6 Points for Reading the Seasonal Movement of Shadows That Cause Power Output Reduction

When a solar power generation system continues to produce low output, it's natural to suspect equipment failure or panel degradation right away. However, in field operations, even if there is no major fault with the equipment itself, the position and length of shadows change with the seasons, and it can appear that generation drops only during certain periods. Shadows that were not noticeable in spring or summer may extend onto the panel surface from autumn into winter. If generation is low only in the morning, the output fluctuates during certain hours in the afternoon, or the figures are lower than those of the same month in the previous year despite clear skies, it's important to include the seasonal movement of shadows among the factors to check.

Table of Contents

• A perspective that suspects seasonal movement of shadows as the cause of low power generation

• Reading the periods when shadows lengthen due to changes in solar altitude

• Find the time periods when shadows are cast from time-series data

• Check the changes in the surrounding environment and the shadows of fixed objects separately.

• Compare the fall patterns for each string with the positions of the shadows.

• Make it possible to keep records for each season and compare them.

• Decisions about shading mitigation should be based on overall operations, not just power generation.

• Summary

Considering Seasonal Shadow Movement as a Cause of Low Power Output

When you feel that power generation is low, the first thing to check is whether the decrease is temporary or a seasonally recurring issue. Solar power generation fluctuates due to multiple factors such as solar irradiance, ambient temperature, weather, dirt on panel surfaces, and the condition of equipment. Among these factors, shading can be comparatively easier to notice on sunny days. On cloudy or rainy days, overall irradiance is weak so differences caused by shading are hard to see, whereas on sunny days it is easier to confirm the difference between shaded and unshaded areas.

The seasonal movement of shadows refers to the phenomenon in which the shadows of the same buildings, trees, fences, utility poles, mounting racks, and surrounding equipment fall in different positions at different times of the year due to changes in the sun’s elevation and direction. In particular, from autumn to winter the sun’s altitude becomes lower, causing shadows from the same obstructions to lengthen. As a result, equipment whose problems were hard to see during summer inspections may receive shadows on the panels during parts of the morning or afternoon in winter.

What field personnel should be aware of is that the impact of shading does not necessarily last all day. It can appear only in the morning, only in the evening, or only for a short period around midday. When you look only at total power generation, it may seem only slightly lower overall, but checking output by time of day can reveal drops that occur only at specific times. These kinds of drops can be a clue not only to overall system degradation but also to shading or to the influence of a specific circuit.

Also, the impact of shadows cannot be judged by area alone. Even a thin shadow on part of a panel can affect power generation depending on the circuit configuration and how the shadow falls. Shadows from thin wires, railings, or the edges of adjacent equipment may look small on-site but can be a cause of output reduction at certain times of day. When investigating the cause of low power generation, it is important to check not only the size of the shadow but also when it occurs, which row or string it falls on, and in which orientation the shadow appears.

To read seasonal shadow movement you need to combine on-site inspection with data verification. Looking only at the site shows only the shadows at that moment. Conversely, looking only at the power generation data can make it hard to determine whether the cause is shading or an equipment malfunction. By reviewing site photographs, time-based generation, per-string values, and changes in the surrounding environment together, you can narrow down the possibility that shading is the cause. Below, six points are explained in order for interpreting seasonal shadow movement that leads to a decline in power generation.

Reading the Periods When Shadows Lengthen Due to Changes in Solar Altitude

To understand the seasonal movement of shadows, the first thing to grasp is the change in solar altitude. When the solar altitude is high, shadows tend to be shorter, and when the solar altitude is low, shadows tend to be longer. In many parts of Japan, the sun’s path is lower in winter than in summer, so the shadows of surrounding objects extend farther. As a result, shadows that stopped short of the panel in summer can reach the panel surface in winter.

A commonly overlooked issue in consultations about low power generation is when the timing of the inspection is misaligned with the period of decline. For example, during an on-site check in spring no shadows may be found and the equipment judged normal, yet the power output actually drops on sunny winter days. In such cases, an on-site inspection in spring alone can miss the cause. If you suspect the influence of shadows, it is advisable to check under conditions close to the same season and time of day as when the power output decreased.

Shadow length is influenced not only by the height of obstacles but also by the distance to the panels and their direction. If there are buildings or trees on the south side of the installation, shadows may extend in winter. Obstacles to the east can affect morning power generation, while obstacles to the west are more likely to affect afternoon power generation. Rather than assuming that objects to the north will never be a problem, be aware that how shadows fall also depends on the terrain, roof orientation, panel installation azimuth, and the height of surrounding objects.

Points to check on site are not limited to the mere presence or absence of obstacles. Observe from which directions shadows extend across the panel surface, how much those shadows are likely to move with the seasons, and whether the area affected by shading varies even within the same row. Shadows that stretch long from a low position in particular tend to affect generation immediately after it starts and just before it ends. Although solar irradiance itself is weak in the morning and evening, when shading effects overlap they can cause differences in total daily generation.

When reading changes in solar altitude, it is important not to rely solely on the field staff’s intuition and to keep seasonal records. Even though it is known that shadows are longer in winter, how far into the rows they actually reach, around what time they clear, and in which month problems begin vary by site. If generation is low at the same time each year, check past data for the same month to confirm when the decline starts and when it recovers. If seasonal movement of shadows is the cause, similar patterns may appear at similar times each year.

However, simply comparing last year’s and this year’s power generation is insufficient. Differences in weather, snow or soiling, equipment outages, inspection or maintenance work, and changes to measurement device settings also have an impact. For that reason, it is important to match comparison days as much as possible to clear-sky days and to look at the output curves for the same season and the same time of day. If similar dips or steps repeatedly appear in the output curves for clear-sky days, it becomes easier to suspect the influence of shading.

Find time periods when shadows are cast from time-stamped data

To detect the effects of shading, it is essential to check not only daily and monthly generation data but also time-of-day data. Daily generation data can show that output is low, but it is hard to tell when the drop occurred. Because shadows move over time, output reductions also vary by time of day. By checking whether the morning ramp-up is weak, whether there is a dip before noon, or whether a step change appears only in the afternoon, you can narrow down the shadow direction and possible causes.

On clear days, the power generation curve generally rises and falls with solar irradiance, peaking around midday and declining toward evening. Of course, it can also vary due to passing clouds, rising temperatures, output control, or protective actions of equipment, but if the output regularly drops in a similar pattern at a specific time each day, it is worth considering the effect of shading. For example, if on clear days from November through February the output is reduced only around 9:00 AM, the shadow of an obstacle on the east or southeast side may be responsible.

When examining time-series data, it's important not to draw conclusions from a single day. If a cloud happens to pass at the same time as a shadow, it can appear like shading. Conversely, on slightly cloudy days overall output is suppressed, making differences caused by shadows harder to detect. If possible, compare multiple clear-sky days to see whether similar dips recur at the same times. Comparing not only low-output days but also high-output days makes differences caused by the presence or absence of shadows easier to see.

If shadows are the cause, a distinct pattern may appear at the start and end of the output drop. When a shadow begins to fall on a panel, the output drops, and when the shadow moves away it recovers. For this reason, the curve may show a valley-like shape or a step-like change. If equipment has stopped, the decrease may be recorded as a sudden, clear drop, but with shading only part of the output may be suppressed. The actual appearance varies depending on the system configuration, so judgment is made by combining time-series data with on-site conditions.

Also, to read seasonal shifts in shadows you need to look not only at the time of day but also at month-to-month changes. For example, if a dip that was not noticeable in September begins to appear in October, becomes larger in December, and smaller in March, you should suspect a relationship between changes in solar altitude and the shadows. Such changes appear different from equipment failures or persistent degradation. While degradation or soiling often affects performance relatively continuously, seasonal shadows tend to be confined to specific times of year and times of day.

When viewing power generation monitoring screens or logbooks, don’t judge solely by the monthly generation; it’s useful to keep a time-of-day curve for a representative sunny day. For days when generation seems low, note the day’s weather, temperature, work history, outage history, and any shadows observed on site—this makes it easier to trace the cause later. Because shadows can disappear by the time you arrive at the site, it’s more efficient to narrow down the time window in the data before going for an on-site inspection.

When reading time-series data, be careful not to confuse shading with output control, equipment protection actions, communication gaps, or measurement update intervals. Before concluding that low generation is due to shading, also check stoppage histories, fault histories, communication gaps, and differences in measurement units. In particular, when data are missing partway through or values remain flat for a certain period, the issue may be due to measurement or communication problems rather than an actual drop in generation. Determining shading is more accurate when you combine time-series data, on-site photographs, and equipment status.

Check changes in the surrounding environment separately from shadows cast by fixed objects

When investigating the causes of shadows, it is necessary to consider separately shadows caused by changes in the surrounding environment and shadows cast by pre-existing fixed objects. Fixed objects are relatively unchanging items such as buildings, walls, mounting racks, utility poles, signs, rooftop equipment, handrails, and so on. Changes in the surrounding environment include tree growth, construction on adjacent land, installation of temporary structures, changes to material storage areas, vegetation overgrowth, and additions of equipment. If power generation is lower than before, these changes may be creating new shadows.

Trees are often overlooked as a cause of shading. Branches and foliage change how they spread with the seasons, and height can change over the course of several years. In spring and summer, leaf growth can make shadows denser, while in winter the lower sun angle can cause trunks and branches to cast much longer shadows. Even deciduous trees can affect power generation if the thin shadows of their branches fall across panels. On-site inspections take into account not only whether leaves are present, but also the position of branches and their future growth.

Changes in neighboring land are also important. Even if there were no problems when a solar power system was installed, shadow conditions can change if buildings or structures are added nearby afterward. Especially during periods of low solar altitude, shadows from structures a short distance away can reach the equipment. If the time when generation suddenly drops coincides with surrounding construction or the addition of equipment, it is important to check site inspection records and photos to identify the source of the shading.

For rooftop installations, attention must also be paid to protrusions on the same roof. Ventilation equipment, antennas, snow guards, handrails, level differences, and adjacent roof surfaces can cast shadows. Even small protrusions can cast long shadows when the sun's altitude is low, shading parts of the panels. Shadows that were considered acceptable at installation may turn out to have a greater impact than expected when generation data is reviewed after operation.

In ground-mounted power plants, shadows between mounting racks should also be checked. Even if the spacing between rows appears sufficient, at the low solar elevation in winter the shadow from the front row can reach the rear row. Especially in the morning and evening and around the winter solstice, shadows tend to fall on the lower parts and edges of the rows. When the conditions assumed at the design stage combine with the actual terrain, construction tolerances, and nearby obstacles, more shading than expected can occur. If rows with reduced power output can be identified, carefully verify the relative positions of each row.

When checking for changes in the surrounding environment, it is effective to keep taking photos at the site from the same direction. Taking them from different angles each time makes comparison difficult, but if you record photos from the same position, the same height, and the same orientation, it becomes easier to confirm tree growth or the presence of new obstructions. In addition to photos taken during periods of low power generation, keeping photos from periods with minimal shadows makes it easier to explain seasonal differences.

Also, when the cause of shading is in the surrounding environment, it may not be possible to remove or alter it immediately. Neighboring buildings, public property, or trees owned by others may not be something your company alone can address. Therefore, it is important to first accurately identify the source of the shading and record during which periods and to what extent it is having an impact. If records remain vague, it will be difficult to explain the situation to stakeholders or to consider countermeasures. As a practical first step, organize not only the fact of reduced power generation but also the shadow’s location, time of day, season, and range of impact.

Cross-check the fall of each string with the position of its shadow

To more concretely understand the effects of shading, check not only the system's total power generation but also string-level and circuit-level data. Looking only at the total power generation makes it difficult to determine whether the drop is caused by the entire system or by a specific row of panels. Because shading often occurs locally, it can appear as reduced output only on certain strings or as a widening of differences during specific time periods.

For example, among several strings installed with the same orientation, the same tilt, and similar conditions, if only some strings show lower output in winter mornings, consider the possibility that an object is casting a shadow in that direction. If a different string is low only in the afternoon, check for obstacles on the west side or inter-row shading. If the strings that are low change with the seasons, it may be that the pattern of shading changes with the sun’s path.

When reading per-string data, it's important not to simply assume that lower values indicate an anomaly. If the installation orientation or tilt differs, the way they receive solar irradiance will also differ. Also, if the number of panels, circuit configuration, or connection conditions are different, you need to be careful when comparing outputs. Compare targets whose conditions are as similar as possible. If you compare items with different conditions side by side, you may mistakenly interpret design differences as anomalies rather than shading.

To correlate shadow locations with string configurations, site drawings, circuit diagrams, and panel layout drawings are useful. If you know which panels are connected to which strings, you can link the shaded areas with the circuits showing low output and verify them. Conversely, without knowing the correspondence between layout and circuits, it is difficult to accurately assess the impact of shading. When investigating the causes of low power generation, you need to check not only the on-site visual appearance but also the electrical connection information.

The effect of shading can impact the output of the entire circuit even if it only covers part of a panel. Therefore, even if it feels on site like “there’s only a little shade,” it can show up as a large difference in the data. Extra caution is needed especially when a long, narrow shadow crosses a panel or when shadows span multiple panels. The way the impact appears changes depending on whether the shadow only slightly touches the edge of a panel or extends broadly along the rows of cells.

Shadows and equipment malfunctions can also occur at the same time. For example, in winter where some strings appear depressed due to shadowing, if connector faults or equipment abnormalities coincide, it becomes difficult to identify the cause. In such cases, separate and organize time-of-day data from clear days, comparisons by string, on-site shadow inspections, and equipment inspection results. If a string remains low even during periods when the shadow has lifted, suspect factors other than shading. If it is low only during the times when shadows are present and approaches other circuits once the shadows pass, the influence of shading is likely the primary cause.

When checking by string, comparing with historical data is also effective. If the same string is dropping at the same time of day compared with a similarly sunny day in the same month of the previous year, seasonal shading may be repeating. On the other hand, if a particular string has suddenly become lower starting this year, you should also check for new shading, dirt, damage, connection faults, and so on. While keeping the perspective of reading the seasonal movement of shadows, it is important not to narrow down the cause of reduced power generation to a single factor.

Make it possible to keep records by season and compare them

Seasonal movement of shadows can be difficult to assess from a single on-site observation. Even a slight difference in the time of a site check can greatly change the position of shadows. If the season is different, the length and direction of shadows change even at the same time of day. Therefore, to continuously verify the causes of low power generation, it is important to keep records for each season so they can be compared later.

What should be recorded are on-site photographs, the date and time they were taken, the weather, the shooting location, the area covered by shadows, power generation data, and observations made during inspection. If you only keep photographs, they are hard to use for comparison if you don't know when they were taken. Conversely, if you only keep data, it can be difficult to identify causes if you don't know the position of the shadows at that time. Linking photos and data to the same date and time makes it easier to explain the relationship between shading and power generation.

Fixing the shooting position is also important. If you take photos from different places each time, it becomes hard to tell whether differences are caused by changes in shadows or by different shooting angles. If possible, decide on positions that allow you to view the entire facility, identify the source of shadows, and check the panel surface where shadows fall. When shooting on roofs or at height, prioritize safety and avoid awkward postures or dangerous movements.

On-site photos are useful if you keep not only those with shadows but also those without. This is because photos showing only shadows make it difficult to judge how unusual those shadows are. Recording multiple conditions—short shadows in summer, long shadows in winter, morning, midday, and afternoon—makes it easier to explain seasonal trends in shadow movement. Comparative records are also effective when explaining to stakeholders why power generation is low during a particular period.

In addition, records must be kept in a form that remains understandable even when personnel change. Investigations into the causes of shading tend to rely on the memories of staff who know the site well. However, when staff are transferred or the contractor is changed, the history of past events can become unclear. Organizing photos, notes, power generation data, and layout diagrams makes it easier for successors to trace past conditions. Because addressing a decline in power generation may not be resolved in a single attempt, continuity of records is important.

In inspection records, it is useful to note not only the presence or absence of shadows but also any points that were difficult to judge. For example: "In the morning shadows from trees on the southeast reached part of the row, but cleared before noon"; "In winter, the front-row mounting rack may cast a shadow on the lower part of the rear row"; "Ongoing monitoring is required for a newly constructed structure on the neighboring property." Even at a stage when you cannot be definitive, separating observed facts from points that need follow-up will improve the accuracy of the next inspection.

When responding to inquiries about low power generation, the speed of the investigation depends on whether past records exist. If there are no records, you need to begin with an on-site inspection; however, if seasonal photographs and data are available, you can suspect the effects of shading at an early stage. In particular, when managing multiple power plants or many rooftop installations, listing the shadow risk for each site makes it easier to address reduced generation in winter.

Countermeasures for shading should be assessed based on overall operation, not just power generation

Even if it is determined that shading is causing a drop in power generation, it is not always necessary to immediately undertake large-scale measures. Options for addressing shading include tree pruning, relocating obstacles, revising panel layouts, reconsidering circuit configurations, adjusting cleaning and inspection frequency, and changing monitoring settings. However, the measures that can be implemented vary depending on site conditions, rights issues, and safety. Rather than judging solely by recovery of power generation, it is important to choose an appropriate response within the context of overall operations.

The first thing to check is the duration, the time of day, and the extent over which the shadow effects appear. The priority of countermeasures changes depending on whether shadows occur for short periods on only a few days a year, or for long periods every day on sunny winter days. Rather than acting solely on the impression that power output is low, organize the trends in reduction caused by shadows, the affected strings, and the periods when they occur. If the impact of shadows is limited, continuing to record and monitor may be a realistic response.

If trees are the cause, consider pruning or removal. However, you need to check ownership, legal requirements, landscape considerations, safe working procedures, and management of regrowth. Even for trees on your own property, professional judgment is required if the work involves heights or the risk of a tree falling. For trees on neighboring property, you cannot take action without permission. It is advisable to organize photos of the shading and power generation data and be able to explain the extent of the impact before consulting with the relevant parties.

If buildings or structures are the cause, relocation is often difficult. In such cases, quantify the impact on power generation and determine whether it falls within acceptable operational limits. Even when shading is unavoidable, management can be improved—for example, by scheduling inspections for seasons when shadows are most likely, reviewing monitoring alert thresholds on a seasonal basis, and accounting for the seasonality of shading when evaluating power generation. If the cause is identified, unnecessary fault responses and excessive emergency dispatches can potentially be reduced.

Revising panel layout and circuit configuration requires careful consideration. If the areas susceptible to shading are clearly identified and significant declines persist over the long term, facility upgrades may become an option. However, modifications to existing equipment involve construction constraints and safety verifications. Not only power generation but also work risks, downtime, maintainability, and future changes in the surrounding environment should be taken into account. Prioritizing only short-term recovery of power output can, in fact, make operations more complicated.

Reviewing monitoring settings is also effective. At sites where the seasonality of shading is known, you can organize how alerts are interpreted on the assumption that power generation will be lower during specific time periods in winter. However, it is dangerous to treat shading as uniformly harmless. Even at sites with shading, equipment faults, soiling, and damage can occur separately. It is important to have criteria by time of day, by circuit, and by season so that known reductions due to shading can be distinguished from new anomalies.

When considering measures against shading, it is also important to ensure a shared understanding among stakeholders. If on-site staff, managers, maintenance personnel, and owners are each looking at different information, judgments about reduced power generation will fluctuate. Organize site photos, generation data, the timing of shading occurrences, and mitigation policies, and make sure everyone can discuss based on the same materials—this will make responses smoother. The seasonal movement of shadows is a visible phenomenon, yet it is a factor that is difficult to explain without records. That is why, as soon as you feel power generation is low, starting to record and compare early will be practically helpful.

Summary

The seasonal shifting of shadows that leads to reduced power generation is one of the causes that can be easily overlooked in the operation of solar photovoltaic systems. Even if the equipment itself shows no major faults, changes in the sun’s altitude can lengthen shadows and cause generation to drop only during specific seasons or times of day. In particular, from autumn to winter, when shadows from buildings, trees, mounting racks, or surrounding equipment begin to reach the panel surface, generation can fall short of expectations even on clear days.

The important thing is not to judge solely based on the fact that power generation is low. Instead of looking only at daily or monthly generation figures, combine and check time-of-day data, string-level data, on-site photos, and changes in the surrounding environment. If shading is the cause, you may see patterns such as output dropping during specific times of day, a greater reduction in certain seasons, or impacts concentrated on specific rows or circuits. Organizing this information makes it easier to distinguish between equipment faults, soiling, communication failures, and seasonal shading.

Also, checking for shading just once is not enough. It is important to keep seasonal records and create conditions that allow comparisons from the same position, the same orientation, and similar times of day. If you associate photographs with power generation data and store them, it will be easier to explain the sources of shading and the extent of their impact later. Even if the person in charge changes, it will be easier to hand over the situation, and you will be able to respond calmly to drops in power generation that occur at the same time each year.

Measures against shading need to be evaluated not only for restoring power generation but also in terms of safety, workability, rights and ownership, and maintainability. In some cases pruning trees or relocating obstacles may be effective, while in others it is more practical to strengthen recording and monitoring and manage with consideration of seasonal variations. The important thing is not to judge the presence or absence of shadows by intuition, but to be able to explain it by linking the site and the data.

To accurately trace the causes of low power generation, a perspective that manages on-site shading, generation data, and equipment layout together is indispensable. If you want to record seasonal changes in shading and establish operations that quickly identify the factors behind generation declines, it is effective to set up a management system that can continuously cross-check on-site inspection records, time-of-day data, and per-string data.