoring items. Don’t dismiss low power generation as simply “due to age”; making stepwise judgments based on history and data leads to practical measures that reduce generation losses.

Summary: Isolate the causes of low power output step by step

To distinguish a drop in power generation from degradation due to aging, it is important not to judge solely by the fact that generation is low. First, check whether the decrease is temporary or persistent, and then separate the effects of weather and solar irradiance. After that, by checking in order panel soiling and shading, the condition of power conditioners and electrical equipment, differences among strings and areas, inspection history, and years since installation, you can more easily narrow down the cause.

Degradation over time is an unavoidable issue when operating solar power systems for long periods, but not all causes of reduced power generation are due to aging. Dirt, vegetation, shading, equipment shutdowns, communication failures, measurement anomalies, output control, poor connections, and other factors that can be corrected may be involved. Conversely, if a decline continues over the long term and cannot be explained by external factors, you should suspect degradation of the panels or peripheral equipment and arrange for a professional inspection.

In practice, as soon as you notice low power output, it's important to record and correlate the data with on-site conditions. There can be shadows and soiling that monitoring data alone may not reveal, and there are time-of-day generation curves that site photos alone can't show. Combining both improves the accuracy of assessment. Especially when a power plant is large or multiple sites are being managed, having a system to quickly identify locations with anomalies and to prioritize inspections and responses is important.

If a decline in power output is left unaddressed, generation losses can accumulate even if the cause is as simple as dirt or shading. Even when the decline is due to aging, early detection makes it easier to plan repairs or replacements. When you suspect low power output, don’t rely on intuition alone: organize solar irradiation, weather, site environment, equipment condition, history, and area-specific data, and then isolate causes step by step.

If you want to continuously monitor on-site power generation status and detect early signs of decline, combining daily data checks with records of on-site conditions is effective. When organizing the causes of a decline in power output, it is important to centralize monitoring data, inspection photos, cleaning records, alarm history, and site notes so they can be compared later. By accumulating records, it becomes easier to determine whether the issue is due to age-related degradation, environmental changes, or a temporary fault, which leads to improved accuracy in power plant management.