Five Revisions to Solar Power Generation Calculations Reflecting the Effects of Shadows

When calculating solar power generation, looking only at solar irradiance, panel capacity, and installation angle can result in a large discrepancy from the actual generated output. In particular, shading can manifest as a reduction in output even if it occurs only for short periods. Causes of shading differ from site to site—buildings, trees, utility poles, railings, adjacent equipment, mountains, and slopes. Furthermore, the same shadow can affect a system differently depending on the season and time of day, so simply treating it as "a little shading" may not be adequately reflected in generation calculations.

This article organizes five items that practitioners searching for "太陽光発電量 計算" should review when reflecting the effects of shading in power generation calculations. Rather than simply looking for reasons why output is low, it explains from a practical, user-friendly perspective where in the calculation conditions shading should be included, how thoroughly site information should be verified, and how to reconcile the calculations with actual measured values.

Table of Contents

• Organize shadows not merely by their presence or absence, but by time of day and by season.

• Review the positional relationship between the shadow-causing object and the panel surface

• Confirm decreased power output at the string or circuit level

• Correct the calculated loss rate by comparing it with measured values

• Retain the review conditions to include future changes in shading.

• Continuously improve solar power generation calculations that account for shading

Organize shadows not merely by presence or absence, but by time of day and season

The first thing to re-examine when handling shadows in solar power generation calculations is whether you are judging shadows with a binary "present" or "absent" decision. During on-site inspections, when you see part of a panel shaded, you tend to lump it together as "shaded." However, the impact on power generation varies depending on the time the shadow occurs, its duration, the season, the intensity of solar irradiance, and which part of the panel is shaded. If the shadow only extends during times of low solar altitude at dawn and dusk versus when it falls during periods of relatively strong solar irradiance, the effect on generation can differ even for the same one-hour period.

Particular attention should be paid to cases where, even though monthly solar irradiation conditions are used in annual generation calculations, shading conditions are handled as a constant annual loss rate. Because shading is governed by the sun’s path, the extent of shading changes between summer and winter. In winter, when the sun’s altitude is lower, shadows from buildings, trees, racking, and surrounding structures tend to extend farther. In summer, when the sun’s altitude is higher, those same obstacles may cast shorter shadows onto the panel surface. At sites where shadow positions change seasonally, calculating based only on an annual average can lead to overlooking wintertime declines or, conversely, overestimating losses through the summer.

Also, organizing by time of day is important. Solar power generation is not constant over the course of a day; output tends to increase during hours of strong sunlight. Therefore, a short shadow in the morning and a shadow around midday carry different weights in calculations even if they cover the same area. For example, if an adjacent building casts a shadow only during some morning hours, and the shadow clears during the main daytime generation hours, the impact on annual generation may be limited. Conversely, if part of a panel is continuously shaded around midday, even a visually small shaded area should be regarded as a factor that will reduce generation.

In practical work, it is important not to judge shadows solely by the date and time when they were observed, but to organize observations at least by representative seasons and times of day. Shadows extend differently in the mid‑spring or mid‑autumn period, near the summer solstice, and near the winter solstice. When conducting on‑site surveys, record the time the photo was taken, the orientation, the weather, and the sun's position so they can be determined later, making it easier to reflect them in power generation calculations. Even if only photographs remain, if the shooting time is unknown it is difficult to judge how representative those shadows are for the year.

When revising power generation calculations, first organize the patterns of shadow occurrence by month or season, and further by time-of-day categories such as morning, midday, and evening. Then consider the extent to which the periods when shadows occur contribute to power generation. Rather than treating all shadows with the same weight, carefully evaluating shadows that occur during time periods with high power output is important for improving calculation accuracy.

However, the more you try to examine the effects of shadows in detail, the more the lack of on-site information becomes a problem. If estimates of solar irradiance, panel layout, orientation, tilt, heights of surrounding structures, and so on are unknown, subdividing the calculations will be less likely to reflect reality. Therefore, rather than proceeding straightaway to advanced calculations, it is practical to carefully organize the times and extents of shadows that can be confirmed on site and decide the level of granularity that can be reflected in the calculation conditions. Don’t stop at simply whether shadows exist; separating when, where, and to what extent they occur is the starting point for review.

Review the positional relationship between objects causing shadows and the panel surface

To reflect the effects of shadows in power generation calculations, it is necessary to understand the positional relationship between the object casting the shadow and the panel surface. Even if shadows are visible on site, the countermeasures and how calculation conditions are set change depending on whether the cause is a nearby handrail, a distant building, trees, or terrain. Nearby obstacles tend to produce well-defined shadow boundaries and can sharply shade parts of the panel. Shadows from distant obstacles or terrain can affect a wide area depending on the time of day or season. Both can lead to reduced power generation, but the items to check for reflecting them in calculations are not the same.

First, what you need to check are the height, width, location, and distance from the panel surface of the shading objects. Building upstands, rooftop equipment, fences, lightning protection devices, piping, signs, and nearby trees may appear small on site but can cast long shadows when the sun is low. Especially on roofs and in confined sites, small protrusions that are easy to overlook at the time of installation can become sources of shade in the mornings, evenings, or during winter. In energy yield calculations, these should not simply be lumped together as surrounding obstructions; you must verify which rows and which panels they might cast shadows on.

Next, the orientation and tilt of the panel surface are important. Even with the same obstruction, the way shadows fall changes depending on the direction the panels face and their tilt. Installations that are close to south-facing, split east–west, have a shallow tilt, or are near a wall receive sunlight at different times of day. An east-facing surface will be more affected by morning shadows, while a west-facing surface is more likely to have problems with afternoon shadows. You need to review not only the objects causing the shadows but also which times of day the panel surface is primarily designed to receive sunlight.

Shading from trees is a particularly difficult factor to handle. The density of branches and leaves changes with the seasons, and their height and spread change as they grow. For deciduous trees, shade may weaken in winter, but fine shadows cast by the branches can still occur. For evergreen trees, shading effects tend to persist year-round and also vary depending on pruning conditions. When accounting for tree shading in power generation calculations, it is necessary to consider not only the current height but also planned maintenance and future growth. If tree shading is checked only once and set as a fixed condition, the gap between actual performance and calculations may widen after a few years.

Shadows cast by terrain and nearby buildings should not be overlooked. In mountainous areas, on slopes, or in urban areas where adjacent buildings are close, there can be periods when direct sunlight does not readily reach the panel surface even when the sun is out. Such shadows can be hard to notice from a brief on-site inspection because there may be no obstacles immediately adjacent to the panels. If actual power generation is lower than predicted in calculations, it is necessary to check not only the vicinity of the panels but also obstructions over a wider surrounding area. In particular, if the morning and evening ramp-up or drop-off occur earlier than predicted, there is reason to suspect the influence of distant terrain or buildings.

Also check whether the objects causing shadows are fixed or may be moved or altered. Temporary structures, parked vehicles, material storage areas, seasonal equipment, scaffolding from nearby construction, and the like may exist at the time of the survey but could change over the long term. Conversely, buildings or vegetation that do not currently exist may be added in the future. When revising power generation calculations, it is important to distinguish between permanent shadows and temporary ones. Overestimating temporary shadows as annual losses will make the calculations overly conservative. Conversely, treating permanently occurring shadows as temporary will leave a persistent gap between estimates and actual performance.

In this way, reviewing the positional relationship between the objects causing shadows and the panel surface is not merely an on-site check but a process of establishing the basis for the calculation conditions. If you can organize which obstacles affect which surface, at what time of year, and to what extent, it becomes easier to explain the loss conditions used in power generation calculations. To properly reflect the impact of shading, it is essential not only to look at the shadows themselves but also to understand the structures that create those shadows.

Check for decreased power generation at the string or circuit level

When re-evaluating the impact of shading in solar power generation calculations, it is important to consider not only the shadow on each individual panel but also the electrical connections. Solar panels may appear to generate power independently, but in reality multiple panels are connected in series and parallel to form a combined unit within the generation system. Therefore, shading on some panels can affect not only those panels but also the output of the group of panels connected to the same circuit. Even if the visibly shaded area is small, depending on the connection configuration the reduction in generation can appear large, so this point should not be overlooked in calculations.

If you consider the effect of shading purely in terms of the proportion of panel area, you can end up with discrepancies from actual performance. For example, assuming that because only a few percent of the total area is shaded the drop in power generation will be of the same magnitude is not sufficiently cautious. How output reduction manifests depends on which circuit the shaded area belongs to, at what times the shading occurs, and how it interacts with other panels in the same connection unit. It becomes especially difficult to evaluate using only simple area ratios when a narrow, elongated shadow crosses part of a panel or when a shadow moves across multiple rows of panels.

What you need to verify in practice is whether the panel layout diagram and the electrical wiring diagram match and are being managed consistently. Panels that appear adjacent on the site are not necessarily connected to the same circuit. Conversely, panels that are physically distant may belong to the same circuit. If you revise the power generation calculations without cross-checking the shading locations and the connection units, you may incorrectly determine the extent of the decline. When the performance is low only for a particular row or section, it is important to check not only the shading location but also which connection unit that section is included in.

Also, when checking actual power generation, it is useful not to look only at the total sum but to break it down by circuit, by equipment, and by section units as granularly as possible. Looking only at the total generation can cause localized drops due to shading to be masked by other normal sections. If there are patterns such as a particular section coming up slowly only in the morning, a particular section’s output falling only in the afternoon, or waveform disturbances affecting only a part of the system even on sunny days, these can be clues to suspect a relationship between shading and the connection configuration. When reviewing power generation calculations, it is important not only to adjust the loss rate using overall values but also to identify which unit is experiencing the decrease.

It is also important not to confuse declines caused by shading with those caused by equipment faults or soiling. Shading tends to occur in patterns linked to time of day and season, whereas equipment faults can cause persistent reductions on specific installations. Soiling varies with rainfall, cleaning, and the surrounding environment. Of course, these factors can overlap. Even when you think you have reviewed shading, connection failures, equipment-side control issues, soiling, degradation, or missing communications data may actually be mixed in. Before adding shading losses to power generation calculations, you need to look at the timing patterns and differences between sections to confirm whether the behavior is consistent with shading.

When viewed at the string or circuit level, the effect of shading is not only a matter of "where the shade is" but also "which electrical grouping the shade falls on." Without this perspective, you may include shading losses in calculations yet find they don't match actual results. To improve power generation calculations, it is important to link panel layout, connection configuration, and actual performance data, and to clarify how shading propagates to output.

Adjust calculated loss rates by comparing them to measured values

When incorporating the effects of shading into energy yield calculations, many practitioners express the condition as a loss rate. The loss rate is convenient for keeping calculations manageable, but if it is set without clear justification, it becomes difficult to explain the calculation results. In particular, shading losses tend to mix with other losses such as soiling, temperature, wiring, equipment conversion, and ageing, so shading can be overestimated or, conversely, treated as too small. When reviewing energy yield calculations, it is important not to continue using the desk-based loss rate as-is, but to cross-check and correct it against measured values.

The first thing to do is check actual performance data for conditions close to clear-sky days. On cloudy or rainy days solar irradiance fluctuates greatly, making it difficult to separate the effects of shading. Select days with clear skies and stable solar irradiance, and by checking the time variation of power generation you can more easily see the characteristics of shading-related reductions. For example, if there are trends such as output dropping at the same time every day, a delayed startup only in certain seasons, or only some sections showing lower output compared with surrounding equipment, these provide a basis for reflecting the effects of shading in the calculation conditions.

Next, when comparing predicted and actual values, it is important to look not only at daily totals but also at differences by time of day. Daily totals can partly mask a morning shortfall with favorable midday conditions, or make an afternoon decline less apparent. Because shading effects often appear unevenly across time periods, comparing by time of day reveals which calculation conditions should be reviewed for which hours. If actuals fall below predictions only in the morning, check for eastern obstructions or terrain; if they fall short only in the evening, check for western obstructions; if they fall significantly short in winter, check for shadows that extend at low solar altitudes—this helps clarify the likely direction of the cause.

When adjusting loss rates, it is preferable not to immediately apply a single large annual value, but to consider the periods and times of day when shading occurs. Reducing the value uniformly throughout the year will lead to underestimation even in periods with little shading. Conversely, if you consider only periods with heavy shading and apply the same loss to the entire year, the calculation may be lower than reality. If you can separate shading effects by month, season, and time of day, the resulting calculation will be closer to actual performance. Even if detailed calculations are difficult, at minimum specify the conditions under which losses occur—such as reductions in winter, reductions in the morning and evening, and reductions in specific zones—so that the results are easier to review later.

When reconciling with measured values, attention must also be paid to the reliability of the measurement data. If power generation data are missing, there are communication delays, aggregation units have changed, or the equipment unit does not match the calculation unit, shading losses cannot be corrected properly. If you adjust only the loss rate while the data remain incomplete, you may end up treating non-shading factors as shading effects. Before revising the power generation calculation, it is necessary to confirm the data acquisition interval, the presence of missing data, the scope of the target equipment, and consistency with the capacity used in the calculation.

Also, when using actual performance data, it is important not to judge based on a single year. If the weather in a given year differs greatly from the average, power generation will vary. If you want to assess the effect of shading but the data are mixed with weather differences, snowfall, yellow dust, soiling, downtime, and so on, adjustments to the loss rate will become unstable. If possible, check multiple years of data and focus on declines that repeatedly occur in the same season or at the same time of day. When operation has just started and there are few performance records, it is practical to carefully verify representative days and treat them as provisional correction conditions.

The loss rate used in calculations is a tool to simplify estimates of power generation. However, because the impact of shading varies greatly from site to site, simply using a generic value is not sufficient. By cross-checking with measured data and confirming the period, time of day, and extent when shading occurs, then applying corrections, power generation calculations will better reflect reality. The important thing is not merely to adjust the loss-rate number, but to be able to explain which site conditions that number represents.

Retain review conditions that include future changes in shading

In reviewing solar power generation calculations, it is necessary to consider not only current shading but also future changes in shading. Because generation equipment is used for a long period, shadows that were minor at the time of installation can become a factor in reduced power generation several years later. Shading conditions are not necessarily fixed: new nearby buildings, tree growth, addition of equipment, changes in land use, or development of adjacent land, among others. If you only record the conditions at the time of calculation and stop, it becomes difficult to track what changed when future actual performance diverges from the calculations.

Tree growth in particular is a typical factor for future shading. Even if shadows do not reach the panel surface at the time of installation, branches can grow and cast shade after a few years. If there are trees around the site, it is advisable to check not only their current height and position but also who manages them, the frequency of pruning, and their potential for further growth. If trees cannot be fully controlled, it is important to record, as a caution in the calculation conditions, that shading may increase in the future. Conversely, if regular pruning is planned, organize it together with the maintenance conditions so as not to overestimate shading losses.

It is also necessary to check for future changes to surrounding buildings and structures. In urban areas, around factories, warehouses, and farmland, the use of adjacent land can change. If new buildings or equipment are added, shadows may occur during times that previously had none. Of course, it is impossible to predict all future developments exactly. However, recording known plans, directions likely to be affected, and distances to site boundaries will make judgments easier during future reviews. Power generation calculations should not be treated as a one-time task; it is desirable to manage them on the assumption that they will be updated to reflect changes in site conditions.

Additions or modifications to the facility itself can also cause shading. When equipment is added on the roof, walkways or handrails are altered, monitoring devices are installed, or maintenance equipment is added in the surrounding area, the shadows on the solar panels can change. When carrying out construction or renovations around a power generation facility, a system is needed to confirm that the shading conditions assumed in the power generation calculations have not changed. Rather than searching for the cause after power output has declined, if the impact of shading can be checked at the stage of changes, it becomes easier to reduce discrepancies between calculated and actual performance.

To address future changes in shading, it is important to keep the conditions for reassessment documented. Record when (date and time) the on-site inspection was performed, what kind of obstructions were present in which directions, which season’s shading was assumed, which loss rate was adopted, and which areas were left unverified. If these records are not kept, then a few years later when the power generation seems low you will not be able to determine whether shading was anticipated from the start or whether shading increased afterward. By keeping not only the numerical values in the calculation sheets but also the underlying assumptions and the reasons for decisions, the power generation calculations will function as management documents during operation.

Also, past photos and inspection records are useful when conducting future reviews. Records taken from the same location and in the same direction make it easier to compare tree growth and changes in surrounding equipment. Comparing actual power generation data with records of shadows makes it easier to identify when a downward trend began. Reviewing shadows is not a task that is completed by a single site visit, but an ongoing process of accumulating records to understand changes.

When factoring future changes into calculations, it's also important not to adopt overly pessimistic assumptions. If you assume large shadows that may or may not occur in the future, the estimated power generation will be lower than necessary. Conversely, it is not appropriate to ignore trees that are clearly expected to grow or planned nearby construction sites. The important thing is to record confirmed conditions, possible conditions, and unverified conditions separately. This makes the explanation of the power generation calculation transparent and makes it easier to update the conditions later.

Continuously improve solar power generation calculations that account for shading

Calculations of solar power generation that reflect the effects of shading are not something you can review once and be done with. Site conditions, the surrounding environment, equipment condition, solar irradiance conditions, and operational data change over time. Therefore, it is important to regularly compare calculations with actual results and update the conditions as necessary. Especially when generation is lower than expected, confirming whether the shading conditions are appropriately reflected in the calculations—rather than immediately considering equipment replacement or major renovations—will lead to a more measured decision.

The first step in continuous improvement is to make the baseline calculation conditions clear. If panel capacity, azimuth, tilt, installation extent, loss conditions, solar irradiance conditions, and shadow assumptions are not organized, then even when comparing to actual results you won’t know what to review. If calculation conditions are scattered across multiple documents or old drawings do not match the current site, it becomes difficult to correctly account for the effects of shading. To use power generation calculations in practice, you need to align the conditions used in the calculations with the on-site situation and document them in a way that anyone can verify.

Next, it is important to decide how to view the performance data. Whether you only look at monthly generation, examine daily or hourly data, or can compare by section will affect how easily shadows can be detected. Because shadows tend to appear at specific times, looking only at monthly totals can make it difficult to identify the cause. In practice, a useful workflow is to check overall monthly trends and, when there are large differences, drill down into daily or hourly data. Regularly reviewing generation profiles on clear days makes it easier to notice when the timing of shadow occurrence changes.

Also, when evaluating the impact of shadows, it is important to combine on-site inspections with data verification. Looking only at the data can lead to mistaking causes other than shading for shadows. Conversely, merely observing a shadow on site makes it difficult to judge how much it affects annual energy production. By checking site photos, the time they were taken, panel layout, connection configuration, and generation performance together, you can grasp the shadow’s impact more realistically. Especially at sites managed by multiple people, it is important not to leave on-site observations as subjective notes only, but to keep them as records that can be verified later.

When reviewing power generation calculations, it is also important in practice to avoid making excessive assertions. Causes of low power generation may include not only shading but also differences in weather, soiling, temperature rise, equipment downtime, output control, the condition of wiring and equipment, missing measurement data, and other factors that can overlap. Just because shading is visible, it is not appropriate to attribute the entire decline to shade. It is necessary to identify the time periods and ranges where shading is likely to have an impact, and to adjust calculation conditions while separating out other factors. This approach enhances the reliability of power generation calculations.

When explaining calculations that reflect the effects of shadows to internal teams and stakeholders, you should show not only the technical figures but also the flow of your reasoning in a clear way. If you can explain which shadows you checked, which time periods you consider they affect, and which loss assumptions you reflected them in, confidence in the calculation results will increase. Simply stating "shading losses were assumed" does not make the basis clear when reviewed later. By linking site photos, layouts, times, and actual performance deviations in your explanation, it becomes easier to share the validity of the calculations.

Ultimately, the key to calculating solar power generation that accounts for shading is not to separate the calculations from the site. You need to do more than estimate generation from irradiance and system capacity; you must understand what shadows actually exist on site, when those shadows occur, what area they cover, and which connection units they affect. By continuously updating calculation conditions while comparing them with actual performance data, generation forecasts will more closely reflect reality.

The impact of shading is visible to the eye, but it is an element that needs to be organized before being incorporated into power generation calculations. Rather than judging only by the presence or absence of shade, reviewing it in combination with time of day, season, obstructing objects, panel layout, electrical connection configuration, actual performance values, and future changes will increase the explanatory power of the calculation results. If you want to use solar power generation calculations for site management and improvement decisions, it is important to continuously record shading information and link the numerical data with current conditions. If you want to efficiently carry out shading checks and reviews of generation calculations, establish a system that allows centralized verification of site records, generation performance, and design conditions, and operate it so that conditions can be updated regularly.