6 checks to make before suspecting installation defects when power output is low

When you feel that a solar power system's output is low, it is common on site to immediately suspect installation defects. Especially immediately after commissioning, after inspections, after renovations, or after taking over used equipment, wiring mistakes, poor connections, and equipment malfunctions tend to come to mind. However, low generation is not limited to installation issues. There are many factors to check, such as weather, solar irradiance, the comparison method, monitor display, power conditioner control, shading, soiling, and grid-side constraints.

If you rush to conclude there was an installation defect, you may move on to contractor checks or re-inspections while the cause is still unclear, which can prolong on-site responses. Conversely, if you first align and confirm the basic conditions, even when an installation defect is a possibility you will have a clear idea of which areas to focus on. In this article, for practitioners searching "low power generation", we organize six items to check before suspecting an installation defect, in an order that is easy to use on site.

Table of Contents

• Basic approach before suspecting installation defects when power output is low

• Whether the weather and solar irradiance conditions match those of the comparison reference

• Whether the comparison period and displayed units for power output are consistent

• Check power output differences for each power conditioner

• Inspect external factors such as shading, soiling, snow, and vegetation

• Check for alerts, shutdown history, and the presence of output control

• Review equipment capacity, circuit configuration, and the assumptions behind expected values

• Procedure for determining the likelihood of installation defects

• Summary

Basic approach before suspecting installation defects when power output is low

When you feel that power output is low, it's important not to assume a single cause from the outset. The power output of a solar power system varies due to multiple factors such as solar irradiance, temperature, the condition of the panel surface, shading, the operating condition of equipment, grid conditions, and the status of monitoring data acquisition. Therefore, it's risky to conclude that there is an "installation defect" based solely on the power output of a single day.

In practice, you first need to confirm whether it is really low. Whether it is lower than the previous day, lower than last month, lower than the expected simulation, lower than neighboring facilities, or lower only in some circuits within the same installation, the points to check vary depending on the criteria used to judge “low.” If a month has continued cloudy or rainy weather, it is natural for the monthly power generation to be below expectations. Even in summer, because the output of photovoltaic modules is affected by high temperatures, a day with strong solar irradiance does not necessarily result in maximum power generation.

Even when there are construction defects, the ways they manifest as reduced power generation are not uniform. Sometimes the output is low overall; sometimes only specific power conditioners are underperforming; sometimes the drop occurs only during certain time periods; sometimes the difference becomes larger only on sunny days; and sometimes abnormalities are more apparent after rain or in the morning and evening. In other words, when confirming low generation, it is important to look not only at total generation but also from multiple angles such as by time of day, by equipment, by circuit, by day, and by month.

The checks you do before suspecting a construction defect are not meant to delay contacting the installation or maintenance company. Rather, they are preparation to improve the quality of your inquiry. If you can organize which day, which display, which equipment, and by how much it was lower, the inspectors can more easily narrow down the cause. Instead of a subjective “the power generation is low,” aim to be able to show—after aligning conditions—that “there is a difference in this range.”

Are the weather and solar radiation conditions consistent with those of the comparison target?

When you feel the power generation is low, the first thing to check is the weather and solar irradiance conditions. Solar power generation is greatly affected by solar radiation, so output can vary significantly between sunny, thinly cloudy, cloudy, and rainy conditions. Even on a day that looks bright, if thin clouds spread overhead the direct solar radiation is weakened and generation may not increase much. Conversely, when strong sunlight temporarily passes through breaks in the clouds, generation can rise for a short time.

It's common in practice to conclude that power generation is lower when comparing with the previous month or the same month of the previous year. However, if the weather during the periods being compared is not the same, this becomes a weak basis for judging installation defects. Even if generation is lower than in the same month last year, if this year's month had more rain or clouds, the primary cause of the decline may be the weather. When looking at daily generation, you should also check the weather and solar irradiance conditions for that day rather than simply comparing dates.

Also, local weather conditions at power generation sites can have an impact. Along the coast, deposits containing salt and humidity; in mountainous areas, fog and snowfall; and in inland areas, high temperatures and dust can affect power output. Conditions such as fog appearing only in the morning, clouds tending to form only in the afternoon, or the low solar altitude in winter causing longer shadows can also lead to reduced power generation. If these conditions are ignored and only installation defects are suspected, the assessment of the cause can be incorrect.

When checking, it is easier to make a judgment if you view the power generation data alongside the weather notes. If the monitoring screen displays daily generation, check whether days with reduced generation coincide with cloudy or rainy weather. If the low-generation days are concentrated on bad-weather days, the influence of weather conditions may be greater than that of installation defects. On the other hand, if generation clearly fails to improve despite continued sunny weather, or is lower than other systems or other circuits under the same sunny conditions, you should proceed to the next verification step.

The influence of ambient temperature should not be overlooked. Solar photovoltaic modules generally tend to experience reduced output as temperature rises. Therefore, on a clear midsummer day you may not see as much energy generation as expected. This is not necessarily an installation defect but a variation inherent to the equipment. In particular, even if solar irradiance is strong around noon, high panel temperatures can make the output appear to plateau. If you make simple comparisons without taking season and temperature into account, you may mistake normal variations for anomalies.

Checking weather and solar irradiance conditions is not intended to rule out the possibility of installation defects, but to establish a common basis for judgment. If there is a decrease that cannot be explained by weather conditions, it becomes more valuable in subsequent checks to look for differences by device, by circuit, and by time of day. The first step is to distinguish whether the low performance is due to bad weather or whether it is lower than can be explained by weather alone.

Are the comparison periods and display units for power generation consistent?

When judging that power generation is low, it is essential to check the comparison period and the displayed units. On monitoring screens and summary tables, numbers that look similar but have different meanings—instantaneous power, daily generation, monthly generation, cumulative generation, electricity sold, and consumption—may be listed. Confusing these can make generation appear low even when there is actually no problem.

First, note the difference between instantaneous power and energy generation. Instantaneous power is the value that indicates how much power is being generated at that moment. On the other hand, energy generation indicates the amount of electrical energy generated over a certain period. Even if you check the monitoring screen in the morning and feel the instantaneous power is low, if the weather improves in the afternoon the daily generation may increase to some extent. Conversely, even if a momentary high value appears during the day, long periods of morning/evening or cloudy weather will result in low daily generation. It is premature to suspect installation defects based solely on the instantaneous display.

Next, confirm that the aggregation periods for daily, monthly, and yearly are correctly aligned. If the monitoring system's displayed period starts partway through, is set to an arbitrary period rather than the beginning of the month, or a time setting offset shifts the date boundaries, the generated power may appear lower. In particular, immediately after equipment starts operating, after replacing communication devices, or after changing monitoring settings, you need to check the aggregation start date and whether any data gaps exist.

Be aware of the difference between sold electricity and generated electricity. In systems that include self-consumption, part of the generated power is used within the facility, so looking only at the amount sold can make the generation appear low. If you do not separate whether you want to check generated electricity or sold electricity, you may misidentify the cause. When comparing the utility’s metered values or billing figures with the system’s generation monitoring data, it is important to confirm that the measurement points and periods match.

Differences in display units also quietly matter in practice. Whether you are looking at the generation of the entire facility, the generation per power conditioner, or the generation per unit of installed capacity changes the meaning of the numbers. If you compare power plants with different installed capacities using only total generation, it is natural that the larger-capacity facilities will appear to generate more. When making comparisons, you need to put them on the same footing—such as generation per unit of capacity or comparisons within facilities under the same conditions.

Also, monitoring data can include communication delays or missing data. Even if the site is generating power, communication faults can prevent some data from being acquired, causing the monitored output to appear low on the monitoring screen. If data is missing for only certain days, the time-of-day graphs are interrupted, or there are blanks in per-equipment displays, suspect that the issue may be on the data-acquisition side rather than an actual drop in generation. Before proceeding to judge a faulty installation, confirm that the monitoring data itself is in a reliable state.

When comparing power generation, it's essential to look at the same period, the same units, the same measurement points, and the same scope of equipment. If you judge the output as "low" without those conditions being met, suspicions of construction defects will be exaggerated. First, clarify what the displays mean and make clear which figures you are using as the basis for judging them to be "low."

Check power generation differences for each power conditioner

When you feel that the overall power generation is low, it is important to check not only the total for the entire system but also the generation differences for each power conditioner. In solar power generation systems, multiple power conditioners are often installed, and each reflects the generation status of the circuits or groups of panels it is responsible for. Therefore, abnormalities that are not visible from the system-wide generation total may become apparent when comparing equipment individually.

If all the power conditioners are similarly low, weather, solar irradiance, output control, grid conditions, or factors common to the entire system may be involved. Conversely, if only certain power conditioners are clearly low, one is more likely to suspect local factors such as the device itself, the circuit to which it is connected, nearby shading, wiring, switches, communications, or module strings. When considering installation defects, it is important to first determine whether the issue is system-wide or localized.

When checking, you need to compare equipment with the same capacity and under the same conditions. If the connected capacity, installation orientation, tilt, or shading conditions differ between power conditioners, differences in power generation are natural. For example, a plot that faces more toward the east and a plot that faces more toward the west will have different peak generation times. It is not appropriate to simply compare a south-facing plot with a plot that is prone to shading and conclude that the lower output indicates poor workmanship.

Also check for differences by time of day. The possible causes change depending on whether a particular power conditioner is low all day, low only in the morning, low only in the afternoon, or only fails to reach its peak during clear-sky peaks. If it is low only in the morning, consider shading on the east side; if it is low only in the afternoon, consider shading on the west side. If it levels off only at midday, output limitations, device control, or temperature conditions may be involved. If it is extremely low throughout the day, also check for circuit-level shutdowns, poor connections, or equipment stoppages.

Another important point is to check whether the values shown on the power conditioner and the monitoring screen match. If the on-site equipment indicates generation while the remote monitoring screen shows a lower value, communication or data aggregation issues may be the cause. Conversely, if both the monitoring screen and the equipment display similarly low values, it is more likely that there is a problem with the actual generation. Do not judge solely on remote data; it is important to cross-check with on-site displays and recorded values as needed.

When checking power generation differences, it's easier to judge if you look at several days rather than just a single day's results. If a difference appears on only one day, it may be due to temporary effects such as shadows, weather, or missing data. Select several sunny days and compare the same devices during the same time periods to determine whether the difference persists. If only a particular device consistently shows lower output, that becomes grounds for raising its inspection priority.

Comparing individual power conditioners is effective as a diagnostic step before suspecting installation defects. If you can determine whether levels are low across the entire installation or only in certain parts, you can narrow the scope of the investigation. Even when an installation defect remains possible, clarifying which equipment or connected range is showing low values improves the accuracy of on-site verification.

Inspect external factors such as shadows, dirt, snow, and vegetation

A commonly overlooked cause of low power output on site is external factors. Because solar power systems are installed outdoors, even if there were no problems at the time of construction, changes in the surrounding environment during operation can reduce generation. Shadows, dirt or soiling, snow accumulation, fallen leaves, bird damage, vegetation growth, and the addition of nearby structures are items to check as potential causes of reduced output.

The impact of shading varies by time of day and season. In winter, because the sun's altitude is lower, shadows that were not a problem in summer may fall on the panels. In the morning and evening shadows stretch longer, so the shadows of fences, utility poles, mounting structures, trees, buildings, and surrounding equipment may fall on some panels. If the periods of low power generation coincide with the periods when shadows occur, it may be the effect of external shading rather than an installation defect.

However, shading is not simply a matter of "slightly darker." When part of a photovoltaic module is shaded, depending on the wiring configuration, the impact on output can extend beyond the shaded area. In particular, if only part of the same circuit is shaded, the power generation graph can show an unnatural drop. Because these phenomena can be difficult to distinguish from installation defects, it is important to compare time-of-day drops in generation with the location of shadows on site.

Dirt can also cause a decrease in power generation. When sand and dust, pollen, yellow sand, bird droppings, fallen leaves, splashed mud, or deposits containing sea salt remain on the surface of panels, they hinder the panels' ability to receive light. If the entire surface is thinly soiled, this tends to appear as an overall reduction in power generation, while heavy soiling in some areas tends to cause localized reductions. Some dirt is washed away naturally by rain, but on installations with a shallow tilt or in places that receive uneven rainfall, dirt can be more likely to remain.

Snow and frost can also be important depending on the season. When snow is present, it can cover the panel surface and greatly reduce power generation. If snow remains only in part, there may be generation differences between circuits even within the same installation. Morning frost or condensation can also temporarily delay the start-up of generation. Before judging a winter drop in output as poor workmanship, it is necessary to check the snow cover, snowmelt conditions, and the condition of the panel surfaces.

Vegetation growth cannot be ignored in long-term operation. Grass that was short at the time of installation can grow and cast shadows; surrounding trees may grow and expand shading seasonally; plantings outside fences may grow beyond the managed area. Especially for ground-mounted installations, power generation can vary depending on mowing frequency and the scope of weed control. Whether output falls suddenly or declines gradually year by year, check for changes in the surrounding environment.

When checking external factors, not only monitoring data but also site photos and inspection records are useful. If you have photos taken at the same time of day, you can later check shadows and soiling conditions. By recording site conditions on days when power generation was low, it becomes easier to rule out external factors when considering the possibility of construction defects at a later date. Before suspecting problems with construction quality, it is important to carefully check whether the site environment is impeding power generation.

Confirm presence of alerts, stop history, and output control

When power generation is low, check whether the power conditioner or monitoring device has any alerts or shutdown history. A drop in generation can occur not only when equipment has completely failed and stopped, but also because of temporary stoppages, restarts, control issues, or communication faults. Even if the site appears to be generating, if it is stopped for certain periods of time the daily generation will decrease.

First, check for any error or warning messages. The items displayed—overvoltage, insulation-related faults, grid abnormalities, temperature rises, communication faults, etc.—vary depending on the equipment or device specifications, but if any alerts are shown, they should be recorded. Even if no alerts are currently displayed, they may remain in the historical logs. Check whether days or time periods with low power generation coincide with shutdown logs or warning histories.

Next, check whether output control or curtailment is in effect. Due to grid-side conditions or equipment-side controls, output can be curtailed even when there is available solar irradiance. In such cases, generation will be low even if there is no problem with the panels or the installation. If, on a sunny day, the generation graph plateaus at a fixed value, if multiple power conditioners are being curtailed in a similar way, or if output drops only during specific time periods, the possibility of control should also be investigated.

Temperature-related protective operation is another point to check. In environments where equipment becomes hot, it may reduce output to protect itself. If conditions such as poor ventilation, frequent exposure to direct sunlight, heat buildup around the equipment, or poor cleanliness around the device are present, power generation can be suppressed during the summer. This may be related to installation conditions, but first verify the equipment’s installation environment and operating history and determine what behavior occurs at which times of day.

Communication failures are also a factor that can easily be mistaken for a drop in power generation. If the monitoring screen’s data is cut off partway, if only certain devices’ displays are not updating, or if there are blanks in the daily data, the issue may be with recording or display rather than actual generation. In some cases data will be filled in after communication is restored, while in others it will remain missing. Before suspecting installation defects, check whether the figures that appear low are the actual generation or just how they appear in the monitoring data.

When checking outage history, it's important to look not only at days with low power generation but also at the days before and after. Histories such as being down since the previous day, delayed recovery in the morning, early shutdown in the evening, or multiple outages lasting from a few minutes to several tens of minutes all affect daily power generation. Even short interruptions can have a large impact on output if they occur during periods of strong solar irradiance. Comparing the timestamps in the history with the power generation graph makes it easier to identify the reasons for the decline.

Alerts and shutdown histories do not directly determine whether installation defects are present. However, they are important information for identifying the likely cause. If the logs show clear anomalies, they help narrow down the areas to prioritize during on-site inspection. When there is no history and a decline cannot be explained by weather or external factors, that situation serves as a basis for deciding to proceed with more detailed checks of circuits, connection status, and equipment performance.

Reassess equipment capacity, circuit configuration, and the assumptions behind expectations

Before suspecting construction defects, it is also important to review whether the equipment capacity, circuit configuration, and the assumptions behind the expected values are correct. When generation seems low, the underlying cause can be a mismatch between actual equipment conditions and the assumed conditions. If you make a judgment without confirming that the design drawings, as‑built documents, monitoring system registration information, on‑site connection status, and the assumptions used in the generation simulation are consistent, you may mistakenly interpret a non‑abnormal discrepancy as a construction defect.

First, let’s clarify how to interpret system capacity. The total capacity of the photovoltaic modules and the capacity of the power conditioner are not necessarily the same. Also, whether the displayed capacity refers to the DC side or the AC side changes how you should view the expected power generation. Instantaneous power does not always reach the same value as the system capacity; it varies depending on solar irradiance, temperature, azimuth, tilt, and control conditions. If you look only at the capacity number and assume it will always generate close to that value, you will overestimate the expected generation.

Checking the circuit configuration is also important. Verify which panel rows are connected to which power conditioners, which sections are treated as the same circuit, and whether the device numbers on the monitoring screen match the on-site device numbers. If display names and on-site numbers are misaligned, the equipment you think is underperforming may actually correspond to a different on-site area. If this mapping is off when issuing inspection instructions or contacting contractors, you may end up checking the wrong location.

Also review the assumptions behind the power generation simulation and the estimated generation. Estimated values are created based on solar irradiation data, orientation, tilt, loss rates, shading conditions, equipment conditions, and so on. If the actual installation conditions or the surrounding environment have changed, deviations from the estimated values can occur. Also, if you mix up annual estimates with monthly actuals, or monthly estimates with daily actuals when comparing, the results may appear lower. Estimated values serve as a guideline for judgment, but when comparing them with actual performance you need to confirm the underlying assumptions.

In projects involving used equipment or handovers, the documentation and the site may not fully match. If equipment replacements, circuit changes, panel replacements, or changes to monitoring-device settings were made in the past, the documents on hand may not accurately reflect the current state. Before concluding that power generation is low, it is important to confirm whether the current equipment configuration matches the documentation and whether the capacity registered on the monitoring screen corresponds to the actual installation.

Setting expectations is also important. Power generation changes with the seasons, with the region, and with equipment conditions. Even with the same installed capacity, if orientation, tilt, shading, temperature, snow, or soiling conditions differ, the output will not be the same. Even if output is lower compared with nearby power plants, you cannot make a simple comparison if equipment specifications or installation conditions differ. If you are going to compare, it is more practical to prioritize comparisons between devices under the same conditions within the same installation, or comparisons with past days that had the same conditions.

By reviewing equipment capacity, circuit configuration, and the assumptions behind expected values, you can prevent misjudgments caused by fundamentally different comparison conditions. Even when construction defects may be possible, clarifying the assumptions makes it clear which drawings, which equipment, which circuits, and which figures should be used as the basis for verification. To streamline on-site responses, confirm the consistency of equipment information before evaluating low power output.

How to proceed when assessing the likelihood of construction defects

If, after checking the six items above, the low power output still cannot be explained, it becomes time to consider a technical inspection that includes possible installation defects. However, even in that case, it is important not to decide subjectively that "the installation is poor" based on a feeling, but to proceed after organizing the inspection results. To determine the likelihood of installation defects, it is necessary to clarify over what range, under what conditions, to what extent, and for how long the decline in power output has persisted.

For example, if a particular power conditioner shows lower output even on sunny days, if there is a persistent difference compared with equipment of the same connected capacity, if output does not increase during times that cannot be explained by shading or soiling, or if there are no alerts but power generation is unusually low on a per-circuit basis, a more detailed inspection is necessary. Items to check include connection status, switches/circuit breakers, terminal sections, wiring routes, circuit polarity, insulation condition, the condition of junction boxes or combiner boxes, and the input status of the power conditioner.

However, internal inspection of electrical equipment requires safety management and specialized knowledge. It is important to distinguish between what on-site personnel can check and what should be requested of qualified personnel or specialist contractors. Forcibly opening panels, approaching live parts, or attempting to make judgments using measuring instruments can create a risk of electric shock or equipment damage. Initial on-site checks should be limited to appearance, labels and indicators, history, photographic records, the surrounding environment, and organizing monitoring data; detailed electrical verification should be carried out under appropriate arrangements.

When sharing a suspected construction defect, it is easier to convey the issue if you summarize the confirmed facts in chronological order. Organize information such as when the values started to be low, which days' data you looked at, what the weather was like, which equipment is showing low readings, whether shadows or dirt were observed, whether there are any alerts or shutdown histories, and what unit is displayed on the monitoring screen. This makes it easier for the construction company or maintenance company to perform reproducible checks.

It should also be noted that the cause of reduced power generation may not be a single factor. Minor shading, soiling, data gaps, output curtailment, and temporary equipment shutdowns can overlap and make the overall output appear lower. Even if there were some construction defects, they may not explain the entire decline. Rather than narrowing the cause down to one item, it is more realistic for on-site response to isolate causes in order of their impact.

The purpose of assessing the possibility of construction defects is not to rush to assign blame, but to accurately grasp power generation losses and use that understanding to drive improvements. Organize the facts, ensure consistent comparison conditions, verify within safe limits, and request the necessary inspections. By following this sequence, you can reduce unnecessary rework and improve the accuracy of root-cause investigation.

Summary

Before suspecting installation defects when generation is low, it is important to first check in order: the weather and irradiance conditions, the comparison period and display units for generation, differences between power conditioners, external factors such as shading or soiling, alert and shutdown history, and the assumptions about system capacity and circuit configuration. If you conclude it is an installation defect without checking these items, cause identification can become unclear, and on-site inspections and communication with stakeholders may take a roundabout path.

A drop in power generation is not necessarily caused by a single factor. In addition to weather effects, shadows, soiling, communication dropouts, output curtailment, and device-to-device differences can overlap. That is why it is important not to look only at total generation but to check by time of day, by device, by day, and by month, and make decisions under consistent conditions. Even if the possibility of construction defects remains, organizing the checklist in advance makes it easier to narrow down the targets for inspection.

What is required of operational staff is to turn a vague impression that power output is low into concrete findings. Which pieces of equipment, at what times of day, and by how much are they lower? Can weather or shading explain it? Are there any alerts or shutdown histories? Do the equipment documents match the on-site configuration? By compiling this information, you can separate cases where installation defects should be suspected from those where other causes should be prioritized for investigation.

To make checking for reduced power output more efficient, it's essential to keep field records, generation data, and equipment information organized in an easy-to-read way on a regular basis. Rather than scrambling to search for documents after noticing a drop in power output, managing them in a form that's easy to compare day-to-day will make early detection of anomalies and isolation of causes easier. When you feel the power output is low, adhering to a set sequence and calmly checking is what ultimately leads to the quickest response.

If you want to improve the accuracy of status assessment and on-site verification of solar power generation systems, it is effective to manage not only generation data but also site photos, location information, and inspection records together. If you can link causes of decreased power output to on-site conditions, it becomes easier to turn inspection findings into next actions. Systematizing checks that precede suspecting construction defects and making decisions based on objective records leads to more stable equipment management.