Five Items to Adjust in Solar Power Generation Simulations After a Site Survey

Solar power generation simulations should not end with only a rough estimate made before a site survey. What matters more is adjusting the simulation to reflect the actual conditions found during the site survey—roof or land conditions, obstacles, shading, maintenance access routes, equipment locations, and power usage conditions—so that the simulation matches reality. Even if the initial simulation shows favorable generation, if the installable area or shading conditions change after the site survey, the annual generation, self-consumption, surplus electricity, and project economics will change. This article explains, from a practical perspective for professionals who search for "solar power generation simulation", five items that should always be reviewed after a site survey.

Table of contents

• Importance of adjusting solar power generation simulations after a site survey

• Item 1: Adjust installable area and final layout

• Item 2: Adjust shading and obstacle conditions

• Item 3: Adjust assumptions on azimuth, tilt, and elevation differences

• Item 4: Adjust generation losses and maintenance conditions

• Item 5: Adjust estimates of self-consumption and surplus electricity

• What to check in a re-simulation after the site survey

• Points to note when modifying vendor proposals

• Accurate recording of site information increases re-calculation accuracy

• Summary

Importance of adjusting solar power generation simulations after a site survey

In the initial study phase, solar power generation simulations are sometimes created based on drawings, aerial photos, facility overviews, and power usage data. At this stage, the simulation can be useful to grasp the site’s generation potential and rough equipment sizing. However, information available before the site survey cannot fully reflect the actual state of the roof or land.

A site survey reveals conditions that drawings cannot show: additional rooftop equipment, piping, inspection hatches, drains, guardrails, penthouses, shadows from surrounding buildings, trees, shifts in site boundaries, terrain elevation differences, drainage conditions, and the need for maintenance access routes. All of these affect the assumptions of a solar generation simulation. In other words, if you do not revise the simulation after the site survey, the figures used for the installation decision may deviate from reality.

A common pitfall is using the initial simulation’s annual generation unchanged for the final decision. If the site survey reduces the installable area, requires avoiding heavily shaded areas, or mandates securing maintenance routes, the equipment capacity and generation will change. Even if the initial proposal shows high generation, if that figure does not match the final layout, actual performance after installation may differ significantly.

Revisions after the site survey are not meant to simply lower generation estimates. They are intended to bring the plan closer to one that can actually be installed, managed over the long term, and matched to the facility’s power usage. A simulation reflecting actual site conditions may show somewhat lower generation, but that makes the documentation more useful for internal explanations, vendor comparisons, pre-construction checks, and post-installation performance management.

To use solar power generation simulations effectively in practice, it is important to improve accuracy through a flow of: pre-survey rough estimate, post-survey revision, and final pre-construction confirmation. Below, we review the five items that should be corrected after the site survey.

Item 1: Adjust installable area and final layout

The first item to adjust after the site survey is the installable area and the final layout. In a solar generation simulation, where and how many panels are placed directly determine equipment capacity and generation. In the initial stage you assume installable areas from roof plans or site area, but a site survey clarifies areas that cannot be used or should be avoided.

For rooftop projects, confirm rooftop equipment, HVAC units, exhaust equipment, piping, inspection hatches, drains, guardrails, penthouses, waterproofing condition, and inspection spaces for existing equipment on site. It is common to find equipment not shown on drawings or added after completion. Avoiding these areas when laying out panels can reduce the installable area compared to the initial simulation.

Also on roofs, securing maintenance access routes is important. Packing panels across the entire roof to maximize generation can make inspections, waterproofing repairs, drain cleaning, and emergency response difficult. After the site survey, revise the layout to reflect not only generation but also long-term maintenance, including walkways and work spaces that can be used without difficulty.

For ground-mounted projects, revising the installable area is also critical. On site you may find differences from initial materials in site boundaries, slopes, elevation differences, trees, existing structures, drainage channels, access routes, and ground conditions. Even if the site area is large, not all of it may be usable for solar. Securing paths for weeding, inspection, cleaning, equipment replacement, and drainage management can reduce the actual installable area.

After adjusting the installable area, recalculate equipment capacity. Changes in panel count affect annual and monthly generation, self-consumption, and surplus electricity. If you keep the initial proposed capacity as a premise, it will not match the post-survey reality. Especially when excluding shaded or hard-to-maintain areas, total generation may decrease, but generation efficiency per unit capacity and long-term operability may improve.

When revising layouts after the site survey, the goal is not to chase maximum capacity but to approach a capacity that is realistic to install, maintain, and consistent with the facility’s purpose. The first revision point in the simulation should be the assumptions about the panel layout that produce generation, not the generation figure itself.

Item 2: Adjust shading and obstacle conditions

The second item to adjust is shading and obstacle conditions. One common cause of gaps between simulated and actual post-installation performance is overlooking shading. Pre-survey simulations sometimes estimate shading from drawings or photos, but on site unexpected obstacles and surroundings can affect generation.

On roofs, rooftop equipment, penthouses, guardrails, piping, exhaust ports, antennas, signage, and surrounding buildings cast shadows. Small equipment can affect panel generation if it is close to panels. In winter, when solar altitude is low, shadows that are insignificant in summer can become long and impactful. After the site survey, reflect in the simulation the position, height, and distance to candidate installation surfaces of equipment that cast shadows.

For ground-mounted projects, trees, utility poles, neighboring buildings, slopes, surrounding structures, and terrain elevation differences are shading sources. Consider not only current tree heights but future growth. After confirming tree positions and heights, whether cutting or trimming is possible, and distances to surrounding structures during the site survey, reflect these details in the simulation.

The impact of shading cannot be judged simply by whether shading exists. Crucial is when the shading occurs: only in the morning, also around midday, for extended periods in the evening, or primarily in winter—each scenario affects generation differently. For self-consumption projects, it is also important whether shading overlaps with times of high facility demand.

After the site survey, checking the difference between generation with and without shading makes the impact clear. Annual generation may fall when shading is reflected in the model, but that means the simulation is closer to reality. It is not uncommon for an initially optimistic generation estimate to decrease after accounting for site shading. In fact, failing to account for shading while retaining high generation estimates is more likely to produce post-installation gaps.

Obstacles affect not only shading but also maintainability. Placing panels around rooftop equipment affects not only generation but also the ease of inspections and repairs. On ground sites, placing arrays near trees or structures can increase leaf and dirt accumulation and the burden of weeding. After the site survey, reflect shading and obstacles both as generation loss and as operational risks.

Item 3: Adjust assumptions on azimuth, tilt, and elevation differences

The third item to adjust is assumptions on azimuth, tilt, and elevation differences. In solar generation simulations, the direction panels face and their tilt angle greatly affect generation. Initially you may assume azimuth and tilt from drawings or map data, but after the site survey you should correct to values closer to actual measurements.

On roofs, the orientation and slope of roof surfaces can differ from drawings. Buildings with multiple roof surfaces may have south-, east-, west-, or north-facing aspects mixed, each producing different generation patterns. After the site survey, reflect the azimuth, tilt, and installable capacity for each surface separately in the simulation. Treating the whole roof as a single surface risks misjudging generation breakdowns and compatibility with self-consumption.

For flat roofs or ground installations, you may be able to set the rack angle. However, do not decide the angle based on generation efficiency alone; consider row-to-row shading, wind effects, spacing, maintenance access, and constructability. After confirming roof or land area, surrounding obstacles, wind exposure, and maintenance space on site, adjust to a practical angle.

For land sites, elevation differences and terrain conditions are important. Land treated as flat in initial simulation may actually have slopes or steps. South-facing slopes can enhance generation, but north-facing slopes or land lower than surroundings change insolation and shading. Ignoring slopes and elevation differences in the simulation can cause discrepancies with actual generation and constructability.

Correcting azimuth and tilt changes monthly and time-of-day generation patterns. East-facing surfaces generate more in the morning; west-facing surfaces generate more in the afternoon. South-facing surfaces tend to have larger generation around midday. Depending on when the facility’s power demand is highest, contribution to self-consumption changes.

After the site survey, review azimuth, tilt, and elevation differences not only as "generation conditions" but as factors that determine "when power will be available." This improves accuracy not only of annual generation but also of self-consumption and surplus electricity estimates.

Item 4: Adjust generation losses and maintenance conditions

The fourth item to adjust is generation losses and maintenance conditions. Simulations must account for losses from temperature, wiring, power conversion, shading, soiling, snow, equipment downtime, and aging. Before the site survey a general loss rate may be used, but after the survey you should review loss assumptions to match site realities.

Temperature losses are especially important for rooftop projects. Roofs tend to become hot, and panel temperature varies with ventilation conditions. After confirming roof material, surrounding equipment, ventilation, panel mounting height, and layout during the site survey, realistically estimate generation reduction due to temperature. If summer generation looks optimistic, recheck temperature losses.

Losses from wiring and power conversion should also be adjusted after the site survey. Panel layout, inverter locations, and connection equipment positions affect wiring distance and routes. If the realistic equipment positions post-survey differ from the assumed ones in the initial proposal, wiring losses and constructability assumptions change.

Soiling losses should be revised to match the site environment. Sites near many trees, prone to dust, affected by birds, or close to unpaved areas are more likely to experience generation reductions due to dirty panels. Low roof pitch can make rain less effective at cleaning panels. After the site survey, revise the loss rate considering ease of cleaning and inspection.

In snowy regions, consider snow impacts. Roof pitch, nearby snow storage areas, snow fall direction, and ease of snow removal for ground-mounted systems influence winter generation. If initial simulations did not sufficiently account for snow, adjust winter generation after the site survey.

Maintenance conditions also affect generation losses. Insufficient inspection routes, difficulty cleaning, inaccessible equipment, or challenging weed and drainage management can affect long-term generation retention. A layout that maximizes generation may not be best for long-term performance. After the site survey, revise the simulation to reflect maintainability as well as generation.

Item 5: Adjust estimates of self-consumption and surplus electricity

The fifth item to adjust is estimates of self-consumption and surplus electricity. If installable area, equipment capacity, shading, generation losses, azimuth, or tilt change after the site survey, not only annual generation but also self-consumption and surplus electricity will change. This revision is especially important for projects aimed at self-consumption.

Self-consumption is the portion of generated solar power actually used within the facility. Even if generation increases, without facility demand at the same time the power becomes surplus. Conversely, slightly lower generation may provide stable benefits if it matches daytime demand well. After the site survey, recheck how corrected generation overlaps with the facility’s power usage data.

During the site survey you can often confirm facility operating hours, holidays, seasonal variations, major equipment operation times, daytime base load, and future equipment expansion plans. Reflecting these details improves the accuracy of self-consumption estimates. If the initial estimate used only annual consumption, it is desirable to revise to monthly and time-of-day conditions after the site survey.

Recalculate surplus electricity as well. If equipment capacity is reduced, surplus may shrink. When shading reduces generation, surplus may fall but generation during times needed for self-consumption may also decline. The key is not to assume reduced surplus is automatically good; check how self-consumption changes.

If you are considering batteries, re-evaluate both battery-less and battery-included scenarios. Changes in generation and surplus after the site survey alter the energy available for battery charging. If surplus becomes smaller, opportunities to use batteries may decrease. Conversely, if the site survey reveals ample daytime surplus, considering batteries or load control may become effective.

Self-consumption and surplus electricity determine installation effects and viability. If you revise generation after the site survey but do not update self-consumption and surplus estimates, practical decisions will be skewed. It is important to revise not just how much can be generated but how much can be consumed and how much will remain as surplus.

What to check in a re-simulation after the site survey

After revising the simulation following the site survey, it is insufficient to look only at whether annual generation rose or fell. In a re-simulation you need to check where differences from the initial simulation originate. This makes installation decisions, internal explanations, and coordination with vendors easier.

First, confirm changes in equipment capacity. If the installable area changed after the site survey, panel count and capacity will change. A reduced capacity may lower generation, but if that reduction comes from excluding shaded or hard-to-maintain areas, it is a realistic correction. If generation per unit capacity improves, the layout quality may have increased.

Next, check monthly generation changes. Adjusting shading, azimuth, or tilt can change generation in specific months. If winter generation falls, it may reflect winter shading or snow. If summer generation changes, temperature loss or tilt adjustments may be reflected. It’s important to check not only annual generation but which months changed.

Also check time-of-day generation. If morning generation falls, evening generation changes, or midday peaks shift, shading or orientation corrections may be the cause. For self-consumption projects, confirm how the overlap between facility demand and generation periods has changed.

Changes in self-consumption and surplus electricity are also important. Even if generation decreases, surplus may drop significantly while self-consumption is little affected. Conversely, if generation reductions concentrate during peak facility demand times, the impact on self-consumption and savings can be large. In re-simulation, check changes in usable power as well as total generation.

A re-simulation after the site survey does not deny the initial proposal. It updates initial hypotheses with site realities to reach a more reliable installation decision. Organizing the reasons for corrections also makes it easier to explain to stakeholders.

Points to note when modifying vendor proposals

When modifying vendor proposals after the site survey, review the entire proposal’s assumptions, not just generation. If installable area or shading conditions change, you cannot simply adjust annual generation. The changes affect equipment capacity, self-consumption, surplus electricity, inverter sizing, wiring routes, battery effectiveness, constructability, and maintainability.

First, clarify differences between the initial and revised proposals. Identify which installable areas were excluded, which obstacles were reflected, how shading evaluation changed, and how much capacity changed. If generation changes without an explanation of these differences, internal explanations and decision-making can become confused.

Next, review how loss rates are handled. If the site survey clarified shading, soiling, temperature, or wiring conditions, you should also revise the loss rate breakdown. Leaving the loss rate the same as in the initial proposal may mean the site survey findings are not fully reflected. Especially when shading, wiring routes, or equipment locations change, confirm generation loss assumptions.

Do not forget to revise self-consumption estimates. If generation changes, self-consumption and surplus electricity change too. If the site survey clarified facility operating hours, holidays, or major equipment usage, revise power usage assumptions as well. It’s desirable to improve from annual-only estimates to monthly and time-of-day estimates.

For proposals including batteries, recheck battery effects. When generation or surplus change, battery chargeable energy also changes. Don’t update only the battery-included scenario; compare again with the battery-less case to verify how much additional benefit the battery provides.

When modifying vendor proposals, do not treat a reduction in generation as inherently bad. If generation decreases after the site survey, it may reflect correct accounting for shading or maintenance conditions. Converting to realistic figures reduces post-installation gaps. What matters is that the reasons for revisions are clear and the final proposal aligns with site conditions and operational objectives.

Accurate recording of site information increases re-calculation accuracy

To correctly revise simulations after the site survey, accurately recording site information is indispensable. If the confirmed site information remains vague, you cannot fully reflect it in the re-simulation, and the survey’s value is lost. Record not only photos but also positions, dimensions, heights, distances, azimuths, and object types.

For rooftop projects, record rooftop equipment, guardrails, piping, penthouses, drains, inspection hatches, HVAC and exhaust equipment, waterproofing-sensitive areas, and inspection access routes. Especially for equipment that could cast shadows, record position and height. Clearly recording which roof areas can and cannot receive panels makes it easier to reflect the final layout.

For land projects, record site boundaries, trees, utility poles, slopes, elevation differences, drainage channels, existing structures, access routes, candidate grid-connection points, and equipment installation candidate locations. Unclear tree or structure positions make shading evaluation unstable. Unclear site boundaries or elevation differences complicate assessments of installable area, wiring routes, and maintenance paths.

Accurately recorded site information helps not only revise vendor proposals but also supports internal explanations and pre-construction checks. It becomes easier to explain why capacity changed from the initial proposal, why certain areas were excluded, and why generation decreased. If actual generation after installation differs from expectations, site information helps identify causes.

Site information is the input for re-simulation. Accurate positional information allows precise assessments of shading, installable areas, wiring routes, maintenance access, and equipment placement. Conversely, vague site information yields vague re-simulations.

To perform high-accuracy re-calculations after a site survey, record confirmed site details on the spot and ensure stakeholders can share the same information. High-quality records of site information are a major key to bringing solar power generation simulations closer to reality.

Summary

Do not stop at a pre-survey rough estimate for solar power generation simulations; always revise after the site survey. The site survey clarifies installable area, obstacles, shading, azimuth, tilt, elevation differences, generation losses, maintenance conditions, and actual power usage, so if you do not update the initial simulation assumptions, post-installation performance may deviate significantly.

The first item to revise after the site survey is installable area and final layout. Reflect rooftop equipment and maintenance routes, site boundaries and access paths, and revise to a layout that can actually be installed. The second is shading and obstacle conditions—reflect shadows from rooftop equipment, surrounding buildings, trees, utility poles, and terrain, and anticipate seasonal and time-of-day generation decreases.

The third is assumptions on azimuth, tilt, and elevation differences—reflect each roof surface’s orientation and slope and land slopes or steps to better match generation. The fourth is generation losses and maintenance conditions—reflect temperature, wiring, conversion, soiling, snow, downtime, aging, and ease of inspection and cleaning. The fifth is estimates of self-consumption and surplus electricity—recalculate because usable and surplus power change with generation.

In re-simulations after the site survey, check not only annual generation but monthly and time-of-day generation, self-consumption, surplus electricity, and generation per unit capacity. Organizing differences from the initial proposal clarifies why generation changed and which conditions affect installation decisions.

When modifying vendor proposals, clearly state reasons for revisions as well as changes in generation. A drop in generation after the site survey is not necessarily bad; if it reflects shading and maintenance conditions, it is a necessary correction to reduce post-installation gaps.

To improve re-simulation accuracy, accurately record site information. If you can accurately record candidate installation ranges, rooftop equipment, obstacles, trees, site boundaries, maintenance routes, and connection candidate points, the simulation premises become clear and you can consistently proceed from proposal revision to pre-construction checks and post-installation maintenance management.

If you want to accurately record candidate installation ranges, rooftop equipment, obstacles, trees, site boundaries, and maintenance routes on site and correctly revise solar power generation simulations after the site survey, using LRTK—an iPhone-mounted GNSS high-precision positioning device—is effective. High-precision site position data allow accurate organization of shading, obstacles, installable ranges, wiring routes, and maintenance routes, making it easier to update from the initial simulation to the final layout. To bring solar power generation simulations closer to reality, establish a system to accurately record site survey information and reliably reflect it in re-calculations.