7 Signs of Degradation to Look for in Low-Output Used Solar Systems

If you operate a used solar power plant and feel its generation is low, it is premature to immediately conclude the equipment has reached the end of its life or suffered a major failure. There are multiple factors that can reduce generation, such as solar irradiance, temperature, shading, vegetation, snow accumulation, grid-side controls, and how measured values are interpreted. On the other hand, with used equipment the elapsed years since installation, past maintenance, replacement history, and the effects of disasters or construction defects can overlap, causing signs of degradation to manifest as reduced generation. What matters is not to judge low generation by intuition alone, but to link visible on-site changes with generation data to verify the cause. This article explains, following the flow of on-site inspections, seven degradation signs that field personnel should prioritize when dealing with used solar power plants showing low generation.

Table of Contents

• When a used solar power system has low power output, distinguish degradation from differences in conditions.

• Sign 1 The surface of the solar panels shows noticeable dirt, scratches, and discoloration

• Sign 2: There are traces of corrosion and moisture on the back of the panel and around the mounting frame.

• Sign 3: Large output differences among power generation strings

• Sign 4: The power conditioner repeatedly shuts down, triggers warnings, and experiences temperature rises.

• Sign 5: There are traces of heat and looseness around the junction box, cables, and terminals.

• The light-receiving conditions of the support structure for Sign 6 are being compromised by its tilt, settlement, and shadows from vegetation.

• Sign 7 Measurement values, amount of electricity sold, and on-site conditions are not consistent

• Inspection procedure after finding signs of degradation in used solar photovoltaic systems

• Summary: Low-output used solar power systems should be evaluated by combining visible signs of degradation with data

When power generation is low in used solar PV systems, distinguish degradation from differences in conditions

When you feel a used solar power plant is underperforming, you should first determine whether equipment degradation is really the cause. Power generation is not determined solely by solar panels or inverters. Even with the same equipment, generation varies during months of persistent cloudiness, on hot days, in seasons when shadows from nearby trees lengthen, in periods with heavy yellow dust or pollen, and during times when snow or fallen leaves remain. Especially with used solar installations, the historical generation records or assumed generation estimates may not have been sufficiently verified before acquisition, and the benchmark figures themselves may not match the actual site conditions.

When assessing whether power generation is low, it is important not to judge based on a single day alone. If generation drops for just one day, it could be due to weather, temporary controls, power outages, maintenance work, or similar factors. Checking whether the same trend continues over several days to weeks, and comparing with the same month of the previous year, makes it easier to narrow down the possibility of an anomaly. For used solar PV systems, unless you examine past generation records, insolation conditions, system capacity, inverter capacity, panel azimuth, tilt angle, and the surrounding environment together, the simplistic impression that "generation is low" can take precedence.

Also, degradation does not necessarily reduce power generation to zero all at once. Partial soiling of panels, poor terminal contact, a rise in the power conditioner’s temperature, partial shading from vegetation, a slight tilt of the racking, and similar issues can gradually depress power output. For that reason, in the early stages these are often overlooked as “it might be the weather” or “just a bit lower than last year.” With used equipment, several of these small declines may overlap. That is why it is important to combine visual inspections, generation data, device warning histories, and on-site photographs to pick up signs of degradation early.

In practice, when a drop in power generation is detected, rather than immediately considering parts replacement, you first determine the scope of the decline. The suspected causes differ depending on whether the entire plant is underperforming, only specific inverters (power conditioners) are underperforming, or only particular strings are underperforming. If the shortfall is overall, check the weather, soiling, shading, measurement conditions, and grid-side control. If only a part is low, suspect localized faults such as panel rows, cables, junction boxes, inverters, terminals, and fuses. Visiting the site without this isolation will broaden the inspection scope and make it harder to identify the cause.

A decline in power generation of used solar PV systems can be related not only to the aging of the equipment itself but also to insufficient maintenance by the previous owner, reduced frequency of grass cutting, poor drainage, unrepaired damage after typhoons, bird and small animal damage, and malfunctioning measuring instruments. If past inspection records are not available, it is necessary to check on-site signs of deterioration one by one and standardize the current condition. From here, we will go through the 7 signs of deterioration you should especially not overlook in used solar installations with low power output.

Sign 1: Dirt, scratches, and discoloration on the surface of solar panels are noticeable

The first thing to check is the condition of the solar panel surfaces. In used solar installations, long-term outdoor exposure can lead to accumulation on the panel surfaces of sand and dust, yellow sand (Asian dust), pollen, bird droppings, fallen leaves, rain streaks, mud splashes, and so on. When panel surfaces become soiled, the amount of light they can receive is reduced, which can lead to lower power generation. If a thin layer of dirt is spread across the entire surface, it is often seen as a drop in output for the whole plant; if soiling is localized, it can appear as output differences in specific strings.

Particular attention should be paid to cases where dirt remains in streaks along the bottom edge of the panel or near the frame. Dirt washed down by rain can accumulate at the lower part and continuously cover portions of the cells, which can affect power generation more than mere cosmetic soiling. Solar panels can suffer reduced output, depending on the connection units, even if only a portion experiences poorer light conditions. For used solar panels that "as a whole are operating but produce less power than expected," it's worth checking whether such partial soiling has been left unattended for a long time.

Scratches and cracks are also important signs of degradation. Check not only for obvious cracks in the surface glass but also for fine linear scratches, impact marks from debris, chipping at the edges, and internal discoloration. Even if they appear small externally, they can allow moisture ingress, reduce insulation performance, and cause localized heating. If a drop in power output coincides with abnormalities that occur more readily after rain or on high-humidity days, moisture affecting the panel surface or edges should be suspected. However, internal degradation cannot be conclusively determined from appearance alone, so visual inspection results must be evaluated together with power generation data and the findings of specialist inspections.

Discoloration should not be overlooked. Conditions such as yellowing of the panel surface or cell areas, whitening or cloudiness, partial brown appearance, or variations in cell color may be related to long-term use, heat effects, deterioration of the encapsulant, or dirt adhesion. With used solar panels, these visual changes often progress gradually the longer they have been installed. If areas of reduced power output overlap with the same visual changes, those areas should be prioritized for inspection.

At on-site inspections, we not only take a distant view of the entire panel array but also check how soiling varies by row, differences by orientation, differences between lower and higher panels, and differences caused by surrounding environments such as the road side or the field side. If soiling is uniform across the whole array, environmental factors may be the cause; if only certain rows are heavily soiled, influences such as drainage, wind direction, nearby trees, or places where birds congregate should be considered. With used solar installations, cleaning histories are sometimes unknown, so keep photographic records of the condition and ensure they can be compared with generation trends, which makes later assessment easier.

Sign 2: Corrosion and moisture marks on the back of the panel and around the mounting frame

In used solar installations with low power output, check not only the panel surface but also the rear and the mounting structure. Even if the surface looks clean, cables, connectors, junction boxes, frames, and mounting hardware on the rear may have deteriorated. Especially in used equipment, years of rainwater, humidity, condensation, poor drainage, salt, dust, and accumulated leaves can cause corrosion and poor contacts to develop in hard-to-see areas. If the decrease in power output is concentrated in some circuits, such rear-side degradation may be one of the causes.

On the back of the panel, check for discoloration or lifting around the terminal box, for cracks or hardening in the cable sheathing, and for moisture marks or adhered dirt on the connectors. The terminal box and connectors are critical parts through which DC-side electricity flows. If moisture enters here or contacts become unstable, it can lead not only to reduced power generation but also to safety risks. You cannot determine the energized condition by appearance alone, but if there is discoloration, burn marks, unusual odors, or melted-looking traces, do not touch them and have them inspected by a professional.

Corrosion around the racking can also indirectly affect power generation. If the racking fastenings loosen, supports settle, or metal fittings corrode, the panels’ tilt and orientation can change slightly. Even without large deformations, differences in tilt between rows lead to variations in sunlight reception. In addition, deformation of the racking can pull on cables or cause connectors to sag into positions closer to puddles. For second‑hand solar installations, low power output should not be seen as a problem of electrical components alone; the condition of the supporting structures must also be checked.

Moisture marks are a particularly important sign. In environments where rainwater remains for long periods on the rear of panels or under the mounting racks, drainage channels are clogged, the ground is waterlogged, or weeds and fallen leaves retain moisture, electrical components and metal parts are more likely to deteriorate. Places where wet marks remain not only immediately after rain but also after a stretch of sunny weather may indicate drainage or ground problems. If areas with low power generation overlap with areas that tend to retain moisture, their inspection priority should be increased.

When carrying out inspections, do not touch hazardous electrical parts carelessly. Second-hand solar PV systems can have components carrying voltages higher than they appear. The purpose of finding signs of deterioration is not to force repairs on the spot, but to narrow down possible causes while avoiding danger. Take on-site photographs and record them linked to the location, the panel string, the inverter (power conditioner) number, the junction box number, and so on, so that subsequent professional inspections and repair decisions proceed smoothly.

Sign 3: Large differences in output between power generation strings

When power generation from used solar PV systems is low, looking only at the total exported electricity can obscure the cause. In practice, it is important to check the output differences for each generation string. A string is a unit in which multiple solar panels are connected in series. If strings with similar orientation, tilt, and sunlight conditions show large output differences, suspected causes include panel degradation, dirt, shading, cable faults, poor connections, blown fuses, or abnormalities inside the junction box.

In used solar PV systems, even within the same plant the panel installation dates and replacement histories may differ. If only some panels were replaced in the past, only certain sections were repaired during disaster recovery, or components of different specifications are mixed, differences in generation trends by string can appear. A difference itself is not necessarily abnormal, but if the difference has widened compared with before, a particular circuit suddenly shows lower output, or the difference does not close even on sunny days, it should be treated as a sign of degradation.

When examining output differences, it is important not to judge based only on instantaneous values. Passing clouds, moving shadows, or differences in measurement timing can temporarily produce discrepancies. If possible, check values during periods of stable solar irradiance on clear days, trends over multiple days, and comparisons with adjacent strings under the same conditions. If values are low only in the morning, only in the afternoon, or only during a particular season, shading or orientation effects should be considered. If values are low throughout the whole day, equipment faults, soiling, or connection problems are more likely.

On site, identify the panel row corresponding to the low-output string and inspect appearance, dirt, shading, vegetation, cables, connectors, and junction boxes. The important point here is to match the generation data with the on-site location. If the drawings and management numbers are outdated, you won’t know which row the low-output string actually is, and the inspection will take longer. In used solar installations, transfers and changes of management company can leave drawings and on-site labels mismatched, so it is important to reconcile management numbers first.

String output differences are the entry point for localizing the causes of reduced power generation. The efficiency of inspections varies greatly depending on which inverter, which combiner box, or which panel string you narrow the overall low power generation issue down to. With used solar PV systems, it is more realistic to identify the areas that are underperforming and address them in order of priority rather than replacing or repairing everything at once. Regularly recording output differences makes it easier to see the locations where degradation is progressing.

Sign 4: Repeated inverter shutdowns, warnings, and temperature rises

For used solar installations with low power output, checking the condition of the power conditioner (inverter) is essential. Even if the solar panels are generating electricity, if the power conditioner is not converting it stably, the amount of electricity sold to the grid and the usable energy will not increase. In used equipment, years of operation can cause degradation of internal components, reduced cooling performance, poor ventilation, dirty filters and intake/exhaust areas, and higher ambient temperatures at the installation site, which can combine to make shutdowns and warnings more likely.

If there is an abnormality in the inverter (power conditioner), reductions in power output tend to appear depending on the time of day. It may operate normally in the morning but stop during the high-output daytime hours, have its output curtailed on hot days, repeatedly trip with warnings and then recover, or show low amounts of electricity sold only for certain inverters—these conditions require inspection. Protective actions triggered by temperature rise can work to protect the equipment, but if they occur frequently they lead to lost generation opportunities. If you feel that a used solar PV system is producing less power, check the inverter (power conditioner) history and see whether the stop times coincide with the times of output decline.

On-site, check the power conditioner (inverter) unit's display, warning history, operation indicator lamps, dirt on intake and exhaust vents, surrounding ventilation conditions, exposure to direct sunlight, discoloration of the enclosure, and any abnormal noises. If nearby vegetation or materials are too close, heat dissipation may be impeded. For outdoor installations, ingress of rainwater, intrusion by insects or small animals, humidity, and sand and dust can also have an impact. Even if there are no visible abnormalities, if the same entries repeatedly appear in the warning history, you should arrange a professional inspection promptly.

In used solar systems, the power conditioner (inverter) replacement history and maintenance records may be unclear. You cannot simply assume a failure based only on the installation year, but the longer a device has been in operation, the more attention should be paid to degradation of cooling fans, internal electronic components, and connections. Also, panel-side defects can manifest as warnings on the power conditioner. Just because a power conditioner displays a warning does not necessarily mean the cause is the unit itself. Check by isolating the DC side, AC side, grid side, and the temperature environment.

Abnormalities in the power conditioner (inverter) are related not only to reduced power generation but also to the risk of shutdown. If small warnings are left unaddressed, you may later discover that a specific section has been offline for a long time. Do not rely solely on remote monitoring readings; cross-check with the on-site display and record the date and time the warning appeared, the recovery status, and the weather conditions to make it easier to estimate the cause. For used solar installations, making the condition of the power conditioner a central item in regular inspections leads to early detection of generation decreases.

Sign 5: Evidence of heat or looseness around junction boxes, cables, and terminals

For used solar PV systems with low power generation, we also inspect for degradation around junction boxes, combiner boxes, cables, terminals, and connectors. These are the pathways that collect and transmit the generated electricity; they may look inconspicuous, but they are critical areas that can affect power output and safety. In used equipment, connection points may be degraded due to years of temperature fluctuations, vibration, rainwater, ultraviolet exposure, insufficient tightening during installation, or the quality of past repairs.

Signs of thermal damage to watch for include discoloration around terminals, scorch marks, deformation of resin parts, hardening or melting of cable insulation, unusual odors, and localized blackening inside the enclosure. If connections are loose or there is poor contact, resistance increases and heating is more likely to occur. As a result, this can appear as unstable power generation, low output in specific circuits only, or warning indications. Because these areas can be hazardous, if an abnormality is suspected do not touch them while energized; they should be inspected using proper professional procedures.

Cable condition is also important. Cracks in the cable sheath, contact with the ground, abrasion against sharp components, loose cable ties, sagging, gnawing by animals, and UV degradation can lead to insulation faults or poor connections. In used solar installations, cables may be nicked during mowing, sag beneath mounting racks and lie close to puddles, or have wiring routes disrupted by past repairs. In sections with low power generation, check not only the panels but also the condition of the wiring.

Degradation around terminals and connectors can be difficult to see from the outside. Even if the appearance of the junction box looks normal, terminals may be loose inside, fuses may be abnormal, moisture may have penetrated, or insects and dust may have accumulated. If a reduction in power generation is concentrated in a specific string, it may be necessary to inspect the inside of the junction box. However, because the inside of a junction box presents an electrical hazard, it should not be opened or worked on by anyone without the appropriate qualifications or knowledge. Operational personnel should record appearance, warnings, power generation data, unusual noises, unusual odors, and the surrounding environment, and, if necessary, request inspection by a specialist.

In used solar power systems, degradation of the electrical pathways progresses gradually, so declines in power generation can also appear gradually. Pay attention not only to sudden failures but also to changes such as “slightly lower output than the previous year under the same weather,” “the peak during sunny conditions not reaching previous levels,” and “only certain sections showing unstable waveforms.” Problems at connection points are related not only to reduced output but also to risks of overheating and shutdowns, so it is important not to ignore signs of degradation when they are found.

Sign 6: Tilt, settlement, and vegetation shadows on the mounting rack are compromising the light-receiving conditions

Low power generation in used solar PV systems is not caused by electrical equipment alone. The tilt of the mounting racks, ground subsidence, unevenness of the developed surface, growth of vegetation, and shadows from surrounding structures can change the light-receiving conditions from those at the time of installation. Solar power output varies even with the same installed capacity depending on how stably it can receive solar irradiance. In used solar power plants, locations that were fine at installation can experience changes in the surrounding environment within a few years, leading to reduced generation.

Tilt or settlement of the racking can be difficult to detect from appearance. From a distance it may look aligned, but you can find conditions such as different heights by row, twisted panel surfaces, ground around support posts that has sunk, or scour marks around foundations. Even a slight change in angle can increase shading between rows and worsen drainage so that dirt tends to remain. If rows with low power output coincide with locations of racking deformation, treat this not as a mere cosmetic defect but as a sign related to power generation performance.

Shadows from vegetation are also an important item to check. In used solar installations, during periods when maintenance frequency has decreased, weeds and shrubs can grow and cast shadows on the lower parts and edges of panels. Because even partial shading can affect the output of an entire string, it can reduce energy production more than it appears. Especially in mornings and evenings, in winter, and when the sun’s elevation is low, shadows from trees, fences, utility poles, buildings, and slopes that normally go unnoticed can lengthen and appear as reduced generation during specific times of day.

During on-site inspections, it is important to check for shadows not only around noon but also during the time periods when power generation drops noticeably. If remote monitoring data shows lower output only in the morning, only in the afternoon, or varying by season, checking for shadows on site at those times makes it easier to identify the cause. Trends such as generation improving only immediately after grass cutting, drops becoming noticeable only in winter, or differences from adjacent plots changing with the time of day are also grounds to suspect changes in light-receiving conditions.

For used solar installations, changes in the surrounding environment may not be reflected in the documents from the time of contract or acquisition. Even if photos taken before acquisition show little shading, trees may have grown, new structures may have been added on neighboring land, or the condition of fences and weed-control sheets may have changed. When power generation is low, it is important to check not only for internal degradation of the equipment but also whether the on-site spatial conditions have changed. Regularly photographing the site from the same position makes it easier to compare later for changes in vegetation and racking.

Sign 7: Measured values, amount of electricity sold, and on-site conditions are not consistent

One easily overlooked sign of degradation in used solar installations is a lack of consistency between measured values, feed-in amounts, and on-site conditions. Even if you feel generation is low, your assessment depends on which numbers you are looking at. If the generation reported by remote monitoring, the inverter display, the feed-in meter, monthly statements, and the on-site operation display do not match, the issue may be measurement or communication-related rather than equipment degradation. Conversely, unstable measurements can also delay the detection of a drop in generation.

In used solar PV systems, the continuity of data can be disrupted by changes such as switching management companies, updating communication equipment, stopping monitoring devices, or changing meter-reading methods. Even if you try to compare the same month year-over-year, you cannot make a straightforward comparison if the measurement method changed partway through. When inverter-level data are missing, communication errors are frequent, time settings are off, or there are large discrepancies between sold electricity and monitored values, it is necessary to separate an actual decline in generation from data-quality issues.

Consistency with on-site conditions is also important. If generation values are extremely low despite clear skies; if an inverter is actually operating but appears as stopped on the monitoring system; if warnings are present on-site while the management screen shows everything normal; or if the amount of electricity sold is falling even though panel and inverter data show no abnormalities — then the scope of checks that need to be performed widens. Leaving these inconsistencies unaddressed can lead to missing actual degradation or, conversely, incorrectly suspecting parts that are not faulty.

During data verification, align the data for the entire power plant, by inverter (power conditioner), by string, power sales records, and meteorological conditions over the same period as much as possible. Even if measured solar irradiance data are not available, comparing sunny days with each other, the same season, or days with similar irradiance conditions makes it easier to detect anomalies. For used solar PV systems, past data may be insufficient; in such cases, it is important to organize records based on the current condition so they can be used for comparison in future inspections.

Inconsistencies in measured values are not always evidence of equipment degradation themselves. However, they are a very important sign for operational staff. To identify the cause of low power generation, accurate data is required. If measurements remain unstable, it becomes difficult to determine whether panel degradation, inverter faults, shading, soiling, or poor connections are to blame. To manage used solar PV systems reliably, it is essential to create a situation in which on-site observations and the data are connected.

Inspection steps after finding signs of degradation in used solar panels

After finding signs of degradation, organize the verification procedure instead of immediately attributing it to a single cause. First, grasp the extent of the generation decline. By distinguishing whether it is the entire plant, a specific inverter, or a specific string, you can prioritize inspections. If the decline is overall, broadly check solar irradiance conditions, soiling, shading, grid-side control, and measurement conditions. If the decline is partial, focus on the affected panel rows, junction boxes, cables, inverters, and terminal sections.

Next, examine the time of day and the conditions when the issue occurs. Depending on whether output is low only at the peak on sunny days, only in the morning and evening, warnings increase after rain, or the system shuts down during high summer temperatures, the suspected causes will differ. If output does not rise at peak, check for inverter temperature rise or output curtailment, panel degradation, and soiling. If output is low in the morning/evening or in winter, check for shading. If abnormalities appear after rain, check for moisture ingress, insulation issues, connectors, and around the junction box.

Then link the onsite photos with the data. Consolidate into the same record the power conditioner numbers with low output, combiner box numbers, string numbers, the position of the panel rows, and any dirt, corrosion, shading, vegetation, or racking deformation found by visual inspection. For used solar installations, drawings and labels may be outdated, so also verify that the onsite numbers match the data numbers. If these are misaligned, it becomes difficult to pinpoint the source of the problem.

Do not forget safety. Panels, junction boxes, inverters, and cables pose electrical hazards. If there are burn marks, unusual odors, damage, moisture ingress, or abnormal noises, do not let on-site personnel judge and handle it by themselves; request inspection by a specialist who holds the necessary qualifications and follows the proper procedures. For used solar installations, past repair histories may be unknown, so it is important not to judge safety by appearance alone.

Finally, don't treat the inspection results as a one-time event. Degradation of used solar PV systems should be viewed not as isolated points but as changes over time. If you record the current power generation, appearance, warning history, shading conditions, and vegetation condition, you can determine at the next inspection whether things have worsened or improved after countermeasures. Even when cleaning, mowing, terminal checks, inverter inspections, or component replacements are carried out, recording the date performed and the change in power generation will improve management accuracy.

Summary Used solar installations with low power output should be evaluated by combining visible signs of degradation and performance data

For used solar installations with low power output, it is important to check signs of degradation one by one while isolating weather and seasonal variations, shading, soiling, equipment faults, and measurement errors. The condition of used equipment can vary greatly not only by the number of years since installation but also by past maintenance practices, repair history, and changes in the surrounding environment. Rather than rushing to replace components or carry out major repairs based only on a low output result, determining where, when, and to what extent the output has declined will lead to more efficient, less wasteful countermeasures.

Particular signs of degradation to watch for are dirt, scratches, and discoloration on the panel surface; corrosion and moisture marks on the rear side and around the mounting structure; output differences between strings; inverter shutdowns, warnings, or temperature rises; heat or looseness around junction boxes and cables; tilt of the mounting structure or shading from vegetation; and discrepancies between measured values and sold electricity. These can individually reduce power output, or several can combine to depress the profitability of the entire plant. For that reason, on-site inspections and data checks should not be separated; they need to be linked and evaluated within the same record.

In used solar installations, the cause of reduced power generation is not always immediately apparent. However, by photographing onsite signs of degradation, cross-checking them with inverter- and string-level generation data, and reviewing warning histories and changes in the surrounding environment, you can gradually narrow down the list of possible causes. What’s important is not to dismiss it simply as “it’s old, so it can’t be helped.” By detecting small dirt, shading, looseness, corrosion, or measurement discrepancies at an early stage, you can more easily prevent prolonged declines in power generation.

To stably manage used solar PV systems with low power generation, it is important to visualize on-site conditions and organize inspection results so they can be continuously compared. If you have a system that allows consolidated review of site photos, generation data, equipment layout, and inspection history, you can reduce missed signs of deterioration and more easily prioritize repairs, cleaning, mowing, and detailed inspections. Declines in the power generation of used solar PV systems should be assessed by combining visible deterioration with changes in the data, and, when necessary, connected to specialist inspections to enable early identification of causes and appropriate improvement decisions.