Causes of Low Solar Power Output and 6 Steps to Check Before Making Improvements

When you feel that a solar power system’s output is low, it’s natural to immediately suspect a malfunction or equipment failure. However, in many cases the apparent low output is the result of multiple overlapping factors such as weather, season, irradiance conditions, shading, dirt, settings, and how measurements are interpreted. If you proceed with cleaning, inspections, part replacements, or setting changes without isolating the cause, you may end up spending time on areas that aren’t the real problem and find it difficult to judge the effectiveness of any improvements.

This article organizes into six steps the sequence that field personnel who feel the power output is low should follow before starting corrective work. The specifics vary depending on the size of the power plant and the equipment configuration, but the explanation is given from a general perspective so it is easy to use for on-site initial checks, review of management documents, and preparations before maintenance inspections.

Table of Contents

• Prerequisites to check before concluding that power generation is low

• Step 1: Align the comparison period with the expected power generation

• Step 2: Separate the effects of weather and solar irradiance

• Step 3: Check for shading and changes in the surrounding environment

• Step 4: Check the panel surface for dirt and damage

• Step 5: Check for signs of equipment shutdowns or power output reduction

• Step 6: Organize records and prioritize improvements

• Summary: Linking confirmation of reduced power generation to ongoing management

Preconditions to check before concluding that power generation is low

To determine whether the output of a solar power generation system is low, you must first clarify what it is being compared to. Whether it is lower than the same month of the previous year, lower than the expected value assumed at the design stage, lower than nearby installations, or simply perceived as low by the person in charge will change what needs to be checked.

Solar power generation is not a system that always produces the same amount of electricity just because it is sunny. The amount generated varies day to day depending on solar irradiance, temperature, season, cloud movement, installation angle, orientation, shading, the operating condition of equipment, and other factors. Because daily generation is especially affected by the weather, judging output as low based on a single day can lead to treating fluctuations that have no real cause as abnormalities.

There are also multiple ways to view generation output. Sometimes you look at the total generation of the entire power plant, and other times you look at generation per installed capacity. If you are looking at the amount sold or the amount consumed, those figures can differ from the actual generation. For systems that include self-consumption, the way the management dashboard appears can change depending on how much of the generated power was used and how much was exported. When you feel the generation is low, it is important to check whether the figure you are viewing is generation output, the amount sold, or the surplus remaining after consumption.

On site, consultations such as "generation this month is low," "it's lower than last year," or "output isn't increasing despite sunny conditions" are common. However, the problems indicated by each phrase are not the same. If generation this month is low, monthly solar irradiation and the number of rainy days may have a major impact. If it's lower than last year, in addition to differences in weather conditions, check for age-related degradation, soiling, shading, and equipment downtime history. If output isn't increasing despite sunny conditions, candidates include passing clouds, output reduction due to rising temperatures, equipment limitations, and string-level anomalies.

The important thing is not to jump to conclusions about the cause. Many issues with solar power systems are hard to judge by appearance alone. Even if it looks like output is low because the panels are dirty, the main cause might actually be an equipment shutdown. Conversely, even if no equipment alarms appear on the monitoring screen, output may be gradually declining due to shading or dirt. Checking before making improvements is meant to reduce assumptions and narrow down the necessary countermeasures.

When checking for a decrease in power generation, it is easier to structure the investigation by looking in order not only at the overall figures but also at the time period, weather, by facility, by equipment, the surrounding environment, and the inspection history. This helps distinguish whether the change is a natural fluctuation, a management issue, or an abnormality that requires on-site verification. Below, I explain in order the six verification steps you should carry out before making any improvements.

Step 1 Align the period used to compare against the expected power generation

The first step is to align the comparison baseline and the period. When you feel that power output is low, there are many points of comparison—same month last year, the previous month, the assumptions at the design stage, past averages, nearby installations, and so on. However, if you make a judgment without aligning the comparison conditions, even normal differences can appear abnormal.

For example, when comparing this month’s power generation with the previous month, it is necessary to take into account seasonal differences in hours of sunlight. The generation trend for the same installation can change significantly between the period from spring to summer and the period from autumn to winter. In winter, days are shorter and the sun’s altitude is lower, so installations that are susceptible to shading tend to experience reduced generation. Conversely, although hours of sunlight are longer in summer, higher temperatures can sometimes reduce the output of solar modules. Therefore, it is not sufficient to simply assume that generation should be higher just because there are supposedly more sunny days.

Comparing with the same month in the previous year is useful, but it is not foolproof. If the previous year benefited from particularly clear weather, this year’s power generation may appear lower. Conversely, if there were equipment outages or recording omissions in the previous year, this year’s power generation may appear higher. Before making a comparison, checking the weather, equipment outage history, inspection history, cleaning history, and whether any construction work took place for the same month in the previous year will improve the accuracy of your assessment.

When comparing actual generation with the expected power output assumed during design, it is important to check the assumptions behind those expected values. Expected generation is produced based on factors such as installation location, orientation, tilt angle, system capacity, loss conditions, and solar irradiance data. In actual operation, additional factors come into play, including changes in the surrounding environment, soiling, aging degradation, equipment downtime, and year-to-year weather variability. Therefore, you should use the expected values as a reference for verification rather than treating them as absolute guaranteed figures.

Also, it is easier to judge trends in power generation on a monthly basis than on a daily basis, and on a yearly basis than on a monthly basis. A drop that occurs on a single day is easily affected by clouds or a temporary shutdown, making it difficult to identify the cause. By checking whether the same trend continues over several days to several weeks, it becomes easier to distinguish whether the change is due to natural variability or to equipment or environmental problems. In practice, checking daily graphs, monthly aggregates, and comparisons with the same month in previous years side by side makes it easier to narrow down when an anomaly occurred.

It is also important to look at generation per unit of installed capacity. If you only look at the total generation of a plant, differences caused by the scale of installed capacity become mixed in. When comparing multiple plants or sections, converting to the same capacity basis makes it easier to identify locations that are underperforming. Even when there are multiple power conditioners or circuits on the same site, comparing generation by device or by circuit reveals whether the decline is overall or only partial.

At this stage, what you need to verify is whether the power generation is actually low, or if it only appears low because of the way it's being compared. If you begin cleaning or repairs while this remains unclear, it will be difficult to determine afterward whether the measures were effective. The starting point for identifying the cause of a decline in power generation is to align the comparison periods, establish clear benchmarks, and organize the figures on a daily, monthly, and per-equipment basis.

Step 2 Isolate the effects of weather and solar radiation

Next, check the influence of weather and solar irradiance. The power output of photovoltaic generation is heavily dependent on solar irradiance. When investigating the causes of low generation, before suspecting an equipment malfunction, you should first confirm how the irradiance conditions compared with typical years or with the assumed conditions.

If there are many cloudy or rainy days, power generation tends to be lower. Even with thin clouds, if sunlight is weakened, generation will decrease. On days that appear sunny, if there are thin clouds aloft or if it was cloudy in the morning or afternoon, the total daily power generation is unlikely to be high. Even if it was sunny only during the times staff were at the site, it may have been cloudy in the morning, evening, or around midday. Therefore, when judging a drop in generation, it is important not to rely solely on the impressions of the person in charge, but to also check daily weather records and trends in solar radiation.

Looking at the relationship between solar irradiance and power generation makes it easier to judge whether there is an equipment malfunction. If power generation is low on days with low irradiance, it is more likely to be a natural variation. Conversely, if irradiance is high but power generation is clearly not increasing, that is a reason to suspect problems with the equipment or the surrounding environment. In particular, if the drop in power generation is larger compared with installations under similar conditions in the same region, it is necessary to investigate the specific causes.

You shouldn't overlook the influence of ambient temperature. Solar panels tend to produce more power the stronger the sunlight, but their output tends to drop as panel temperature rises. As a result, on a clear summer day instantaneous output may not reach expected levels. This is not necessarily a malfunction; it can be a natural decline due to the panels' temperature characteristics. Of course, if the output is extremely low you should investigate other causes, but it's premature to judge it as abnormal simply because it doesn't reach its maximum output on a sunny day.

Regional and seasonal factors such as snowfall, frost, yellow sand, pollen, salt from sea winds, volcanic ash, and fallen leaves also affect power generation. If snow covers the surface of the panels, power generation drops significantly even when there is sunlight. In regions where frost remains in the morning, the morning ramp-up can be delayed. During periods with heavy yellow sand or pollen, dirt on the panel surface combined with weaker sunlight can make power generation appear lower. These seasonal factors should be accounted for separately from equipment faults.

When isolating the effects of weather, it's useful to look not only at daily generation but also at the output trend by time of day. If the generation graph on a sunny day forms a smooth bell-shaped curve, the system is likely operating naturally overall. If there is a sudden drop during a certain period, suspect passing clouds, shading, or a temporary shutdown of equipment. If the drop happens at the same time every day, the likelihood increases that shading from buildings or trees is the cause. If the drops are irregular, check for intermittent weather effects or equipment operation.

The important point in this procedure is not to look at power generation in isolation. A low power generation result can sometimes be explained simply by low solar irradiance. Conversely, if solar irradiance conditions were good but power generation did not increase, that provides a basis to proceed to the next checks. By compiling weather and solar irradiance data, you can exclude natural variability and more easily focus on issues that require on-site inspection.

Step 3: Check for changes in shadows and the surrounding environment

When a drop in power generation cannot be explained by weather alone, the next things to check are shading and the surrounding environment. In solar power generation, even partial shading of a panel can affect output depending on the circuitry and system configuration. Shading is easy to overlook, and because how it appears changes with the time of day and season, it must be carefully checked as a cause of reduced generation.

Causes of shading include buildings, trees, utility poles, signs, fences, adjacent equipment, antennas, rooftop structures, weeds, and so on. Even equipment that had no problems at the time of installation may later be affected by shading if surrounding trees grow or new buildings or equipment are constructed. For ground-mounted installations, weeds and low shrubs can grow in front of the panels and cast shadows. For rooftop installations, shadows from chimneys, railings, neighboring houses, and roof shapes can become noticeable depending on the season.

When checking shadows, it is not sufficient to look just once during the daytime. Because the sun's position moves over time, shadows may fall only in the morning, only around midday, or only in the evening. Also, even if shadows are not a concern in summer, the sun's altitude is lower in winter and shadows can extend much farther. If a drop in power generation is noticeable in a particular season, it is important to inspect while taking that season's solar altitude and the way shadows extend into account.

Signs of shading can also be read from the power generation graph. If output drops at the same time every day despite clear sunny conditions, shading is suspected. For example, the output may fail to rise for a certain period in the morning, suddenly fall in the afternoon, or show a localized valley-like dip around noon. Declines caused by clouds tend to vary in timing from day to day, whereas shadows from fixed objects tend to recur at the same times.

Changes in the surrounding environment are easier to assess when confirmed with management records and on-site photographs. Comparing installation photos, past inspection photos, and current photos makes it easier to spot tree growth, increased weeds, and changes to surrounding structures. In particular, for equipment that has been in operation for a long time, the assumptions made at installation may no longer hold. When investigating the causes of low power generation, it is important not to assume that because there was no problem before there isn’t one now, and to recheck the current situation.

Also, attention needs to be paid to shading between panels. For ground-mounted and flat-roof installations, panels in the front row can cast shadows on those in the back row. Even if this was taken into account during design, shading can still occur depending on the season or time of day. If it remains within the normally anticipated range it is unlikely to cause problems, but if there is any shift in the racking, changes in surrounding ground conditions, or the addition of equipment, check whether the impact of shading has become greater than originally expected.

Countermeasures for shading vary depending on the cause. If it’s weeds, implement weed control; if it’s trees, prune them or consult the site manager; if it’s nearby structures, determine the times of day when shading occurs and the extent of its impact. However, before starting remedial work, you need to verify whether shading is truly the main cause of the reduction in power generation. Even if shading is visible, the bulk of the output reduction may be caused by equipment outages. Conversely, shading can cause chronic losses even without any equipment alarms. Because shading is a cause that can be observed on site, it is important to assess it in combination with power generation data.

Step 4: Inspect the panel surface for dirt and damage

Next, check the condition of the panel surface. Because solar panels are installed outdoors, they are exposed to rain, wind, sand and dust, bird droppings, fallen leaves, pollen, yellow sand, mud splashes, salt, and other factors. If dirt adheres to the panel surface, light may have difficulty reaching it sufficiently, which can lead to a decrease in power generation.

The effects of soiling depend on the type, extent, location of the soiling, and the system configuration. If a light layer of dust is spread across the entire array, power output may gradually decrease. If strong localized shading occurs from bird droppings or fallen leaves, some panels or circuits may be disproportionately affected. On ground-mounted systems, dirt tends to accumulate at the lower edge, and when the tilt is shallow so that rain does not easily wash the soiling away, deposits are also more likely to remain.

When you feel that power output is low, it's natural to suspect dirt as the cause, but you should avoid judging based solely on appearance. Even if something looks slightly dirty, the impact on power generation can be small. Conversely, dirt that is hard to see from a distance can accumulate in specific areas. Before cleaning, it's important to confirm what kind of dirt is present in which areas and whether the timing matches the decline in power generation.

It is also necessary to check for damage and deterioration. Cracks on the panel surface, deformation of the frame, sagging cables, abnormalities at connection points, shifting of the mounting structure, and loosening of fastenings affect not only power output but also safety. If there is damage clearly visible on visual inspection, do not touch it; arrange for a professional inspection. Especially because this is electrical equipment, on-site personnel should avoid carelessly touching wiring or the interior of devices.

When checking for dirt or damage, keeping photographic records makes later decisions easier. Record the photo date, photo location, the panel in question, the extent of the soiling, and whether shadows are present so you can compare before and after cleaning or inspections. If a drop in power generation continues, place the photos alongside the generation data to check whether the period when soiling increased is close to the period when output fell. This makes it easier to explain the need for and the priority of cleaning.

When considering cleaning, it is important not to carry it out immediately but to consider safety and effectiveness. Work on roofs, at heights, or around electrical equipment carries risks of falls, electric shock, and damage. Incorrect use of water or cleaning methods can also damage equipment. In practice, it is necessary to decide on an approach after confirming equipment management rules, maintenance contracts, scope of work, and safety standards. Even when cleaning to improve power generation, it is pointless to cause other problems through unsafe or unreasonable work.

Checking for dirt and damage is an important procedure that allows you to visually identify causes of reduced power output. However, it is also important not to assume a single cause. The panel surface may be dirty while some equipment is simultaneously out of service. Shadows, dirt, and equipment faults can overlap. It is realistic to treat surface condition checks as part of the overall cause analysis together with power generation data and equipment information.

Step 5 Check for signs of equipment shutdown or reduced output

If checking the weather, shading, and soiling does not clarify the cause of reduced power generation, check the condition of the equipment. A solar power system is composed not only of panels but also of multiple pieces of equipment such as power conditioners, junction boxes, wiring, protective devices, measurement devices, and communication equipment. If any part stops or experiences reduced output, the overall power generation may decrease.

First, check the power generation and operating status of each piece of equipment. If you only look at the plant’s total power output, it can be easy to miss that some devices have stopped. In facilities with multiple power conditioners, compare the output of each unit. If, among devices in similar conditions, only a particular unit has lower output, suspect an abnormality around that unit. If all units are similarly low, consider influences such as weather, solar irradiance, overall control, and measurement conditions.

The shape of the power generation graph is also useful. Even on sunny days, if the output drops to near zero partway through, caps out at a constant upper limit, fluctuates frequently, or fails to resume after a specific time, you should check for equipment shutdowns, curtailment, communication faults, protective operations, and the like. In particular, if the output is completely interrupted during periods with solar irradiation, it is difficult to explain that solely by weather.

However, care is needed when judging solely by the figures on the monitoring screen. Even if the generation equipment itself is operating, data may be missing due to communication or measurement faults. It is important to distinguish whether the generation appears low only on the management screen or whether it is actually not generating. Comparing multiple sources such as the export meter, on-site displays, monitoring data, and daily reports makes it easier to tell data loss apart from an actual shutdown.

When alarm or error logs are available, compare the time they occurred with the time of the power output drop. An alarm does not necessarily indicate the primary cause of the output decrease, but if the times match it becomes an important clue. Conversely, there may be no alarm even when power output has dropped, because minor output reductions, shading, soiling, or communication issues may not appear as a clear alarm.

When inspecting equipment, drawing clear safety boundaries is also important. While checking displayed readings, inspecting the appearance, and reviewing records can be carried out easily by management personnel, electrical measurements, internal inspection of equipment, wiring inspections, and parts replacement may require specialized knowledge, qualifications, and proper procedures. When investigating the causes of low power generation, it is safest not to forcibly open equipment or handle wiring; instead, organize the information you can confirm and then arrange for a professional inspection.

Output curtailment or grid-side constraints may also be involved. Even if the equipment is operating normally, output can be temporarily limited due to external conditions or operational rules. In this case, because it is not an equipment fault, cleaning or replacing parts will not improve it. If low generation occurs in a specific pattern during certain time periods, you need to check not only for equipment shutdowns but also for operational restrictions and how it appears in the measurements.

Confirming the equipment side often directly leads to identifying the cause, but if judged incorrectly it can easily lead to unnecessary work. It is important to organize, in order, generated power, output per device, alarm history, communication status, local displays, and inspection records, and to narrow down the scope of the abnormality. Clarifying whether the whole system is low, only part of it is low, whether it changes by time of day, or whether it is continuous makes it easier to decide the next countermeasures.

Step 6 Organize records and set priorities for improvements

The final step is to organize what has been checked so far and decide the priority of improvements. The cause of low power generation is not necessarily a single factor. In addition to weather effects, there may be shading, soiling, and a shutdown history in part of the equipment, so multiple factors can overlap. Therefore, if you decide on countermeasures based on intuition without recording the inspection results, you may overlook the causes that would have the greatest impact.

First, determine when the decline in power generation began. Review daily, monthly, and time-of-day generation data to confirm whether the decline is temporary, persistent, or concentrated in specific periods. Once you know the start of the decline, it becomes easier to investigate events that occurred before and after. Check whether output improved after cleaning, declined after construction work, changed following typhoons or heavy snowfall, or coincides with periods when the surrounding environment changed.

Next, we classify the candidate causes. We separate natural variations due to weather and solar irradiance; surrounding environmental factors such as shading and weeds; panel surface issues such as dirt and damage; equipment-side problems such as device shutdowns and communication failures; and issues related to output limits or the way measurements are interpreted. By classifying them, it becomes clear which can be dealt with immediately, which require specialist inspection, and which should be assessed through continuous monitoring.

It's easier to prioritize improvements by considering their impact on power generation, safety, ease of response, and likelihood of recurrence. If damage or equipment abnormalities affecting safety are suspected, consider addressing them promptly even if their impact on power generation appears small. If a partial shutdown of equipment has been confirmed, you may prioritize checking operational status and arranging specialist inspections before cleaning. If shading from weeds or fallen leaves is evident, it may be easy to confirm the effectiveness of weeding or cleaning.

You should also decide the comparison conditions for before and after the improvement. After implementing measures, it is important to determine in advance which period's power generation you will use to assess the effect, how to account for weather conditions, and at what equipment unit you will make the comparison. If you carry out measures without deciding anything, even if it appears to have improved, it may simply be because the weather was better, making it difficult to judge whether the measures were actually effective. Keeping pre-improvement generation data, on-site photos, and inspection records makes it easier to explain the results after the measures.

Ideally, records should be kept in a form that remains understandable even if the person in charge changes. When you feel that power generation is low on a given day, leaving notes on the data you checked, the conditions you observed on site, photos, alarm history, actions taken, and any unchecked items will speed up future decision-making. Especially when managing multiple power plants, relying on memory makes it easy to repeat the same checks or to overlook important changes.

At this stage, decide whether to proceed immediately with corrective work, require additional inspection, or monitor the situation for a while. If a decrease in power output can be explained by weather conditions, avoid excessive intervention and opt to continue monitoring. If shading or soiling is a likely cause, plan on-site work. If equipment malfunction is suspected, prioritize safety and arrange for a specialist inspection. The important thing is to link the possible causes with the appropriate response policy.

The lower the power generation, the stronger the desire to fix it quickly. However, if you skip checks and start working, it becomes difficult to decide what to do next if the power generation does not recover. Conversely, if you conduct thorough preliminary checks, the aim of the countermeasures becomes clear and it is easier to verify their effectiveness after improvements. It is important to treat the response to reduced power generation as a single process that includes both investigating the cause and confirming the effect after countermeasures.

Summary: Linking confirmation of power generation decline to ongoing management

When you feel that solar power generation is low, it is important not to immediately assume a fault or soiling, but to check in the order of comparison conditions, weather, shading, soiling, equipment condition, and record organization. Since generation fluctuates with solar irradiance and the seasons, judging an anomaly based only on short-term figures can lead to unnecessary actions. On the other hand, overlooking signs of decline can prolong the loss of generation opportunities.

In practice, what’s important is to break down the result of low power generation into verifiable pieces of information. We examine, in order: since when it has been low, which time periods it is low, whether the whole system or only parts are low, whether it can be explained by weather, whether there have been changes on site, and whether it matches the equipment history. Establishing this sequence makes it easier to separate natural variability, environmental factors, equipment-related factors, and measurement-related factors.

Carefully carrying out pre-improvement checks not only reduces wasted effort but also helps when explaining the situation to stakeholders. In consultations about reduced power generation, multiple parties may be involved, such as the manager, the construction company, maintenance personnel, and the facility owner. In those cases, if power generation data, site photos, inspection records, the time of occurrence, and possible causes are organized, it becomes easier to decide on the next steps. Conversely, if the information remains fragmented, rechecking and delays in decision-making are likely to occur.

Also, responding to a decline in power generation is not a one-time task. By continuously monitoring seasonal solar irradiance trends, changes in the surrounding environment, dirt accumulation, and equipment operating history, you can detect signs of decline earlier. Keeping regular records from the same perspective improves the accuracy of comparisons with the same month of the previous year and before-and-after comparisons of improvements. Formalizing the verification procedures to follow when you suspect low power output as management rules helps maintain long-term generation efficiency.

On-site, it is particularly effective to record not only power generation figures but also photos and location information together. If you can later check where shadows are occurring, which panels have concentrated soiling, and which area was inspected, you can reduce the chance of overlooking countermeasures. Rather than trying to determine the cause of low power generation from a single inspection, continuously accumulate power generation data, on-site conditions, and inspection history and use them to inform the next decision; this leads to more stable management.