How to Determine Roof Orientation in Solar Power Generation Simulations

Table of Contents

• The purpose of checking roof orientation in solar power generation simulations

• Basic effects of roof orientation on power generation

• Points missed when judging by south-facing only

• How to evaluate east- and west-facing roofs

• Cautions for north-facing roofs and complex roof shapes

• How to read roof slope and orientation together

• How to check shadow effects together with roof orientation

• How to organize input conditions used in practice

• Criteria for judgment when viewing simulation results

• Precautions to avoid mistakes in design decisions by roof orientation

• Why on-site inspection and positional accuracy become important

• Summary

The purpose of checking roof orientation in solar power generation simulations

The purpose of judging roof orientation in solar power generation simulations is not simply to decide “south-facing is good” or “north-facing is bad.” In practice, it is necessary to determine which roof surface is most reasonable to install on by considering roof orientation, slope, area, surrounding shadows, the number of panels that can be installed, how electricity is used, and future operation and maintenance. Especially for houses and small facilities, roof shapes are often divided into multiple surfaces rather than being a single simple plane, so it is important to use generation simulations to compare differences for each roof surface.

If roof orientation is judged only by intuition, you may overlook east- and west-facing surfaces that can generate sufficient power, or conversely place excessive expectations on surfaces that do not produce much power. Solar power systems are used for long periods, so early-stage judgment errors affect annual generation, self-consumption, surplus electricity, payback plans, and maintenance plans. Therefore, in simulations it is important not to treat roof orientation as just one input item but to interpret it as a key axis of the overall design judgment.

Also, evaluating roof orientation is not only for maximizing generation. For example, if a building uses a lot of electricity during the day, a south-facing surface that generates well around noon may be advantageous. Conversely, for buildings with high morning or evening usage, east- or west-facing roofs can be meaningful. In commercial facilities, operating hours matter; in residences, at-home time and how electrified equipment is used change the value of the same generation amount. In other words, when judging roof orientation in solar power generation simulations, it is important to look not only at the annual generation numbers but also at when generation occurs, where it is consumed, and which surfaces generate stably.

Basic effects of roof orientation on power generation

Roof orientation directly affects the irradiance solar panels receive. Generally in Japan, south-facing roofs tend to receive more sunlight throughout the year and thus tend to have higher generation. This is because the sun rises in the east, travels through the southern sky, and sets in the west. If panels are installed on a south-facing roof at an appropriate slope, they can efficiently receive light over a long period during the day.

However, generation is not determined by orientation alone. If the roof slope is shallow or too steep, the impact of orientation changes. On gently sloped roofs, sunlight can be received even if the roof is not exactly south-facing, reducing differences between east and west. Conversely, on steep roofs, orientation differences are more likely to affect generation. Especially in winter, when solar altitude is low, the way a roof surface receives sunlight changes greatly depending on its orientation and angle.

In simulations, it is common to input roof orientation as an angle: using true south as a reference, set how far it is rotated to the east or west, and combine this with roof slope to calculate annual generation. In practice, you should not judge only by the orientation symbol on drawings; confirm the building’s actual orientation, each roof surface’s orientation, and relationships with surrounding obstacles. Drawings’ orientation marks may be old, simplified, or not aligned between the site plan and the roof plan, so trusting results without checking simulation assumptions is risky.

An important point in understanding the effect of roof orientation is not to think of orientation superiority as fixed. While south-facing is often advantageous, a wide, low-shadow east- or west-facing surface can outperform a small south-facing area in total generation. Also, if the south face has many projections or shadows, choosing another surface that allows neat installation can yield more stable generation. Simulations should be used as a tool to compare multiple conditions numerically.

Points missed when judging by south-facing only

When considering roof orientation for solar power, many people initially prioritize south-facing. Indeed, south-facing tends to be advantageous for generation and often yields high simulation results. However, practitioners should be careful not to decide optimality solely because a surface faces south. In actual designs, south-facing roof surfaces may be narrow, have complex shapes making panel placement difficult, have many ridges, valleys, vents, or equipment, or receive shadows from surrounding buildings.

A common oversight with south-facing roofs is that even if solar conditions are good, installation efficiency can be poor. If the roof surface is broken into small parts, trying to force panels on may reduce the number of rows or leave insufficient maintenance space. Placing panels too close to roof edges can cause problems with wind loads and constructability. Even if simulations favor installing many panels, that is meaningless if construction conditions cannot be met in reality.

Furthermore, south-facing roofs tend to have large peak generation during the day, which may not match the building's consumption pattern. For residences that are often unoccupied during the day or facilities not operating on weekends, much of the generated power may not be used on site, resulting in excess. While there are uses for surplus electricity, if self-consumption is prioritized, you need to consider not only total generation but also generation timing. In judging roof orientation, do not just select the surface with the highest annual generation; confirm how well the building’s power demand matches the generation curve.

When evaluating south-facing roofs, it is easier to judge by checking simulation results by month, by time of day, and by roof surface rather than relying on a single number. A roof may generate well in summer but suffer large shadows in winter. Even if generation around noon is high, it may not match morning or evening demand. Thus, while south-facing is a strong candidate, it is not always the optimal solution. In practice, use south-facing as a baseline and compare with east- and west-facing and other surfaces to determine the most reasonable overall layout.

How to evaluate east- and west-facing roofs

East- and west-facing roofs tend to have lower annual generation than south-facing ones. However, accurate comparison through simulations shows that east- and west-facing roofs can still have significant value. Especially when a roof surface is wide, has few shadows, and allows orderly panel placement, it can outperform a small south-facing surface in total generation.

East-facing roofs generate more in the morning and late morning. For homes with high morning electricity use or offices, stores, and factories that operate from the morning, east-facing generation may match self-consumption well. West-facing roofs generate more in the afternoon and early evening and can be valuable for facilities that operate until late afternoon or buildings with high afternoon cooling demand. In summer, when afternoon air-conditioning loads can be large, west-facing generation can coincide with demand.

When evaluating east- and west-facing roofs, do not judge them only by simple annual generation differences with south-facing. Even if annual generation is slightly lower, a dispersed generation timetable can increase usable on-site energy. Concentrating only on south-facing surfaces can create large midday peaks and more surplus, while combining east and west surfaces tends to flatten the generation curve from morning to evening. This characteristic is important when self-consumption rate is a priority.

Also, consider the ratio of use between east and west faces. If morning demand is high, allocate more to the east; if afternoon demand is high, allocate more to the west. In simulations, separate the generation per roof surface and examine not just aggregate values but time-of-day output trends. This allows judgment from the perspective of “does it fit the building’s usage” rather than simply “east/west is inferior to south.”

Cautions for north-facing roofs and complex roof shapes

North-facing roofs are generally considered unfavorable for solar power. Because the roof surface faces away from the sun that travels through the southern sky, it receives little direct sunlight and annual generation tends to be low. Steep north-facing roofs in particular can see a significant decrease in generation. Therefore, when evaluating north-facing roofs in simulations, you should carefully verify not only generation but also investment decisions, construction risks, reflection impacts on surrounding areas, and ease of maintenance.

However, north-facing roofs should not be automatically excluded. If the roof slope is very shallow, if installation capacity must be secured due to building use, or if other surfaces are unusable due to shadows or equipment, the north-facing surface may be a candidate. The important point is not to overestimate generation on a north-facing surface. Enter orientation and slope correctly in simulation conditions and check how much generation drops month by month and whether there is an extreme decline in winter.

In complex roof shapes, orientation may not be represented by a single value. In hip roofs, gable roofs, stepped roofs, or facilities where multiple roof sections are connected, each surface has different orientation and slope. In such cases, representing the entire building with one orientation can make simulation results diverge from reality. It is desirable to separate roof surfaces for input and set orientation, slope, installable area, and shadow conditions for each.

In complex roofs, the number of panels that can be arranged also greatly affects generation. Even with the same orientation, conditions such as a roof surface being close to triangular, having many steps, or containing obstacles limit the number of panels that can be placed efficiently. If you input only theoretical installation capacity in simulations, generation may appear larger than reality. In practice, confirm usable dimensions for each roof surface and calculate based on capacities that assume constructible layouts. The more you deal with north-facing or complex roofs, the more consistency between site conditions and input conditions determines result reliability.

How to read roof slope and orientation together

When judging roof orientation, you must always check roof slope along with orientation. Roof slope describes how much a roof surface tilts relative to the horizontal plane. Because panel generation changes depending on the angle at which sunlight is received, results differ even for the same south-facing surface if slopes differ. Similarly, for the same east-facing orientation, a gently sloped roof and a steep roof show differences in annual generation and seasonal generation trends.

Generally, on gently sloped roofs the differences caused by orientation tend to be smaller. Near-horizontal surfaces receive a reasonable amount of light regardless of whether the sun is to the east, west, or south. Conversely, on steep slopes the effect of orientation is stronger. A steep south-facing surface tends to receive more sunlight, while north-facing or heavily east/west-tilted surfaces see generation vary greatly by time of day and season.

In simulations, comparing multiple patterns that combine orientation and slope is effective. For example, calculating an annual generation for a steep south-facing surface, a gentle east-facing surface, and a wide west-facing surface may yield results that differ from simple orientation impressions. Check not only annual generation per roof surface, but generation per unit capacity to compare surface efficiency. Large-capacity surfaces tend to produce more total generation, so separate efficiency and total volume when evaluating.

Also, roof slope affects rain and dirt runoff, snow retention in snowy regions, and maintainability. Though generation may be acceptable on gentle slopes, continued soiling in certain environments may cause long-term generation decline. While steep slopes sometimes improve generation conditions, they also increase complexity of construction and maintenance. Judging roof orientation becomes more realistic by combining simulated generation with on-site constructability and maintainability.

How to check shadow effects together with roof orientation

When judging roof orientation in solar power generation simulations, you cannot separate shadow effects. No matter how good the orientation or slope, generation will decrease if shadows are cast by surrounding buildings, trees, utility poles, chimneys, rooftop equipment, or offsets in adjacent roofs. Shadows especially lengthen in winter and during morning and evening, and even a shadow on part of a roof surface can affect generation.

In shadow assessment, it is important to check changes throughout the year. Even if shadows are minor during summer daytime, substantial shadows may appear in winter mornings or afternoons. East-facing roofs face morning shadows, west-facing roofs face afternoon shadows, and south-facing roofs are susceptible to midday shadows from southern obstacles. Since the times when shadows frequently occur differ by roof orientation, simulations should make time-of-day shadow conditions as realistic as possible.

In practice, drawings often do not fully capture shadows. Surrounding building heights, distances from site boundaries, tree growth, and rooftop equipment locations are often discoverable only through on-site inspection. Especially for installations on existing buildings, it is not uncommon for as-built conditions to differ from past drawings. Even with correct roof orientation input, lax shadow assumptions make results optimistic. Conversely, overestimating shadows may exclude roof surfaces that are actually usable.

When judging shadow impacts, do not assume the entire roof surface generates under identical conditions. If only part of a roof is shaded, the impact may spread depending on panel arrangement and electrical groupings. In the design stage, avoid shaded parts in layout, prioritize low-shadow surfaces, and separate generation tendencies by surface. The goodness of a roof orientation should be judged not by theoretical values assuming no shadows, but by how stably that surface can actually receive sunlight.

How to organize input conditions used in practice

To correctly judge roof orientation in solar power generation simulations, organizing input conditions is indispensable. In practice, you collect multiple pieces of information before calculation: roof orientation, roof slope, roof surface area, installable range, surrounding shadows, panel layout, system capacity, and regional irradiance conditions. If any of these are left ambiguous, the numbers may look attractive but the reliability of the judgment declines.

First confirm the building’s exact orientation. Even if drawings have orientation symbols, the building’s actual orientation may not match exactly. Cross-check site plans, roof plans, aerial photos, and on-site positioning data and organize orientations for each roof surface. Especially when a building is slightly rotated east or west of true south, that difference influences generation timing. Rather than broadly treating it as south-facing, it is desirable to capture the angle as precisely as possible.

Next, check slope and usable area for each roof surface. Roof slope may be readable from drawings, but for renovated buildings or extensions on-site confirmation may be necessary. For usable area, consider not the simple roof area but the range where panels can be safely installed. Reflect real usable ranges in input conditions by considering roof edges, ridges, valleys, snow stops, inspection spaces, rooftop equipment, and allowances required by laws or construction standards.

It is also important not to treat different conditions for each roof surface collectively. If there are south, east, west, and north faces, organize orientation, slope, shadow conditions, and installable capacity separately for each. Configure simulations to output per-roof-surface results so that later design changes or capacity adjustments are easier to evaluate. For example, an initial plan might use south and west faces, but if shadows significantly affect the west side you can more easily consider reducing west and increasing east.

When organizing input conditions, keep the basis for numbers documented. Record which source was used to confirm orientation, whether slope is from drawings or on-site verification, what period was assumed for shadows, and which constraints were considered for installable area—this helps with internal and client explanations and design changes. Simulations are not a one-time task; refine accuracy by updating conditions. Organizing input conditions makes roof orientation judgments reproducible.

Criteria for judgment when viewing simulation results

When viewing simulation results by roof orientation, do not focus too much on annual generation alone. Annual generation is an easy-to-understand metric, but it does not fully judge the quality of a roof surface. In practice, check generation per roof surface, generation per unit capacity, monthly generation, time-of-day generation trends, reductions due to shadows, and compatibility with self-consumption.

First, look at generation efficiency for each roof surface. Large roof surfaces may appear to have high total generation but may also require large installation capacity. Comparing generation for the same capacity makes it easier to evaluate the roof surface conditions. Even if the south face has high efficiency but small capacity, combining east and west faces may secure more generation for the whole building.

Next, check monthly generation. Even if annual differences seem small, some surfaces drop significantly in winter. In winter solar altitude is low and shadows lengthen, so orientation and surrounding conditions have a greater impact. For facilities where heating and lighting increase power use in winter, winter generation decreases can affect business plans. When judging roof orientation, check not only annual averages but how much can be secured in months with low generation.

Time-of-day generation trends are also a major practical decision factor. South-facing roofs concentrate generation around noon; east-facing peaks in the morning; west-facing peaks in the afternoon. Which roof surface is valuable changes with the time-of-day electricity demand. For example, south-facing is clearly advantageous for facilities with large midday consumption, but east-facing can be better for operations starting early, and west-facing for buildings with large afternoon cooling loads. If self-consumption is prioritized, look at whether generation occurs at usable times, not only its amount.

Also, simulation results contain uncertainties in assumptions. Irradiance varies by year, and temperature, soiling, snow, aging, equipment stoppages, and changes in the surrounding environment affect actual generation. Therefore, avoid making definitive judgments on small numeric differences between roof surfaces. When differences are minor, include constructability, maintainability, future roof renovation, ease of electrical wiring, and lower shadow risk in the overall decision. Simulations are not precise predictions but comparison tools to improve design decisions.

Precautions to avoid mistakes in design decisions by roof orientation

A frequent failure when making design decisions by roof orientation is prioritizing simulation numbers while neglecting on-site conditions. A roof surface that looks like high generation may be difficult to construct on, hard to inspect, likely to require removal for future roof renovation, or vulnerable to strong winds or snow—making it disadvantageous in the long term. Since solar systems are used for a long time after installation, it is important to consider operational aspects in the design stage.

For south-facing roofs, be careful not to over-concentrate panels simply because generation looks high. Trying to fit as many panels as possible and pushing to roof edges can affect constructability, safety, and maintainability. Also, placing panels near rooftop equipment may later create shadow or inspection path problems. Do not aim for maximum capacity simply because a face is south-facing; optimize generation within safely installable ranges.

For east- and west-facing roofs, excluding them only because they generate less than south-facing can also lead to mistakes. Utilizing east and west surfaces can broaden the generation timing and better match self-consumption. In commercial projects in particular, the overlap between operating hours and the generation curve is important. Even if annual generation is slightly inferior, if more generated energy coincides with consumption the practical value can be high.

For north-facing or shaded roofs, it can be important to avoid forcefully increasing installed capacity. Increasing capacity may raise total simulated generation, but installing on inefficient surfaces can worsen investment efficiency and maintenance burdens. Including low-generation surfaces can also obscure overall expectations. Prioritize by checking generation efficiency per surface.

Comparing multiple patterns is effective in design decisions. Run simulations for south-only, south + east, south + west, east/west-centered, excluding shaded surfaces, etc., to make differences by roof orientation clearer. When comparing, change only one condition at a time; changing orientation, capacity, shadow conditions, and slope simultaneously makes it hard to identify which factor affected results. In practice, set comparison conditions that are easy to explain and create decision materials that stakeholders can accept.

Why on-site inspection and positional accuracy become important

When judging roof orientation in solar power generation simulations, there are limits to drawings and desk-based data. Actual buildings may not be oriented exactly as shown in drawings, and rooftop equipment or obstacles may have been added later. New nearby buildings, trees, signs, and utility poles can all cause shadows. Therefore, on-site inspection is indispensable for accurate roof orientation judgment.

On-site inspection confirms roof surface orientation, slope, obstacles, and surrounding environment. Orientation is a basic simulation condition but surprisingly prone to error. Even if a drawing looks south-facing, the actual surface may be rotated southeast or southwest. Even small differences affect east/west surface evaluation and time-of-day generation judgment. Accurately grasping orientation increases trust in simulation results.

Accurately obtaining positional information for each roof surface makes it easier to assess shadows and organize installable ranges. Recording surrounding elevation differences, the positions of adjacent structures, site boundaries, and rooftop equipment reduces rework after design. Especially for complex roofs or large facilities, accurately organizing which surface faces which orientation and where installations can be placed directly affects simulation accuracy.

For practitioners, the important thing is to properly reflect on-site findings in simulation inputs. If shadow factors noticed on site or roof constraints are not included in calculation conditions, results will diverge from reality. Conversely, carefully incorporating on-site information makes roof-orientation comparisons more persuasive. When explaining to clients or internal stakeholders, having on-site-verified evidence makes it easier to present decisions as based on reality rather than desk calculations.

Summary

When judging roof orientation in solar power generation simulations, it is important to understand the basic advantage of south-facing while not drawing conclusions from that alone. Roof orientation strongly affects generation, but in actual design you must evaluate roof slope, area, shadows, constructability, maintainability, and the building's power usage patterns together. While south-facing tends to yield higher generation, if the south face is small, shaded, or hard to layout, combining east and west faces may be more reasonable.

East- and west-facing roofs may look inferior by annual generation alone, but for buildings that can use morning or afternoon generation they may better match self-consumption. North-facing and complex roofs require careful judgment, but depending on slope and shadow conditions they may still be worth considering. The key is to separate roof surfaces in simulations and comprehensively view annual generation, monthly generation, time-of-day generation trends, and generation per unit capacity.

Also, simulation accuracy depends heavily on the accuracy of input conditions. Judging only by drawing-based orientation and roof area risks overlooking on-site shadows, rooftop equipment, orientation deviations, and construction constraints. To raise roof-orientation judgments to a practical level, on-site inspection and reflecting that information in simulations are essential. Accurate positional information and as-built understanding determine design quality, especially for buildings with multiple roof surfaces or sites vulnerable to surrounding influences.

Judging roof orientation is not just about maximizing generation. It is a critical design decision to achieve generation that matches building use, create a feasible construction plan, and ensure a solar power system that can be operated stably in the long term. Using simulations enables evidence-based comparisons and explanations instead of intuitive judgments.

If you want to accurately grasp roof azimuths and installation positions on site and improve the precision of simulation conditions, using LRTK—a high-precision GNSS positioning device that can be attached to an iPhone—is effective. If you can obtain high-precision positional information for roof surfaces and site surroundings, it becomes easier to verify orientations, organize shadow factors, record installable ranges, and prepare explanatory materials for stakeholders. To connect solar power generation simulations from desk-based estimates to practical, site-based decisions, it is important to incorporate accurate positioning and on-site records.