6 criteria for not rushing the decision to replace when power output is low

When low power generation persists, you naturally start to suspect that some part of the equipment is faulty and that the equipment should be replaced promptly. In particular, for the operations personnel responsible for managing a power plant, a decline in generation affects electricity sales and equipment valuation, so it is a problem that is difficult to postpone addressing.

However, the power output of a solar power generation system fluctuates due to various factors such as weather, solar irradiance, temperature, snowfall, shading, dirt, vegetation, voltage rise suppression, communication failures, measurement conditions, and system settings. If you look only at the result of low power output and immediately proceed to replace photovoltaic modules, power conversion equipment, cables, or measurement instruments, you risk overlooking causes that could have been remedied without replacement.

To avoid rushing a replacement decision, it is important to sequentially isolate the causes of reduced power generation and confirm whether the malfunction requires replacement or whether the decline is temporary due to operational, environmental, or measurement conditions. This article organizes six criteria to check before making a replacement decision for practitioners who are searching "low power generation" to investigate the causes.

Table of Contents

• An approach to avoid rushing replacement decisions when power generation is low

• Criterion 1: Check whether the decrease can be explained by solar irradiance and weather conditions

• Criterion 2: Identifying output declines caused by temperature and seasonal factors

• Criterion 3: Check external factors such as shadows, dirt, and vegetation first.

• Verify suppression due to controls on the Reference-4 system side and to voltage conditions.

• Criterion 5: Check for discrepancies among measured values, transmitted values, and comparison conditions

• Determine the necessity of replacement based on the inspection results and reproducibility of Standard 6.

• Summary: The lower the power generation, the more replacement decisions should be based on records.

A Mindset for Not Rushing Replacement Decisions When Power Generation Is Low

When you feel that power generation is low, the first thing to avoid is simply deciding “replace it because the equipment is old” or “replace it because some values are low.” Solar power generation systems can produce different amounts of power even for systems of the same capacity, depending on installation azimuth, tilt angle, surrounding environment, installation year, and operating conditions. Also, even with the same equipment, power output varies on sunny days, cloudy days, rainy days, after snowfall, during periods with a lot of yellow dust or pollen, and in high-temperature periods.

There are certainly cases where replacement is necessary. For example, if there is obvious physical damage, burn damage, insulation failure, terminal damage, internal abnormalities, repeated shutdowns, or significant deterioration identified during inspection, replacement or repair should be considered. However, a decline in power generation alone does not allow you to determine which component is causing the issue, whether replacement will lead to improvement, or whether non-replacement measures will be sufficient.

In practice, you must first clarify the basis for judging that it is "low." Whether it is lower than the same month of the previous year, lower than the power generation forecast, lower than neighboring facilities, or lower than another system within the same power plant will change what points you need to check. If the comparison target is left vague, you may mistakenly regard the issue as equipment failure when in fact the solar irradiance conditions were simply poor.

Also, when deciding whether to replace equipment, it is important to distinguish between a "temporary drop" and a "persistent drop." If power generation is low for only one day, possible causes include weather, a temporary shadow, output curtailment, or communication loss. On the other hand, if on sunny days the same system is consistently low, similar drops occur at the same times of day, there is no improvement after cleaning or mowing, or abnormal readings are reproduced during inspections, there is increased reason to suspect a fault on the equipment side.

Not rushing a replacement decision does not mean leaving a fault unaddressed. Rather, by sorting out the causes before deciding, it is a practical approach to ensure necessary replacements are not missed and unnecessary replacements are avoided. The lower the power generation, the more important it is to cross-check numerical data, on-site conditions, inspection records, and weather conditions, and to gather the information needed to make a decision before performing a replacement.

Criterion 1: Confirm whether the decrease can be explained by solar irradiance and weather conditions

When power generation is low, the first things to check are whether the drop can be explained by solar irradiance and weather conditions. Because solar power generation is heavily influenced by solar irradiance, you cannot determine the condition of the equipment by looking at generation alone. Overcast skies, rain, fog, thin clouds, snow cover, and the surrounding atmospheric conditions can all cause large changes in power output. Even on days that appear bright, if there are many clouds and solar irradiance is unstable, generation may not reach expected levels.

In practice, it is standard to view changes in power output together with solar irradiance. If power output is low on days when solar irradiance is low, the cause may be weather rather than an equipment malfunction. Conversely, if solar irradiance is sufficient but power output is low only during particular time periods or only on specific systems, you should suspect problems with the equipment or the surrounding environment.

What you need to be careful about here is the period used for comparing power generation. Even if generation is lower year-over-year for the same month, if the weather that year was poor that can explain the difference as natural. Looking only at monthly generation can make it seem like a large drop, but when corrected for solar irradiance the trend in the system’s output may not have changed much. Conversely, even if there is little difference for the whole month, if only the peak output on sunny days has fallen, there may be problems hidden in the equipment or its surrounding environment.

To avoid rushing a replacement decision, first check whether the decline follows solar irradiance conditions. Examine not only days with low generation but also sunny, cloudy, and rainy days separately. If the power generation curve on sunny days is smooth and there is no significant difference from systems of similar scale, the evidence directly supporting equipment replacement is weak. On the other hand, if there is a sudden drop during periods when irradiance is stable, a particular system shows lower output compared to other systems, or generation falls at the same time every time, you need to proceed to the next checks.

When investigating the cause of low power output, you should avoid both dismissing it as "it was unavoidable because the weather was bad" and assuming "the equipment is faulty." What matters is whether the relationship between power output and solar irradiance is natural. By determining whether output rises and falls in line with irradiance, or whether there are discrepancies that irradiance cannot explain, you can reduce unnecessary speculation before deciding on replacement.

Criterion 2: Distinguishing output reductions caused by ambient temperature and seasonal factors

A commonly overlooked cause of low power generation is output reduction due to ambient temperature and seasonal factors. Photovoltaic modules do not always produce their maximum power simply because solar irradiance is strong. In general, module output tends to decrease as module temperature rises. Therefore, even on clear summer days, when ambient temperatures are high and conditions are affected by heat from roof surfaces or the ground, power generation can fall short of expectations.

In summer, if you feel that "generation is low despite sunny weather," you should consider output reduction due to high temperatures rather than immediately suspecting equipment replacement. In particular, even when the peak around midday, when solar radiation is strongest, is lower than expected, it may not be abnormal given the temperature conditions. Conversely, if a similar drop is observed on sunny days during periods when the ambient temperature is not that high, you need to check for causes other than temperature.

Seasonal factors are not limited to summer. In winter, the angle of incoming solar radiation is lower and daylight hours are shorter. Shadows in the mornings and evenings tend to lengthen, and shadows cast by nearby mountains, buildings, trees, and rows of mounting racks are more likely to affect power generation. In snowy regions, residual snow in some areas can cause differences in power output between systems. Even in spring and autumn, pollen, yellow sand, fallen leaves, bird droppings, and dust from farmland or development sites can adhere to the surface of the panels and affect power generation.

Before proceeding with a replacement decision, confirm whether the deterioration matches natural seasonal patterns. For example, if the same degree of deterioration occurs at the same time each year and can be explained by local environmental factors, cleaning, weeding, checking for shading, and organizing operational records should be prioritized over replacement. On the other hand, if deterioration begins suddenly during a period that was problem-free in previous years, or if only certain sections within the same power plant are showing decline, it may not be attributable to merely seasonal factors.

When examining temperature and seasonal factors, it is useful not only to look at the absolute value of power generation but also to check the shape of the generation curve. Output reductions caused by high temperatures can take the form of somewhat suppressed peaks even during periods of strong solar irradiance. Shadow effects tend to produce specific dips in the morning or evening. Snow cover or soiling can appear as differences between systems and delayed recovery. In this way, by interpreting how the decline manifests, it becomes easier to determine whether it is a fault that requires replacement or variation due to environmental conditions.

When you see a result indicating low power generation, ignoring natural seasonal differences can lead to unnecessary replacement decisions. Conversely, assuming the issue is seasonal can cause you to overlook an actual fault. What is important is to compare past data from the same season, other systems within the same plant, and generation curves from days with similar conditions, to determine whether the discrepancy can be explained by temperature or season.

Criterion 3: Check external factors such as shadows, dirt, and vegetation first

As an important criterion to avoid rushing a replacement decision, first check external factors such as shadows, dirt, vegetation, fallen leaves, bird damage, and surrounding structures. In solar power installations, even if the equipment itself is not faulty, power output can decrease simply because shadows fall on the power-generating surface, dirt accumulates, or weeds grow. These issues may be improved not by replacement but by cleaning, weeding, checking for obstructions, and managing the surrounding area.

The impact of shading requires particular attention as a cause of reduced power generation. Buildings, utility poles, trees, fences, slopes, adjacent equipment, and row shadows from racking can fall on the generation surface depending on the time of day and season. Even locations that had no problems at installation can experience increased shading effects after several years due to tree growth and changes in the surrounding environment. If power generation is low only in the mornings and evenings, or if declines are especially noticeable in winter, checking for shading is essential.

Dirt is also a factor that cannot be overlooked. Sand dust, pollen, yellow sand, fallen leaves, bird droppings, puddle marks, mud splashes, and the like, if left on the generating surface, can affect power generation. The impact of soiling is not necessarily uniform across the entire power plant. Due to wind direction, slope, surrounding terrain, the position of trees, and places where birds congregate, only some sections may show noticeable soiling. Therefore, it is important to correlate systems with low power output with the on-site level of soiling and verify them.

Weeds and vegetation can also cause low power generation. On ground-mounted systems, grass can grow and cast shadows on the front of the modules. Because grass height can increase rapidly during the summer, a system that showed no problems at the previous inspection may be affecting power output by the next inspection. Checking whether power generation improves after weeding makes it easier to determine whether the issue can be addressed by environmental maintenance rather than equipment replacement.

On-site photos and inspection records are useful when checking external factors. Inspecting the site at the same times of day when power generation is low makes it easier to identify the position of shadows and how vegetation affects the panels. A single check during daytime can miss morning and evening shadows. If the generation curve shows a drop at specific times, it is necessary to check what kinds of shadows or environmental changes are occurring at those times.

The reason to check external factors before rushing to decide on replacement is that these causes may not be resolved by replacing equipment. If shading is the cause, replacing the modules will not improve power generation if the same shading occurs. If dirt is the cause, cleaning and managing the surrounding environment should come first. If weeds are the cause, you should consider weeding and weed-prevention measures. When power output is low, it is especially necessary to check the overall plant environment rather than focus only on the equipment.

Criterion 4: Verify suppression by grid-side control and voltage conditions

The cause of low power generation is not limited to faults in the generation equipment. Output can be curtailed by grid-side conditions or voltage conditions. In particular, if generation does not increase as much as expected on sunny days, if output plateaus only during certain daytime hours, or if generation appears low even though the equipment has not stopped, it is necessary to check the control settings and voltage conditions.

Solar photovoltaic systems are affected by voltage conditions and protection settings when exporting generated power to the grid. During times when local generation is high or demand is low, grid voltage can rise. As a result, the system may act to limit its output. In such cases, the equipment is not necessarily faulty; it may be operating in accordance with grid interconnection conditions and settings.

To avoid rushing a replacement decision, check whether the decline in generation is concentrated in specific time periods. If, on sunny days around noon, output fails to rise beyond a certain level, the top of the curve becomes flat, or the pattern repeats not only on particular days but on days with the same conditions, suspect curtailment or configuration settings. Conversely, if, despite solar irradiance, output drops sharply and irregularly, only certain strings approach a near-shutdown state, or there are abnormal indications or records of protective actions, inspection of the equipment is necessary.

Also, when checking voltage conditions, it is important to know where within the power plant the values are being observed. The situation you see can change depending on the measurement location — near the point of receipt, near the distribution board, or near the power conversion equipment, for example. From the result that generated power is low alone, you cannot tell whether it is a grid-side condition or a problem inside the equipment. By checking operation records, voltage records, shutdown history, alarm history, and generation curves together, you can narrow down the likely cause before replacing equipment.

When grid-side control or voltage conditions are involved, simply replacing equipment may not eliminate the root cause. Because output can still be suppressed under the same conditions after replacement, it is important to first organize records and consider verifying settings and grid interconnection conditions, consulting relevant parties, and conducting on-site measurements. As an operations professional, you should not only check whether the equipment is faulty but also confirm under what conditions and how the equipment is being controlled.

Rushing to decide on replacement when generation is low can lead to mistaking curtailment or voltage condition issues for equipment failures. Examining the shape of the generation curve, the time of day, the weather, whether alarms are present, and the voltage trend makes it easier to distinguish cases that require operational checks or condition adjustments rather than replacement. In particular, plateauing during sunny conditions and declines that show a regular time-of-day pattern are points to check before replacing equipment.

Criterion 5: Verify discrepancies in measured values, transmitted values, and comparison conditions

Before concluding that power generation is low, it is also important to verify that the measured values and communication data themselves are being correctly obtained. There are cases where the system is actually generating power but the monitoring screen or summary tables show low values. Communication dropouts, measurement device malfunctions, mismatches in aggregation periods, unit mix-ups, errors in registered equipment capacity, or incorrect time settings can all make generated power appear low.

For example, even if daily or monthly reports show low power generation, on-site cumulative totals or other records may indicate there is no major problem. In such cases, the items to check are not the generating equipment but measurement or communication. If there are periods when communication is interrupted, data for those periods can be missing, causing daily or monthly generation totals to be reported as low. When assessing a drop in generation, it is important to cross-check not only the monitoring data but also the on-site records and cumulative totals.

Be careful about discrepancies in comparison conditions. When comparing with the same month of the previous year, the number of days covered, weather, equipment downtime days, inspection days, whether curtailment occurred, and conditions after snow or typhoons may not be the same. If part of the equipment was out of service during the month, the measurement start date was shifted, or the hours subject to output control increased, a simple comparison will make the generated output look lower. If you do not align the assumptions for comparison, you may mistakenly conclude that a fault requires replacement.

Even when comparing systems within the same power plant, differences in capacity, orientation, tilt, number of connected modules, shading, and installation location can lead to differences in power generation. Rather than looking at simple generation totals, you need to compare generation per unit of capacity and compare during the same time periods under the same conditions. A particular system may appear to be underperforming, but in reality natural differences can arise because the installation conditions differ. Conversely, if conditions are almost identical yet a persistent difference remains, it should be prioritized for inspection.

When checking measured values, pay attention to time misalignments. If the timestamps of power generation data, solar irradiance data, temperature data, site photos, and inspection records are out of sync, you may think you are comparing the same time period, but in reality you are comparing different conditions. In particular, on days with large irradiance fluctuations or fast-moving clouds, even differences of a few minutes to around ten minutes can change how the power output appears.

To avoid rushing the decision to replace equipment, verify that the assessment that the power generation is low is correct. By checking whether any measurement values are missing, communications are functioning properly, the aggregation period matches, the comparison conditions are aligned, and there are no errors in the registered equipment capacity, you can exclude declines that are not due to actual equipment faults. If you skip this and proceed with replacing the equipment, you may end up in a situation where the displayed power generation does not improve even after replacement.

Criterion 6 Determine the need for replacement based on inspection results and reproducibility

The final criteria for deciding whether to proceed with a replacement are the inspection findings and reproducibility. If a decline that is difficult to explain continues even after checking weather, season, shading, soiling, curtailment, and measurement issues, you should suspect a fault on the equipment side. However, even in that case, do not decide on replacement based solely on low power generation; instead, look at the facts confirmed during inspection and whether the decline is repeatedly reproducible.

Reproducibility refers to whether the same anomaly repeatedly occurs under similar conditions. If only the same system shows low output during the same time on sunny days, if the gap does not narrow even on days with sufficient solar irradiance, if there is no improvement after cleaning or weeding, if on-site measurements are low despite no problems in communication or aggregation, or if inspections confirm abnormal readings, then there is more justification for considering replacement or repair.

During inspections, check the appearance, connection points, cables, terminals, insulation condition, any overheating, abnormal indicators or displays, shutdown history, protection operation, and differences in output between systems. If visible damage, discoloration, scorch marks, loose terminals, damage to cable sheathing, or conditions suggesting water ingress are found, carefully assess their relationship to reduced power generation. However, since some faults cannot be identified by appearance alone, it is important to perform appropriate measurements and professional inspections as needed.

It is also important to consider whether replacement is necessary at the component level. Low power generation does not necessarily mean that the entire installation needs to be broadly replaced. The scope of response varies depending on whether the cause is in the connection points, the cables, some of the power-generating surfaces, the power conversion equipment, or the measurement side. Expanding the replacement scope without narrowing down the source can increase unnecessary work and may leave the root cause unresolved.

When making replacement decisions, also confirm the expected improvement. It is important to be able to explain which declines and to what extent they are expected to improve if a particular piece of equipment is replaced. For example, if the output decline in a specific system matches the inspection results and the abnormality in that part can reasonably explain the reduction in power generation, the decision to replace becomes easier. Conversely, if the relationship between the power-generation decline and the replacement target is unclear, it is safer to carry out additional inspections and organize the records first.

It is also important to keep records both before and after the replacement. If you retain the pre-replacement power generation, solar irradiance, weather, inspection results, history of abnormalities, and on-site photographs, you can objectively verify whether there was an improvement after the replacement. If no improvement is seen despite the replacement, you can consider the possibility that the cause lay elsewhere. Conversely, if power generation recovers under the same conditions after the replacement, it becomes easier to justify the decision.

There are situations where replacement becomes necessary when power generation is low, but that decision should be based not on "replace because it's low" but on "replace because other factors have been excluded, inspection results and reproducibility are consistent, and you can explain the expected improvement from the replacement." By following this order, you can avoid unnecessary replacements while making it less likely that necessary measures will be delayed.

Summary: Decide on replacement based on records when power generation is low

To avoid rushing a replacement decision when power generation is low, it is important to isolate causes in sequence. First, check whether the decline can be explained by solar irradiance or weather conditions. Next, look for natural output decreases due to temperature and seasonal factors. Then verify external factors such as shading, dirt, vegetation, fallen leaves, and bird damage. Furthermore, check grid-side controls and voltage conditions, measurement and communication data, and discrepancies in comparison conditions; if the decline still cannot be explained, determine the need for replacement based on inspection results and reproducibility.

A hasty decision to replace equipment can lead to replacing it even when the root cause lies in external environmental factors or measurement conditions. Likewise, overlooking grid-side curtailment or seasonal factors can mean power generation does not recover as much as expected after replacement. Conversely, postponing verification for too long risks delaying responses to faults that genuinely require replacement or repair. What matters is not avoiding replacement, but making replacement decisions grounded in solid evidence.

Field personnel should organize power generation, irradiance, temperature, generation curves, site photographs, inspection results, and anomaly history in the same sequence. Rather than only noting that power generation is low, recording when, under what conditions, in which system, and what kind of decline occurred makes it easier to explain whether replacement is necessary. In particular, comparing sunny days, comparing systems within the same plant, comparing before-and-after cleaning or weeding, and cross-checking with alarm histories helps improve the accuracy of decisions.

When addressing reduced power generation, it is important not to separate on-site inspection and data analysis. Even if shadows or soiling are visible at the site, if you cannot determine how much they affect the data, it becomes difficult to make a decision about replacement. Conversely, even if a drop is visible in the data, failing to check the on-site condition may lead you to misidentify the cause. By combining visible on-site information with less obvious changes in the generation data, the points that should be checked before replacement become clear.

To avoid leaving low power generation unaddressed, while also not rushing a replacement decision, a system for daily recording and comparison is indispensable. By understanding the generation trends for each plant and ensuring that, in the event of an anomaly, past data and on-site conditions can be immediately cross-checked, the initial response can be faster. It becomes easier to determine whether a replacement is necessary, whether cleaning or weeding will suffice, whether additional inspections are needed, or whether curtailment or measurement conditions should be checked.

When you are unsure about whether to replace components because of low power generation, the starting point is to first align the records and eliminate potential causes one by one. If you can link changes in power generation to on-site conditions, you can avoid unnecessary replacements and more easily move forward with the measures that are actually needed. Organizing the decline in power generation of a solar power generation system together with on-site data and improving the accuracy of inspections and decisions leads to evidence-based replacement decisions.