7 Inspection and Improvement Items to Prevent Leaving Low Power Generation Unaddressed

A low power output from a photovoltaic system can sometimes be explained solely by weather. However, behind a decline there can be site-specific factors such as soiling of PV module surfaces, shading, equipment shutdowns or control actions, abnormalities in wiring and connections, missing monitoring data, and insufficient inspection records. If a low-output condition is left unaddressed, identifying the cause can be delayed and opportunities to recover generation that should have been possible may be missed. What matters is not to judge by intuition alone but to combine generation data, weather conditions, on-site verification, and work history to narrow down the cause step by step.

Table of Contents

• First quantify the low power output numerically

• Separate the effects of weather conditions and solar irradiance

• Check the panel surface for dirt and damage

• Inspect shading patterns at different times of day

• Check for abnormalities in wiring and connections

• Review anomalies in inverters and monitoring data

• Record inspection results and use them to drive an improvement cycle

• To prevent leaving low power output unaddressed

First, quantify the low power output numerically.

When you feel that power generation is low, the first thing to do is not to judge based solely on the on-site impression, but to quantify how low it is. There are many situations in which the person in charge feels something is off: generation does not increase even though it is sunny, there is less generation than the previous year, or the output appears lower compared with surrounding facilities. However, if you use that impression directly to determine the cause, differences in weather, season, equipment capacity, operating conditions, and measurement methods can become mixed together, making it difficult to carry out a proper inspection.

First, what you should check is the trend in power generation on a daily, monthly, and annual basis. If the decrease is limited to a single day, possible causes include cloud cover, temporary shutdowns, communication failures, or temporary effects from snow accumulation or heavy soiling. On the other hand, if low generation persists for several weeks to several months, it is necessary to investigate including equipment-related factors. In particular, if output is consistently below past performance for the same season, the priority for proceeding to an on-site inspection increases.

When comparing, it is important to look not only at total generation but also at generation per unit of rated capacity. When comparing multiple power plants with different installed capacities, total generation alone does not provide a correct comparison. Checking generation per 1 kW of rated capacity and metrics that take insolation into account helps normalize differences in scale and makes trends easier to see.

Also, if a single plant has multiple sections, power conditioners, strings, or circuits, comparing generation by unit makes it easier to determine whether a decline is appearing across the whole system or concentrated in specific parts.

To investigate the cause of low power generation, a benchmark expected value is also necessary. By combining past performance, the assumptions made at the design stage, trends from nearby installations under similar conditions, and power generation relative to solar insolation, you can verify whether the current generation is within a reasonable range. However, the design assumptions change depending on input parameters and simulation conditions. Rather than immediately concluding there is an anomaly simply because the output is below the assumed value, it is important to check whether performance is continuously deteriorating by looking at past performance for the same period and generation per unit of insolation.

When quantifying a drop in power generation, you also need to deliberately narrow down the scope of the anomaly. Separate whether the whole plant is producing low output, only certain systems are low, or only specific time periods show low output. If there are time-of-day patterns—low only in the morning, dipping around midday, or showing differences only in the evening—it becomes easier to consider factors such as shading, orientation, tilt, equipment temperature rise, and output control. Conversely, if output is low all day, a wider range of checks is required, including dirt, equipment degradation, device settings, wiring abnormalities, and measurement offsets.

The important point here is not to look only at the fact that power generation is low and immediately proceed to cleaning or parts replacement. If you take countermeasures without understanding the cause, it becomes difficult to evaluate their effectiveness, and you will have no basis for judgment if the same problem recurs. First, quantify the decline in power generation and organize when, where, to what extent, and during which time periods it is occurring—this is the first step to streamlining subsequent inspections.

Disentangling the Effects of Meteorological Conditions and Solar Irradiance

When power generation is low, the first thing often suspected in many cases is the weather. Solar power generation is greatly influenced by solar irradiance, so if cloudy or rainy days persist, generation will be reduced. Therefore, rather than judging an anomaly based solely on generation output, it is essential to compare it with the solar irradiance conditions during that period. To correctly determine the cause of low generation, it is necessary to distinguish whether it is an equipment issue or a natural variation due to weather conditions.

A decline in power generation during periods of low solar irradiance is natural. During the rainy season, the approach of typhoons, snowfall, prolonged rain, or the effects of yellow sand or haze/smog, sunny periods decrease and monthly power generation can fall sharply. In such cases, low generation should not immediately be treated as an anomaly; it is necessary to assess whether the generation is reasonable relative to the solar irradiance. If generation relative to irradiance does not deviate significantly from past trends, it may not indicate an equipment malfunction.

On the other hand, if solar irradiance is sufficient but power generation does not increase, caution is needed. If generation is clearly lower than in the same period in the past despite many sunny days, or if only certain sections produce less than nearby sections under similar conditions, the cause may lie with the equipment. In particular, if power output responds sluggishly to changes in irradiance, you should check for dirt on panel surfaces, shading, equipment control, wiring faults, or missing monitoring data.

When comparing solar irradiance and power generation, it becomes easier to judge if you also look at changes by time of day. If generation is occurring in the morning but suddenly drops in the afternoon, afternoon shadows, equipment temperature rise, or output control may be involved. Conversely, if the morning ramp-up is slow, shadows from the east, shading from mountains or buildings, equipment start-up conditions, or delays in communication records can be factors. Problems that are not visible from daily totals alone are often easier to detect by examining time-of-day data.

When separating meteorological factors, the influence of temperature should also be taken into account. Photovoltaic modules do not always produce more output in direct proportion to stronger irradiance; when module temperature rises their output tends to decrease. If power generation on a clear midsummer day does not increase as much as expected, the effect of temperature rise may be a factor. However, if a drop continues that is difficult to explain by high temperatures alone, soiling, shading, wiring, and equipment faults should also be checked.

Also, in snowy regions how the snow remains is a major factor. If snow remains only on part of a plant, accumulates at the lower edge of panels, or if differences in snowmelt occur due to mounting racks or surrounding structures, power generation can vary even within the same site. After rain, dirt may be washed away, while conversely splashed mud or deposits may remain. Weather affects not only solar irradiance but also the occurrence of soiling and shading, so it is important to take a broad view of the factors behind reduced power generation.

When using a pyranometer or meteorological data, check the condition of the measurement instruments as well. If the pyranometer is dirty, the installation location is shaded, the time settings are off, or communication is interrupted, comparisons between solar irradiance and power generation can be misleading. If you attribute low power output solely to the weather, you may overlook equipment-side abnormalities. Conversely, if you assume equipment failure without considering weather effects, you may waste time on unnecessary inspections or countermeasures. By reviewing solar irradiance, weather, temperature, snowfall, seasonal variation, and the reliability of measurement data, the next on-site items to verify become clear.

Check the panel surface for dirt or damage

If low power output persists, dirt or damage on the surface of solar modules should be prioritized for inspection. Solar modules generate electricity by receiving light, so when dirt adheres to their surface the amount of light received is reduced, which can lead to decreased power output. Even if the impact of dirt appears small at first glance, if it is widespread or remains for a long time it will manifest as a sustained difference in power generation.

The types of soiling vary depending on the site environment. They include soil dust, pollen, yellow dust, bird droppings, fallen leaves, exhaust-derived deposits, mud blown from farmland or development sites, and salt-containing deposits in coastal areas. These may be naturally washed away by rain, but not all soiling will be removed by rain alone. In particular, soiling tends to remain at the lower edges of panels, near the frames, on gently sloped surfaces, and in areas with poor airflow.

When checking for soiling, simply viewing the entire power plant from a distance is not sufficient. Even if it looks clean from afar, streaks of dirt or localized buildup can be found on closer inspection. Also, if only some panels are fouled with bird droppings or fallen leaves, this can affect the output of the circuit that includes those panels. During inspections, check whether the surface soiling is uniform, localized, or biased toward specific rows or directions.

Checking for damage and degradation is also important. Cracks in the surface glass, frame deformation, deterioration of the encapsulant or sealant, discoloration, scorch-like marks, trapped foreign objects, and similar issues can affect not only power output but also safety. If there is visibly obvious damage, do not proceed with work carelessly; carry out a professional inspection and, as necessary, consider verifying operating conditions, replacement, isolation, or other measures. This should not be overlooked not only as a cause of low power generation but also from the standpoint of accident prevention.

When checking for dirt or damage, recording the change in power generation before and after cleaning makes it easier to judge the effectiveness of the improvement. If generation does not improve despite cleaning, there may be causes other than dirt. Conversely, if generation recovers after cleaning, this provides a basis for reviewing future cleaning frequency and inspection timing. However, because weather and solar irradiance may differ before and after cleaning, simple comparisons are not valid, so it is necessary to compare with days that have similar irradiance conditions or with other sections within the same power plant.

During inspections, simply thinking that you can just wash off dirt as soon as you find it is not sufficient. Checking why dirt accumulated, where it tends to accumulate, and what conditions make recurrence likely will lead to the next countermeasures. If the site environment is the cause—such as nearby sources of sand and dust, structures that tend to attract birds, abundant leaf fall from vegetation, or poor drainage causing mud splashing—cleaning alone is unlikely to provide a fundamental improvement.

When cleaning, it is essential to follow the procedures specified by the photovoltaic module manufacturer or maintenance provider. Improper cleaning methods, the use of hard tools, or applying excessive pressure can damage the modules and wiring. Inspections and cleaning carried out on roofs, slopes, at height, or in areas with poor footing involve hazards. Because equipment that is generating power also presents electrical risks, avoid undertaking unsafe work and ensure tasks are performed in accordance with required qualifications and procedures. Balancing improvements in power generation with work safety is the perspective expected of operational personnel.

Inspect the occurrence of shadows by time of day

A commonly overlooked cause of low power generation is shadows that occur at certain times of day. In solar power generation, even partial shading of a solar cell module can affect output depending on the circuit configuration. In particular, shadows that were not noticeable when the system was installed can develop later due to the low solar altitude in mornings and evenings, seasonal changes in the sun’s position, and the growth of nearby buildings or trees. To avoid leaving a low-output condition unaddressed, it is important to inspect for shading not just once but with awareness of the time of day and season.

When checking for shadows, it's important not to judge based only on around noon. Even if there are no shadows in the middle of the day, shadows may extend only in the morning, only in the evening, or only in winter. When you look at power generation data by time of day, you may see generation drop only during specific hours. If you inspect the site during those hours, you may find that buildings, utility poles, overhead lines, fences, trees, adjacent equipment, or mountain ridgelines are casting shadows.

Shading is also related to the overall layout of the power plant. Even within the same site, the ends of rows, sections close to surrounding structures, and locations affected by terrain are more prone to shading. In addition, for ground-mounted installations, overgrowth of weeds can also cause shading. If vegetation grows up to the lower edge of the panels, it may appear as a small shadow to the naked eye, but it can become a factor that continuously reduces power generation. Regular weeding and surrounding-area maintenance are important for maintaining power output.

For rooftop equipment, check for shadows caused by the roof shape and any attached equipment. Exhaust vents, antennas, railings, adjacent buildings, rooftop equipment, and raised sections can cast shadows depending on the time of day. Even items that had little impact at the time of installation can change the length and direction of shadows as the seasons change. In addition, environmental changes—such as new buildings being constructed nearby, signs or equipment being added, or trees growing—can gradually reduce power generation.

For shadow inspections, it is effective to link and store site photos with power generation data. Recording the date, approximate time, location, and extent of the shadow allows you to later correlate it with drops in power output. Because photos alone do not reveal the duration of the shading, it is important to check at multiple times on the same day and compare with the time variation in the power generation data. The priority of countermeasures changes depending on whether the shading is temporary or persists for long periods.

Countermeasures for shading depend on the cause. If vegetation is the cause, weeding or pruning are options. If movable objects are the cause, removal or rearrangement can be considered. When shading is caused by structures or terrain, it may not be easily removed; therefore, it is necessary to understand the impact on power generation and treat it as an operational management issue. If a drop in power generation is assumed to be equipment failure without identifying the cause, it can lead to unnecessary inspections, so shading should be isolated at an early stage.

In consultations about low power generation, the impact of shading is sometimes underestimated. However, unlike solar irradiance or weather, shading can occur persistently as a site-specific condition. If a similar drop appears at the same time every day, it is important to check for the possibility of shading. Examining the time variation of power output, confirming shading on-site, and keeping records will improve the accuracy of improvement decisions.

Check for abnormalities in wiring and connections

If low power output persists, you need to check not only the panels and the weather, but also for abnormalities in the wiring and connections. In a solar power generation system, the electricity generated by the solar modules is sent through wiring to the junction box and the power conditioner. If there are poor connections, broken wires, increased contact resistance, loose terminals, corrosion, or cable damage along the way, this can lead to reduced power generation or a shutdown. Because wiring abnormalities are difficult to see from the outside, it is important to make judgments by combining power generation data with on-site inspections.

A characteristic of wiring anomalies is that only some circuits may have reduced power output. Even if the plant as a whole does not appear to have a major anomaly, a circuit-by-circuit view may reveal that only a particular system has low output. In such cases, it is necessary to inspect the panels, junction boxes, cables, terminals, and protective equipment around the affected system. Because looking only at the plant's total monthly generation can cause small declines to be overlooked, monitoring and comparing at the circuit level is important.

Abnormalities in connection points arise from aging, installation quality, and environmental conditions. Outdoor equipment is exposed to rainwater, moisture, temperature fluctuations, ultraviolet radiation, wind-induced vibration, and the effects of small animals. Looseness or corrosion at terminals can lead to increased resistance and heat generation. If heating progresses, it can result in equipment damage and safety hazards; therefore, inspections are necessary not only to identify the causes of reduced power generation but also from the standpoint of accident prevention.

In cable inspections, check for damage to the outer jacket, chew marks, pulling or overstretching, sagging, inadequate securing, contact with the ground or support structures, and standing water in areas with poor drainage. If cables are not properly secured, they can sway in the wind or rub against the racking, accelerating degradation. Also, for ground-mounted installations, mowing operations or movement of heavy machinery can damage cables. If a sudden drop in power output occurs, checking recent work history and weather events can provide useful clues.

When inspecting wiring and connection points, it is also important not to rely on appearance alone. Even if there seems to be no problem on the outside, poor contact can be occurring internally. If an anomaly is suspected, specialized measurements and safety checks are necessary. It is dangerous for unqualified personnel to carelessly open terminals or touch equipment that is energized. Operational staff are required to organize suspected anomalies and compile the information in a form that can be handed over to the appropriate responsible person or maintenance company.

When examining power generation data, comparing circuits under the same conditions makes anomalies easier to detect. If orientation, tilt, the number of panels, and connection configuration are similar, generation trends will generally be similar. If only some circuits are consistently lower, suspect wiring or connection issues. However, because shading or soiling concentrated in one area can produce the same pattern, do not conclude a wiring fault from data alone; combine data analysis with on-site inspection when making a determination.

Abnormalities in wiring and connection points, if left unaddressed, can lead not only to reduced power generation but also to equipment shutdowns and safety hazards. Even small differences in output are worth checking early if they persist and are concentrated in specific circuits. To improve low power generation, you need to inspect the entire electrical path through which current flows, not just the visible panel surfaces.

Review anomalies in conversion equipment and monitoring data

When investigating low power generation, you need to review not only the panels but also the power-conversion equipment and the condition of monitoring data. In solar power generation systems, the direct current power generated by the photovoltaic modules is converted by equipment such as power conditioners into a form suitable for use or for selling to the grid. If these conversion devices have faults or control limitations, generation can be limited even when solar irradiance is sufficient. Also, communication failures or data loss on the monitoring system side can make generation appear low in the records even though actual generation is occurring.

Abnormalities in power conversion equipment can include shutdowns, output curtailment, temperature rises, protective actions, and problems with set values. For example, if output plateaus during specific daytime hours, the equipment’s capacity, control settings, grid-side conditions, or temperature rise may be involved. When you feel the generated power is low, check not only the total generated energy but also whether the output waveform looks natural and whether there are sudden drops or unnatural flattening.

Be aware of anomalies in monitoring data. If there are periods when communication is interrupted, the power generation for those periods may not be recorded and can appear lower. Missing data, time shifts, differences in aggregation units, or malfunctions of measuring equipment can lead to incorrect assessments of power generation. If the power generation suddenly shows zero while the site was actually operating, or if the values on the monitoring screen do not match the on-site meter, issues on the monitoring side should also be suspected.

When checking power conversion equipment, it is important to review alarm logs and event histories. Temporary shutdowns or protective operations can occur and recover without the operator noticing. Even short shutdowns that do not appear in daily or monthly reports can affect power generation if they occur frequently. Organizing alarm history, shutdown times, restart times, frequency of occurrence, and whether the same equipment is repeatedly affected makes it easier to isolate the cause.

Thermal conditions also affect the performance of power conversion equipment. If the installation site has poor ventilation, is exposed to direct sunlight, allows heat to accumulate around the equipment, or if filters and ventilation openings are clogged with dirt, the equipment may limit its output to prevent a rise in temperature. In such cases, power generation can be reduced even when the panels and solar irradiance are not problematic. Checking the cleanliness around the equipment, the ventilation status, and the installation environment is important for stable operation.

Checking set values and operating conditions must not be overlooked. After equipment modifications, parts replacements, setting changes, or restoration work following inspections, settings may end up different from what was intended. If a drop in power generation began after a specific operation, review the work history. Knowing when, who, which equipment, and what work was performed makes it easier to narrow down possible causes. Low power generation problems can be caused not only by faults that occur naturally but also by inadequate post-work checks.

When using monitoring data, it is essential to verify the reliability of the data. Rather than taking the numbers displayed on the monitoring screen at face value, compare them with on-site measurements, the amount of electricity sold, equipment displays, and historical data. If multiple data sources agree and show a decline, the likelihood of an actual drop in power generation is high. On the other hand, if only the monitoring data is low, suspect measurement or communication issues. Before concluding that generation is low, it is important to confirm that the data are being collected correctly in the first place.

Record inspection results and connect them to the improvement cycle

To improve a low power output, it is important not to perform an inspection just once and stop, but to record the results and use them to inform the next improvement. The causes of a drop in power output are not necessarily singular. Dirt, shading, wiring, equipment, weather conditions, and problems with monitoring data may combine. Therefore, unless you keep records of what was inspected, the abnormalities found, the countermeasures taken, and the changes in power output after the measures, you will lack the information needed to make decisions if the same problem recurs.

The items to be recorded are not limited to the power generation figures. Record the inspection date, weather, solar irradiance conditions, inspected sections, photos taken, whether any abnormalities were found, work performed, the person in charge, and items that will need to be checked at the next inspection. In particular, site photos are useful for confirming conditions afterward. Since dirt, shadows, damage, weeds, and cable conditions are hard to convey with text alone, recording the photo location and time makes them easier to use for later comparisons.

When implementing countermeasures, comparing before and after implementation is important. After carrying out measures such as cleaning, mowing grass, removing causes of shading, inspecting around equipment, and checking connections, confirm how the power generation changed. However, because the weather varies and power generation also changes, it can be difficult to judge by simply comparing the previous day and the next day. Assess the improvement effect by comparing days with similar solar irradiance conditions or by comparing with other sections within the same power plant.

Setting thresholds is also effective for managing declines in power generation. Deciding in advance how large a decline should trigger an inspection, what difference between circuits should be considered a potential anomaly, and how many consecutive days of low output require confirmation makes it less likely to depend on the judgment of individual staff. Of course, thresholds vary with site conditions, so they need to be adjusted according to past performance and equipment characteristics. The important thing is not to leave the criteria for detecting declines ambiguous.

When inspection results are accumulated, site-specific trends become apparent. You can identify each plant’s weaknesses — for example, susceptibility to pollen and yellow sand in spring, grass shadows appearing in summer, fallen leaves accumulating in autumn, longer shadows in winter, and mud splashes remaining after rain. Based on these trends, advancing inspection timing or designating priority inspection areas makes it easier to respond before power generation drops significantly.

Records are also useful for handovers between personnel. Because the operation of power generation equipment spans a long period, personnel may change. If there is no record of what malfunctions occurred in the past and what countermeasures were taken, the new person in charge will have to repeat the same checks from scratch. If inspection records are organized, decisions can be made based on past history, reducing duplicated work and oversights.

To avoid leaving low power output unaddressed, a system that links daily monitoring with regular on-site inspections is necessary. Detect decreases in the monitoring data, confirm the causes on site, implement countermeasures, and record their effects. By repeating this cycle, responding to declines in power output becomes not an ad hoc task but an ongoing continuous improvement activity. Maintaining power output is not something that can be completed with a single inspection; accuracy improves as records and improvements accumulate.

Preventing Low Power Output from Being Left Unaddressed

To avoid leaving a reduced power output unaddressed, it is important to isolate the causes step by step as soon as you notice a decline. First, quantify the decrease in power output and compare it with past performance and generation per rated capacity. Next, check meteorological conditions such as solar irradiance, weather, temperature, and snowfall to determine whether the variation is natural or due to equipment. Then, in order, inspect the panel surface for dirt or damage, shadows at different times of day, wiring and connections, the power conditioner, and any abnormalities in monitoring data.

The causes of low power generation do not necessarily exist in isolation. For example, if dirt accumulates during a period of low solar irradiance and shading occurs during some hours, the monthly power generation can drop significantly. In that case, attributing the issue solely to the weather will cause you to overlook dirt and shading. Conversely, blaming only dirt may lead you to miss problems with equipment or monitoring data. In practice, it is important not to fix on a single cause, but to iteratively verify by going back and forth between the data and the site.

Also, when responding to declines in power generation, early detection and record-keeping are of great importance. Even small drops, if they persist over long periods, accumulate into lost generation opportunities. If there are signs such as only specific circuits showing low output, drops occurring only at certain times of day, no improvement after cleaning, or discrepancies between monitored values and on-site measurements, you should investigate rather than leave them unaddressed. Keeping records at the stage when an anomaly is suspected makes it easier to trace the cause later.

In the operation of power generation facilities, it is necessary not only to note that power output is low, but to visualize why it is low, what to check, and whether improvements had an effect. If site photos, inspection records, power output data, weather conditions, and work history can be managed together, sharing information among responsible personnel becomes easier. By making judgments about power output declines based on objective information rather than relying solely on individual experience, it also becomes easier to set inspection priorities.

There are several issues behind low power generation being left unaddressed: the causes are unknown, on-site verification is time-consuming, records are scattered, and it is difficult to confirm the effectiveness of improvements. To reduce these issues, it is important to link power generation data with on-site information and to streamline the process from inspection through improvement. Detect anomalies during daily monitoring, verify them on-site, record them, and apply the findings to the next inspection. Repeatedly practicing this basic routine leads to operations that do not overlook decreases in power generation.

Both data checks and on-site inspections are essential to stably manage the power output of photovoltaic systems. If you want to quickly identify the causes of low power output and implement improvements, it is effective to establish an operational framework that lets you record on-site conditions while monitoring changes in power output. Preparing a system that enables continuous detection of power output declines, organization of inspection records, and verification of improvement status, and accumulating trends for each installation, is a practical measure to ensure low-output conditions are not left unaddressed.