5 Key Points to Assess the Impact of Shadows in Solar Power Output Calculations

Table of Contents

• Basics to grasp before assessing the effects of shadows

• Point 1 Read shadows by time of day, not simply by whether they exist

• Point 2 Use winter shadows as a baseline when considering annual impact

• Point 3 Separate partial shading from effects on the entire system

• Point 4 Evaluate shadows together with orientation and angle

• Point 5 Read shadows not only by annual values but also by month and time of day

• Steps practitioners follow to estimate the impact of shadows

• Common calculation errors resulting from misreading shadows

• Summary

Basics to Know Before Reading About the Effects of Shadows

In calculating solar power generation, attention tends to focus on installed capacity and local solar irradiance conditions, while the effects of shading are often neglected. However, one of the reasons generation forecasts are prone to variability in practice is precisely how shading is handled. Even with the same installed capacity, actual generation can vary considerably depending on the positional relationships of nearby buildings, trees, fences, rooftop equipment, railings, antennas, and so on. Moreover, shading is not simply a matter of presence or absence; it matters when, on which surface, and to what extent the shading occurs.

The first thing to clarify is the difference between kW and kWh. kW refers to the output capacity of an installation; figures such as 5 kW or 10 kW indicate the size of the installation. In contrast, kWh is the amount of electrical energy actually generated over a given period. Considering the effect of shading is not about changing the installation’s capacity itself, but about estimating how much it reduces the kWh that the installation would have otherwise produced. In other words, shading is not the entry point of generation calculations, but an important correction factor to bring the theoretical value from the entry closer to the actual value.

To correctly assess shadows, it is important to move away from intuitive judgments. Common vague judgments are: there is only a little shadow so it’s probably fine; it’s only in the morning so it can be ignored; the tree is a bit far away so it’s not a problem. However, power generation is determined by accumulation over the year. Even a slight shadow at the same time every day can create a larger difference over the year than you expect. Conversely, a shadow that looks large may actually be limited to certain seasons or short time periods, in which case its impact can be overestimated. In other words, you should not decide based on appearance; you need to assess shadows by time, area, and season.

Also, the impact of shading does not occur in isolation. It is linked to orientation, roof pitch and racking angle, equipment layout, and the timing of demand. For example, an east-facing surface that is shaded in the morning and a west-facing surface that is shaded in the afternoon have different implications even if the shadow is the same. Some cases experience shading only at the low solar altitude in winter, while others have light shading every morning throughout the year. Shading assessment becomes accurate only after understanding the geometric conditions of the site where the equipment will be installed.

For practitioners, the important thing is not to treat shading as all-or-nothing. You need to consider how much the output will fall compared to theoretical values and be able to explain the reasons. This article sequentially lays out five key points to keep in mind for that purpose. Once you can correctly interpret the impact of shading, estimates of solar power generation become much more practice-oriented.

Point 1: Interpret shadows by time of day, not simply as present or absent

The first point when assessing the impact of shading is to evaluate it by time of day rather than simply as present or absent. Solar power generation is not produced evenly throughout the day. Sunlight conditions differ in the morning, midday, and evening, and the effect on generation depends on when the shade occurs. Therefore, treating shading collectively as "a little" can easily lead to misunderstanding the actual impact.

For example, there is a difference between a shadow that only falls in the morning and one that falls around noon. The morning is a time when power generation is just ramping up, while around noon is the period when generation tends to be strongest that day. In other words, even with the same length of shadow, the reduction in kWh depends on which time of day it occurs. The same applies in the afternoon: on west-facing surfaces, afternoon shadows tend to have a larger impact, while on east-facing surfaces, morning shadows tend to have a stronger effect.

A common assumption in practice is that it won’t be a major problem because the shadow only intrudes slightly onto the edge of the building. However, if that shadow coincides every day with the periods when generation tends to be high, the annual difference can be considerable. Conversely, even if shadows are visible at certain times, if the generation during those times is small the annual impact may be limited. In other words, it’s more important how the shadows overlap with generation time periods than the mere presence of shadows.

Therefore, in practice you first check the time windows when shadows occur and see how they overlap with the system's peak power output, which makes it easier to organize the analysis. For projects that have shadows only in the morning, only around midday, or only in the evening, the approach to adjustment differs even for the same installed capacity. When calculating annual generation, whether this time-of-day consideration is included can significantly change the accuracy of the figures.

Also, when considering self-consumption, the significance of the time of day becomes even greater. For example, if a factory or office has a high operational load in the morning, morning shading is likely to lead not only to a reduction in power generation but also to a decrease in the self-consumption rate. Conversely, if demand is high in the afternoon, afternoon shading on the west-facing side becomes more important. In other words, reading shading by time of day helps assess not only power generation but also the usability of the installation.

When you look at a shadow, consider from about what time to what time, on which surface, and how much it affects. Simply having this habit will significantly improve the quality of power generation estimates. Read shadows not as a matter of presence or absence, but by time of day. This is the first important point.

Point 2: Use winter shadows as the reference to assess year-round impact

The second point is to consider the year-round effect using winter shadows as the reference. When checking shadows on site, people sometimes only look on clear, easy-to-see days or at times that are convenient and conclude that "shadows don't seem to be a problem." However, because the sun's altitude changes with the seasons, shadows are longer in winter and shorter in summer. In other words, if you don't check shadows in winter, your assessment tends to be overly optimistic.

In particular, on Japanese buildings and sites, the low winter sun angle can cause shadows from neighboring buildings, trees, fences, and rooftop equipment to extend much farther than expected. Obstacles that were of no concern in summer can cast shadows for long periods in winter. Therefore, when calculating annual power generation, it is important to pay attention to winter shading so you do not underestimate its impact.

However, what you should be careful about here is not to look only at winter shadows and conclude that the entire year is equally bad. Winter is a benchmark for understanding the harshest conditions, and treating it directly as year‑round conditions is another mistake. The correct approach is to determine how much stronger the shading becomes in winter and to consider how much that effect reduces monthly power generation. If it falls sharply only in winter, the impact on the annual average may be limited; conversely, if it coincides with important winter months, it can be highly significant for that project.

In practice, in addition to checking winter shading, being aware of which months and to what extent it has an effect makes your perspective on annual energy production more realistic. If you only look at annual kWh, winter shading tends to be obscured, but if you break it down into monthly generation, the difference becomes quite clear. Especially for facilities and homes with high winter demand, overlooking this difference can easily lead to a misvaluation of the equipment.

Winter shadows are also closely related to orientation and slope. In conditions that are easily affected by the low sun—such as east-facing surfaces in the morning, west-facing surfaces in the evening, and north-facing surfaces—it is advisable to carry out careful winter assessments. On sites with roof pitch or elevation differences, these variations tend to be even more pronounced. In other words, winter shadows are not simply a harsh seasonal condition but an important factor for confirming the appropriateness of equipment placement.

When estimating annual power generation, using winter shading as the reference makes it easier to avoid overly optimistic estimates. On that basis, consider how to translate conditions that occur only in winter into the annual calculation. If you can adopt this two-step perspective, the accuracy of shadow correction is greatly improved.

Point 3: Consider partial shading separately from its impact on the entire installation

The third point is to consider partial shading separately from its impact on the installation as a whole. When people think of shading, they tend to imagine situations where the entire installation becomes significantly darker, but in practice it is more common for shading to affect only a portion. For example, the shadow on a rooftop installation may fall on only a particular row, the shadow of an adjacent building may cover a few panels at the edge, or the tips of tree branches may cast shadows on part of the array. In such cases, the appearance of the shadow can lead you to overestimate a severe degradation of the whole installation, or conversely to dismiss it because it affects only a small area.

In power generation calculations, it is important to separately examine where, how much, and during which time periods this partial shading occurs. Because not all panels receive the same shading, applying a uniform shading condition to the entire system tends to result in either overestimation or underestimation. This is because the effect on the annual output differs depending on whether only the edge of the south-facing side is shaded, only a single row on the west side, or just a few panels in the morning.

Also, partial shading is related to layout and equipment configuration. If you calculate by each surface, you can apply a slightly lower correction only to the shaded surface, and if necessary you can further focus on the parts of that surface where the shading impact is strongest. Conversely, if you view the entire installation only as a single total capacity, you lose visibility of where and how much it is being affected. In practice, simply dividing at least into south-, east-, and west-facing surfaces makes a considerable difference.

The reason this point is important is that if you misjudge partial shading, your assessment of the entire installation can be distorted. For example, in a project where only a few panels at the edge are shaded, concluding that the whole installation is disadvantaged will underestimate the attractiveness of a project that would otherwise be viable. Conversely, if you ignore it because it only affects a small part, you may overlook the cumulative effect of shading that repeats every day. Both are errors you want to avoid in practice.

To separate partial shading from the whole system, it's clearest to first pinpoint "where the shade falls" in terms of position, then "when it falls" in terms of time, and finally consider how much that affects the total kWh. By viewing shading in both space and time, the impact on the entire system becomes much easier to assess.

In other words, the effects of shading do not necessarily apply uniformly to the entire installation. Distinguishing between partial shading and overall effects is an important point for improving estimation accuracy.

Point 4 Evaluate shadows in conjunction with orientation and angle

The fourth point is to evaluate shadows together with orientation and tilt. Even if a shadow appears to occur independently, it is actually strongly linked to the orientation and tilt of the equipment surface. A morning shadow on an east-facing surface and an evening shadow on a west-facing surface have different implications, and on a steeply sloped surface versus a gently sloped one the same obstacle will cast shadows differently. Therefore, treating shadows in isolation makes it easy to misjudge how they will affect the system's overall power generation.

For example, in a project where the east-facing side receives heavy morning shading, that side’s inherent strength—morning generation—is reduced. If the west-facing side experiences heavy afternoon shading, the value of the west-facing side decreases. In other words, the impact of shading carries different weight depending on which orientation is affected. Simply applying a uniform shading correction based on a south-facing standard will not reveal this difference.

Also, the interaction with angle is important. A steep roof pitch can cause shadows to extend more relative to the sun’s elevation in winter, and for ground-mounted installations a large racking tilt can make inter-row shading more likely. In other words, shadow assessment cannot be done with a plan view alone; a three-dimensional perspective that includes the tilt of surfaces is necessary. If you rely on intuition alone here, discrepancies with actual performance can be significant.

In practice, it is easier to think about this by first organizing the orientation and tilt of each surface, and then layering in the effects of shading. Separate the system capacity for the south-, east- and west-facing sides, and simply evaluating how shading affects each side will already yield figures that are quite suitable for practical use. If necessary, you can further subdivide by row or by block. The important point is not to treat the entire installation under a single, uniform shading condition.

Also, this way of thinking extends to how equipment is used. For example, at a facility with high afternoon demand, evening shading on west-facing surfaces may need to be weighted more heavily than the annual total generation. If morning demand is high, morning shading on east-facing surfaces is important. In other words, by considering orientation, angle, and shading together, you can more easily interpret not only power generation but also self-consumption and operational value.

To evaluate shadows correctly, it is important not to ignore orientation and angle. Shadows do not just fall on equipment; they are also a factor that reduces the times of day when that equipment’s value is realized. With this perspective, the accuracy of power generation calculations improves considerably.

Point 5: Assess shadows not only by annual values but also by month and time of day

The fifth point is to assess shading not only on an annual basis but also by month and by time of day. Annual generation figures are useful for getting a sense of a system’s outline, but they can be too coarse to understand the effects of shading. This is because shadows often occur concentrated in particular seasons or times of day. If you average everything into an annual total, it becomes difficult to see which months and which times of day show differences.

For example, in a project where shading is strong only on winter mornings, the annual total might appear to differ by only a few percent. However, if those winter mornings are important for the project, the practical significance is much greater. Conversely, even if it seems to have some effect over the year, if it only affects months or time periods that are not important, it may not be that significant from an equipment evaluation standpoint. In other words, you should consider not only the annual total of shading but also when it has an effect.

Viewed by month, the seasonal differences in shading become easier to understand. In winter, the sun's altitude is low and shadows are longer, so losses tend to be more pronounced. Spring and autumn are intermediate, and in summer the impact of shading can be relatively weaker. Organizing these differences by month makes effects that were not visible in annual values become clear. If you are considering forecasts for self-consumption or power sales, examining monthly impacts is far more practical for operational use.

Evaluating shadows by time of day is equally important. For east-facing installations, morning shadows; for west-facing installations, afternoon shadows; and for south-facing installations, shadows around noon tend to be particularly significant. Because generation time periods directly affect how generation overlaps with demand, time-of-day shading has implications beyond the mere reduction in energy output. For example, even a difference of only a few kWh in generation can greatly affect economics and the self-consumption rate if it occurs during high-demand periods.

In practice, it is clearer to first grasp the outline of a system using annual values, then break it down by month, and, if necessary, by time of day. If you want to interpret the impact of shading in a truly usable way, this breakdown is indispensable. It is crucial for maintaining the accuracy of estimates not to judge the performance of equipment solely on annual values.

Procedure for Practitioners to Estimate the Impact of Shadows

When practitioners estimate the impact of shading, deciding on an order makes it much easier to organize. The first thing to do is to sort out the orientation and angle for each surface where equipment will be mounted. Divide the surfaces into south-facing, east-facing, west-facing, and northward-facing surfaces, etc., and check each one's capacity and slope. If this remains unclear, it becomes difficult to see where and how to apply the effects of shading.

Next, identify the obstacles that cause shading. Nearby buildings, trees, fences, rooftop equipment, antennas, upstands, etc., can all be shading factors at a site. The important thing is not simply to list them, but to organize them with attention to which surface they affect, at what times of day, and in which seasons. If you take winter conditions into account here as well, the annual outlook becomes considerably more stable.

After that, consider partial shading and whole-system effects separately. Determine whether the shading affects the entire installation or only certain rows or individual panels, and think about how to reflect those differences in the corrections. If you assess the entire installation under a single shading condition, it can easily lead to either an overestimation or an underestimation. If possible, it is ideal to organize the assessment by surface and by column.

Next, read that impact by month and by time of day. If you organize whether it only affects winter mornings, only afternoons year‑round, or is only a problem in spring and autumn, the effect on annual kWh becomes clear. Furthermore, for projects that consider self‑consumption and selling electricity, looking at how those time periods overlap with demand makes the significance of the shading much clearer.

Finally, reflect these shading effects in the power generation calculation as correction coefficients. It is important to quantify, in numbers, how much the output will drop compared to the theoretical value and to be able to explain the reasons. In practice, it is crucial not just to have the numbers but to be able to say why those corrections were applied. Simply having this series of procedures in place will make shading assessment considerably more stable.

Calculation Errors Commonly Caused by Misreading Shadows

There are several typical calculation errors that commonly arise from misreading shadows. The most common is judging only whether a shadow is present or not. If you try to apply corrections to power output based solely on subjective expressions like "there’s a little shadow but it seems okay" or "there’s almost no shadow," the numbers become quite coarse. In reality, what matters is what time the shadow occurs, in which season, and on which surface it falls.

Another common mistake is judging the presence or absence of shadows based only on on-site checks in summer. Because the sun’s altitude is high in summer, shadows appear shorter than in winter. Therefore, even if there seems to be no problem in summer, it can have a large impact in winter. Reflecting this in the annual power generation without checking winter conditions tends to produce particularly large discrepancies with actual winter performance.

You can also go too far in underestimating partial shading. If you assume it’s only a few panels at the edges so it won’t matter much, you’re likely to overlook the cumulative effect of shading that repeats every day. Conversely, it’s also wrong to conclude that the entire installation is significantly degraded just because there is shading in one part. Failing to distinguish between partial shading and overall impact makes both types of mistakes more likely.

Furthermore, it is common to think of shadows separately from orientation and angle. Morning shading on an east-facing surface and afternoon shading on a west-facing surface may be the same shadow, but they have different value. If you correct based only on the annual total without considering overlap with demand periods, you can easily misjudge the usability of the system. There are implications that cannot be seen from differences in annual kWh alone.

To prevent such mistakes, it is important to develop the habit of considering shadows in relation to time, season, surfaces, and demand. If shadows are seen not as isolated obstacles but as conditions that interact with equipment and operations, the accuracy of calculations will improve considerably.

Summary

The points for assessing the impact of shading in solar power generation calculations can be organized into five: reading shading by time of day rather than as simply present or absent; using winter shading as the baseline when considering annual effects; separating partial shading from impacts on the entire installation; evaluating orientation and tilt together; and examining not only annual values but also monthly and time-of-day values. Each of these may seem simple, but they are highly effective perspectives in practice.

The effect of shading, unlike installed capacity or regional differences, depends heavily on the positional relationships at the site. That’s why handling it based only on desk-based assumptions can cause power generation forecasts to vary widely. In practice, even with the same installed capacity, the meaning of annual kWh changes depending on the location, orientation, and angle of obstacles and the overlaps in time of day. Whether shadows are read correctly greatly affects the reliability of the estimates.

Especially if you want to properly reflect partial-shading assessments, winter shadow estimates, and differences between surfaces, accurately capturing on-site conditions is essential. Elevation changes, obstacle locations, and variations in proposed equipment positions — which can be difficult to discern from drawings or aerial photos alone — can greatly alter how shadows fall. If you truly want to improve the accuracy of power generation calculations, increasing the accuracy of the input conditions is the most effective approach.

In that regard, LRTK, an iPhone-mounted GNSS high-precision positioning device, is extremely effective as a means of accurately grasping spatial relationships on site. Because it makes it easier to accurately record candidate equipment locations and the positions of surrounding obstructions in the field, it becomes easier to interpret how shadows fall in a way that is closer to reality. If you want the impact of shading in solar power generation calculations to be figures you can actually use, properly capturing on-site positional relationships with a method like LRTK is a major practical advantage.