Six items to check in solar power generation output simulations before a site survey

Solar power generation output simulations are often thought of as something to perform after a site survey, but in practice they play a major role even before the survey. Even with limited information about candidate sites or roofs, checking a rough estimate of generation, installable area, shadow risks, system capacity, and expected self-consumption in advance clarifies which points to focus on during the site survey. This article explains, from a practical perspective for practitioners searching for "solar power generation output simulation", six items to check before a site survey.

Table of contents

• The meaning of checking a solar power generation output simulation before a site survey

• Item 1: Tentatively organize candidate installation areas and usable area

• Item 2: Confirm azimuth, tilt, and roof shape

• Item 3: Identify risks from shadows and surrounding obstacles

• Item 4: Check the reasonableness of monthly generation and solar irradiation conditions

• Item 5: Formulate hypotheses about self-consumption and surplus electricity

• Item 6: Organize information to verify during the site survey

• Assumptions to watch when simulating before the site survey

• Summary

The meaning of checking a solar power generation output simulation before a site survey

Solar power generation output simulations become more accurate when performed based on precise information obtained during a site survey. However, there is still significant value in using simulations before a site survey. If you understand approximate generation prospects and risks beforehand, you can clearly prioritize what to check on the survey day.

Many conditions affect solar generation: roof or land area, azimuth, tilt, surrounding environment, and the facility’s power usage. Information is often incomplete before a site survey, but you can form initial hypotheses using drawings, aerial photos, facility documents, power usage records, site photos, and interviews with stakeholders. Whether or not you have such hypotheses greatly changes the quality of the site survey.

For example, if a preliminary simulation shows low winter generation, you can focus the site survey on causes of shadows to the south or east/west. If capacity cannot be increased relative to roof area, you need to check rooftop equipment, inspection walkways, waterproofing clearances, and drain locations. If generation looks high but surplus is likely large, the survey topics become daytime facility demand, batteries, and load-operation adjustments.

Pre-survey simulations are not final figures for decision-making. They are preparatory documents to form hypotheses to verify on site. Therefore, don’t treat the numbers as absolute; instead, identify which assumptions are uncertain and what to confirm on site to improve accuracy.

When considering multiple candidate sites or buildings, pre-survey simulations also help prioritize. If you can early on identify sites with clearly poor generation conditions, high shadow risk, or poor compatibility with power demand, you can narrow down survey targets. Conversely, for candidates with high generation potential, you can make concrete lists of items to check in detail during the site survey.

What to check before a site survey is not limited to generation volume. The goal is to identify points to verify on site, including installable area, possible shadows, generation losses, compatibility with power usage, constructability, and maintainability. Solar power generation output simulations can be used as preparatory materials to streamline site surveys and reduce rework.

Item 1: Tentatively organize candidate installation areas and usable area

The first item to check before a site survey is the candidate installation area and usable area. In simulations, the area available for panels directly determines system capacity and annual generation. However, total roof or land area is not equal to installable area. Even before a site survey, it is important to tentatively separate areas likely usable from areas likely unusable.

For rooftop projects, you must consider not only the roof area on drawings but also rooftop equipment, railings, piping, drains, inspection hatches, penthouses, air-conditioning equipment, exhaust equipment, waterproofing constraints, and inspection/maintenance access. Even from drawings and photos, listing locations that may affect panel layout clarifies what to verify during the site survey.

For example, a roof that looks wide on drawings may have a centralized concentration of equipment, reducing contiguous usable area. Edge zones and areas around railings may require safety clearances. Placing panels above drains or inspection hatches can hinder future inspections or repairs. Tentatively excluding such areas beforehand helps avoid overestimating initial simulation results.

For ground-mounted projects, check not only site area but also boundaries, slopes, elevation differences, trees, existing structures, drainage channels, maintenance paths, connection candidate points, and areas planned for future use. Even on large lots, slopes, drainage, and maintenance access can limit where panels can actually be placed. Before the site survey, create tentative installation ranges from maps and drawings and then confirm on site whether those ranges are truly usable.

When tentatively organizing candidate areas, separate the range that maximizes capacity from the range that is realistic to operate. A maximum-capacity simulation is useful to understand generation potential, but it may not be directly constructible. Simulating within a realistic range that considers inspection and maintenance makes it easier to estimate generation closer to post-installation reality.

If you tentatively organize usable area before the site survey, the survey-day checks become clearer. On roofs, confirm clearances around equipment, inspection walkways, drainage routes, and waterproofing condition. On land, check boundaries, slopes, trees, drainage, maintenance paths, and connection candidate points. Entering the survey with hypotheses about installable area makes it easier to improve simulation accuracy.

Item 2: Confirm azimuth, tilt, and roof shape

The second item to confirm is azimuth, tilt, and roof shape. Solar generation is greatly affected by which direction panels face and at what angle they receive solar radiation. Even before a site survey, grasping approximate azimuths and roof shapes from drawings, maps, and photos helps form assumptions for generation simulations.

Regarding azimuth, surfaces closer to south-facing generally produce more annual generation. However, in practice you cannot always use only south-facing surfaces. You may combine east- and west-facing roof surfaces. East-facing surfaces tend to contribute to morning generation, west-facing to afternoon generation, and these can be effective depending on the facility’s power usage hours. Before the site survey, broadly organize candidate surfaces’ azimuths and hypothesize which time periods they will generate most.

Tilt angle also affects generation. For rooftop installations, you often follow the existing roof slope and cannot freely set angles. For flat roofs or ground-mounted systems you can set angles via racking, but larger angles influence wind loads, row-to-row shading, installation spacing, and maintainability. Before the site survey, treat tilt not as an ideal condition but with awareness of the practical range of angles that can be constructed.

Roof shape is also important. Gable roofs, hipped roofs, single-slope roofs, flat roofs, and roofs with level differences differ in available surfaces and layout flexibility. For roofs divided into multiple surfaces, consider generation and capacity per surface. Treating the entire roof as a single plane in pre-survey simulations can lead to discrepancies with actual generation and layout.

For land projects, terrain slope and orientation matter. A gentle south-facing slope can be favorable, while north-facing slopes or sites lower than surrounding land change irradiation and shadow conditions. Simulations assuming flat terrain may differ greatly from actual construction conditions.

Confirming azimuth, tilt, and roof shape beforehand clarifies what to measure during the site survey: whether the azimuth on drawings matches reality, whether roof pitch is as assumed, whether steps or upstands create shading, and whether terrain tilt affects installation. The accuracy of generation simulations depends greatly on the accuracy of these basic conditions.

Item 3: Identify risks from shadows and surrounding obstacles

The third item to check is risks from shadows and surrounding obstacles. Shadows strongly influence generation in simulations. Even before a site survey, listing elements likely to cast shadows—surrounding buildings, rooftop equipment, trees, utility poles, signs, and terrain—improves the precision of checks on the survey day.

Shadow effects cannot be judged simply by whether obstacles exist. The obstacle’s position, height, distance to candidate surfaces, azimuth, season, and time of day all change shadow reach. Shadows may be short in summer but lengthen in winter when solar altitude is low. Before the site survey, organize hypotheses on where shadow sources might be and plan to confirm their positions, heights, and distances on site.

For rooftop projects, pay attention not only to surrounding buildings but also to rooftop equipment. Air-conditioning units, exhaust equipment, penthouses, railings, piping, antennas, and upstands around skylights can cast shadows at close range. Even small rooftop equipment can affect generation in winter or at morning/evening times. Before the site survey, check drawings and photos to list equipment locations that might cause shading.

For land projects, trees, utility poles, neighboring structures, slopes, surrounding buildings, and terrain elevation differences are shadow sources. Consider not only current tree heights but future growth. If surrounding land use may change, include that as a long-term shadow risk.

Shadow risks are reflected in monthly and time-of-day generation. If a candidate shows low winter generation, or weak morning generation, or potential afternoon shading, you can identify which directions and time periods to check on site. This is especially important when prioritizing self-consumption: whether shadows occur during the facility’s high-demand times matters.

It may be difficult to fully reflect shadows accurately in pre-survey simulations. However, you can list shadow sources and roughly compare generation with and without shading. This preparation reduces the chance of overlooking shadow causes during the site survey.

Item 4: Check the reasonableness of monthly generation and solar irradiation conditions

The fourth item is to check the reasonableness of monthly generation and solar irradiation conditions. Even before a site survey, checking regional irradiation conditions and monthly generation trends allows a rough evaluation of candidate sites’ generation potential. Annual generation alone can obscure seasonal variations and regional characteristics, so monthly checks are important.

Solar generation varies by season due to differences in sunshine hours, solar altitude, irradiation, temperature, weather, snowfall, and shadow length. Generally, generation increases from spring to summer and decreases in winter due to shorter daylight hours. However, summer can see output reduction due to high temperatures. Regions with rainy seasons, typhoons, or frequent cloud cover may have depressed generation during those periods.

Before the site survey, confirm that the simulation reflects appropriate regional irradiation conditions. Using weather data close to the candidate site versus broad-area averages changes the apparent generation. Mountain areas, coastal areas, basins, snowy regions, and frequently foggy areas can exhibit different conditions even within the same municipality.

Looking at monthly generation makes survey risks more visible. If winter generation is low, check not only daylight hours but also shadows and snow possibilities. If summer generation underperforms expectations, consider temperature losses and surrounding environment effects. If a specific month’s generation is unusually high or low, verify the simulation assumptions.

Monthly generation is also important to assess compatibility with facility power usage. Facilities with high air-conditioning demand in summer benefit directly from high summer generation. Facilities with high winter power demand may face issues if winter generation is low. Overlaying monthly generation and monthly usage before the site survey reveals operational challenges to check on site.

Irradiation conditions and monthly generation are key indicators to gauge a candidate’s generation potential before a site survey. Don’t accept generation numbers at face value; verify that seasonal variations are consistent with the region.

Item 5: Formulate hypotheses about self-consumption and surplus electricity

The fifth item is to formulate hypotheses about self-consumption and surplus electricity. Simulations should assess not only generation but also how much of the generated power can be used within the facility. If you have power usage and operating-hour data before the site survey, you can form hypotheses about self-consumption and surplus.

Self-consumption is the portion of generated power consumed within the facility. Since PV generates mainly during daytime, self-consumption is higher when daytime facility demand is large. Factories, warehouses, stores, offices, and public facilities have varying daytime demand depending on operating hours and equipment.

Surplus electricity is the portion of generated power that cannot be consumed by the facility at the same time. If surplus is large, the system may be oversized relative to demand. Whether surplus is exported externally, stored in batteries, or curtailed changes installation benefits. Tentatively assessing the likelihood of surplus before the site survey helps focus checks on power equipment and operational methods.

Annual usage alone is insufficient to estimate self-consumption. Important are monthly usage, hourly usage, and differences between weekdays and weekends. Facilities that operate on weekdays but not weekends may see increased surplus on weekends. Facilities with seasonal variations in air-conditioning load experience changes in self-consumption by season. Before the site survey, check facility operating hours and major equipment run times to build daytime consumption hypotheses.

Do not judge only by self-consumption rate. Small systems tend to show high self-consumption rates but small absolute self-consumed energy. Large systems may show lower self-consumption rates but larger absolute self-consumed energy. Before the site survey, comparing how self-consumed energy and surplus change with different system capacities helps form hypotheses about appropriate capacity.

When considering batteries, hypotheses about self-consumption and surplus are also necessary. Facilities with daytime surplus and evening/night demand may increase self-consumption through batteries. Conversely, facilities with little surplus cannot make full use of batteries. Simulating with and without batteries before the site survey clarifies which power usage patterns to verify.

Item 6: Organize information to verify during the site survey

The sixth item is to organize the information to verify during the site survey. Pre-survey simulations are used not to produce final figures but to clarify what to check on the survey day. Organizing verification items in advance reduces omissions during the site survey and improves the accuracy of post-survey re-simulations.

For rooftop projects, verify roof dimensions, azimuth, tilt, rooftop equipment, piping, railings, penthouses, inspection hatches, drains, waterproofing condition, inspection/maintenance access, and access to existing equipment. Precisely record locations of equipment that cast shadows on candidate areas or affect construction/maintenance. Many pieces of equipment are not shown on drawings, so on-site verification is crucial.

For ground-mounted projects, verify site boundaries, terrain, elevation differences, slopes, trees, utility poles, surrounding structures, drainage channels, maintenance paths, existing equipment, and connection candidate points. Trees and surrounding structures can cause shading. Elevation differences and drainage conditions affect constructability and maintainability. Distance to connection candidate points and cabling routes also influence later design.

For power usage, check facility operating hours, holidays, seasonal variations, major equipment operating times, daytime base load, and future equipment expansion plans. If emphasizing self-consumption, usable energy is more important than generation volume. Preparing questions for facility managers and equipment personnel in advance makes post-survey estimates easier.

Also, in the site survey, recording not only photos but also positional information is important. If locations of obstacles, equipment, site boundaries, or connection candidate points are ambiguous, it becomes difficult to reflect them in simulations later. Accurate records of where things are enable more precise assessment of shading, installable area, cabling routes, and maintenance access.

Organizing the information to verify before the site survey clarifies the survey day’s objectives. By identifying uncertain assumptions in the simulation and confirming them on site, you can make post-survey generation estimates closer to reality.

Assumptions to watch when simulating before the site survey

Pre-survey solar power generation output simulations are useful preparatory materials, but there are assumptions to watch. The main point is that pre-survey simulations are hypotheses, not final figures. Before on-site confirmation, roof or land conditions, obstacle locations, shadow patterns, inspection access, and connection equipment status may not be fully known.

First, beware of overestimating installable area. Areas may look widely usable on drawings but can be reduced by rooftop equipment, piping, drains, railings, waterproofing clearances, and inspection access. For land projects, site boundaries, trees, slopes, drainage, maintenance paths, and existing structures can limit usable area.

Second, beware of underestimating shadows. Shadows change with time of day and season. Judging from maps and photos alone may overlook winter or morning/evening shadows. Simulations that don’t sufficiently account for shading tend to overstate generation.

Be careful about how generation losses are handled. Generation depends on temperature, cabling, power conversion, soiling, snow, equipment downtime, and aging. Before the site survey, surrounding environment and maintenance conditions may be uncertain, so avoid overly optimistic assumptions about losses.

Also pay attention to the granularity of power usage data when estimating self-consumption. Using annual usage alone may miss daytime demand and weekend surplus. If pre-survey data are limited, understand that estimates will have a wide range.

Pre-survey simulations are for checking candidate sites and planning system directions. After the site survey, re-simulate incorporating on-site findings. Don’t use initial numbers as final decisions; updating assumptions after the site survey is fundamental to reducing post-installation gaps.

Summary

Using solar power generation output simulations before a site survey allows you to organize candidate sites’ generation potential, installable area, shadow risks, monthly generation, self-consumption, surplus electricity, and items to verify on site. Pre-survey simulations are not for producing final figures but should be used as preparatory materials to improve survey accuracy.

The first items to check are candidate installation areas and usable area. Rather than total roof or land area, tentatively organize the realistically installable area considering equipment, inspection access, drains, waterproofing, site boundaries, slopes, and maintenance paths. Next, confirm azimuth, tilt, and roof shape to identify favorable and unfavorable surfaces.

Shadows and surrounding obstacles are also important. Surrounding buildings, rooftop equipment, trees, utility poles, and terrain affect generation by time of day and season. Identifying shadow sources before the site survey clarifies which directions and positions to verify on site. Checking monthly generation and irradiation conditions reveals regional characteristics and seasonal variation.

Formulating hypotheses about self-consumption and surplus electricity is essential. Even with high generation, benefits are limited if the facility cannot use the power. Consider daytime demand, holidays, seasonal variation, and battery presence to estimate how much generated power can be used. Finally, organize items to verify during the site survey so you can comprehensively check roof or land, power usage, connection candidate points, and maintenance access.

However, pre-survey simulations are hypotheses and must have assumptions updated after on-site verification. Installable area, shading, generation losses, and self-consumption become more accurate only after on-site confirmation. Do not treat initial simulations as final decision figures; link them to re-simulations after the site survey.

If you want to accurately record candidate installation areas, rooftop equipment, obstacles, trees, site boundaries, inspection access, and connection candidate points on site to improve simulation and site survey accuracy, using LRTK, an iPhone-mounted high-precision GNSS positioning device, is effective. High-precision on-site positioning enables you to verify pre-survey hypotheses and precisely organize shading, obstacles, installable area, cabling routes, and maintenance access. To use solar power generation output simulations effectively before a site survey, establish a process that connects desktop calculations with accurate on-site positional information.