When considering the installation of solar power generation, many practitioners first check the power generation simulation. By understanding how much can be generated annually, the extent of monthly fluctuations, and whether sufficient effects can be expected given the building or site conditions, you can organize the factors for deciding whether to install. However, simply looking at the annual generation number in simulation results is insufficient. If you do not interpret results including installation conditions, how electricity is used, shading effects, equipment capacity, operations and maintenance, and future changes, you may encounter problems after installation such as “the effect was not as large as expected” or “the assumptions at the time of decision differed from reality.”

This article explains six criteria that practitioners searching for “solar power generation simulation” should check when deciding whether to install. The thinking framework is common whether the subject is a residence, office, factory, warehouse, or vacant land. What matters is not making the generation look large, but translating it into realistic decisions that can withstand post-installation operation.

# Table of Contents

• Solar power generation simulations are comparative material for deciding whether to install

• Criterion 1: Is the annual generation sufficient for the installation purpose?

• Criterion 2: Do the monthly generation and electricity usage patterns align?

• Criterion 3: Are losses from shading, azimuth, and tilt realistically accounted for?

• Criterion 4: Does the balance between self-consumption and selling electricity match the installation policy?

• Criterion 5: Is the equipment capacity and installation space feasible?

• Criterion 6: Can you make a decision that includes maintenance and long-term changes?

• Points to note when using simulation results for installation decisions

• Accurately understanding site conditions is the foundation of generation assessment

• Summary

# Solar power generation simulations are comparative material for deciding whether to install

Solar power generation simulations are an important document for deciding whether to install. However, simulations do not perfectly predict future generation. They are estimates made under certain assumptions based on solar radiation, installation azimuth, tilt angle, panel capacity, surrounding environment, and loss conditions. Therefore, when deciding whether to install, you should not rely solely on “this number makes me comfortable,” but instead adopt a mindset of comparing “on what assumptions was this calculated,” “what risks are incorporated,” and “which of multiple proposals is realistic.”

A common practical mistake is judging only by the size of the annual generation. For example, even if the annual generation looks large, if the times when electricity is actually used do not match the generation times, the effect of self-consumption will be smaller than expected. Conversely, even if the total generation is moderate, facilities with high daytime consumption may see high installation effects. In other words, simulation results must be read not only for “how much is generated” but also for “how that generation can be used.”

Also, simulation results can include design-stage wishes or sales-oriented presentation. Proposals that use the maximum possible area, handle shading simply, or assume optimistic loss rates tend to show higher numbers. If you use simulations for deciding whether to install, it is important to compare not only the best-case numbers but also realistic conditions, slightly conservative conditions, and conditions that consider future variability.

There are six major criteria to check when deciding whether to install: whether the annual generation matches the purpose, whether monthly generation aligns with usage patterns, whether losses from shading and azimuth are incorporated, whether the balance between self-consumption and selling electricity is appropriate, whether equipment capacity and installation space are feasible, and whether the plan holds up including maintenance and long-term changes. By checking these six items in order, you can use the simulation not merely as numbers but as practical material for installation decisions.

# Criterion 1: Is the annual generation sufficient for the installation purpose?

The first criterion to check is whether the annual generation is sufficient for the installation purpose. The purpose of introducing solar power varies by project. It could be to cover part of electricity usage by self-consumption, to advance decarbonization of a facility, to support emergency power supply, to utilize vacant space, and so on. The meaning of the generation amount you should look for changes with the purpose.

Annual generation is the most prominent figure in solar power simulations. However, you cannot decide based only on this figure. What is important is to confirm the proportion of annual generation relative to current electricity usage, whether it reaches the level to meet the installation purpose, and whether the equipment is not oversized. For example, if generation is too small relative to total facility usage, the installation effect will be limited. On the other hand, if generation is overly large, unused electricity may increase and fail to deliver the expected benefits.

When reviewing annual generation, it is easier to judge if you also check generation per unit of installed capacity, not just the simple total. Checking whether generation per capacity falls within a natural range helps detect overly optimistic simulations. For the same capacity, generation varies depending on regional solar conditions, roof orientation, tilt, shading, temperature environment, and equipment configuration. Therefore, rather than simply comparing with past projects, confirm whether it is appropriate for the current installation conditions.

Also, annual generation is not constant over the long term. Equipment performance gradually declines with age, and the surrounding environment may change. New buildings may be erected nearby, trees may grow, rooftop equipment may increase, and dirt may accumulate—factors not easily visible at installation time. Therefore, do not base decisions only on the first-year expected generation; consider whether the purpose will continue to be met over a long period.

When deciding, judge annual generation not as “more or less” but as “a meaningful amount for the purpose.” For projects prioritizing self-consumption, look at where generation impacts electricity usage. For projects prioritizing environmental response, look at the expected reduction effect. For projects emphasizing business viability, check whether the generation is reasonable as a long-term revenue assumption. Linking purpose and generation in this way makes simulation results usable for installation decisions.

# Criterion 2: Do the monthly generation and electricity usage patterns align?

Next important is the compatibility between monthly generation and electricity usage patterns. Even if the annual generation appears sufficient, if monthly generation does not align with actual electricity demand, the installation effect may be smaller than expected. Solar generation varies seasonally. Solar radiation, daylight hours, temperature, and weather patterns change month to month, so generation is not the same each month.

Practitioners should check how much the months with high generation overlap with months of high electricity usage. For example, facilities whose electricity usage increases in summer due to air conditioning or production equipment need to know how much summer generation can be expected. Conversely, facilities with high winter heating or lighting loads need to know how much winter generation contributes to demand. Annual totals do not reveal these seasonal compatibilities.

Moreover, not only monthly but also daytime hourly usage patterns are important. Solar generation is essentially daytime generation. Thus, facilities that use a lot of power during the day can more easily self-consume and are more likely to see benefits. Conversely, facilities with concentrated usage at night or early morning may have many hours when generated power cannot be used directly. In such cases, consideration of surplus power handling or the need for energy storage is required.

When looking at monthly generation, do not focus only on the peak month; it is also important to understand months with low generation. Confirm the minimum level of effect in low-generation months and whether electricity reduction contribution is stable throughout the year. Especially in corporate projects, you will need to explain reasons for large monthly differences in internal reporting and effectiveness verification. Understanding monthly variation during the simulation stage will make it easier to evaluate actual generation after installation without undue concern about seasonal changes.

Also consider future changes in electricity usage patterns, not just the present. Operating hours may change, equipment updates may increase or decrease usage, air-conditioning systems may change, and electrification may progress—facility operation is not fixed. Since solar generation is a long-lived installation, it is desirable to assume usage several years ahead as well as current electricity data.

When deciding whether to install, check monthly generation vs monthly usage and the overlap of daytime generation and daytime demand. The greater the overlap, the more you can expect self-consumption benefits. Conversely, if generation and demand are largely misaligned, you need to review surplus power utilization and equipment sizing. When reading simulation results, do not be satisfied with large annual figures; digging into monthly and hourly alignment improves decision accuracy.

# Criterion 3: Are losses from shading, azimuth, and tilt realistically accounted for?

One factor that often leads to errors in installation decisions based on solar generation simulations is the handling of losses due to shading, azimuth, and tilt. Even though ideal conditions can promise high generation, actual sites have various shading factors such as surrounding buildings, trees, utility poles, signs, rooftop equipment, railings, level differences, and adjacent structures. If these are not sufficiently considered, simulation results can appear larger than reality.

The impact of shading cannot be judged solely by the shaded area. The size of generation loss varies depending on the time of day when shading occurs, the season, the shaded position, and the effect on circuit configuration. Even short-duration shading can be significant if it occurs during high-generation hours. Also, because the sun altitude is lower in winter, shading that was not a problem in summer can extend substantially in winter. When deciding whether to install, it is important to check shading changes throughout the year.

Azimuth also greatly affects generation. Generally, orientations that receive more sunlight produce more power, but the available roof or site orientation is often not freely selectable. East- or west-facing installations can be effective depending on the match with electricity usage patterns. For example, east-leaning generation helps facilities with high morning demand, while west-leaning generation suits facilities with high afternoon demand. Azimuth should be judged not only by deviation from the ideal but also by compatibility with usage.

Tilt angle is similar. Sometimes the roof slope becomes the installation angle, and sometimes a mounting structure is used to adjust the angle. Changing the tilt not only changes generation but also affects wind load, installation spacing, shading behavior, constructability, and inspectability. If maximizing generation alone leads to excessive installation density or poor maintainability, long-term operation can suffer. When deciding, you need to balance generation with constructability and maintainability.

When checking losses in simulation, see how concretely loss items are set. Confirm whether shading losses, temperature-related losses, wiring and conversion losses, soiling losses, and aging are appropriately considered. Even if you cannot calculate everything in fine detail, it is important that the assumptions about which losses are included and how are explainable.

Especially in projects with complex site conditions, it is risky to proceed with installation decisions based solely on simple simulations. For buildings with complex roof shapes, sites surrounded by tall structures, rooftops with many level differences or equipment, or sloped-site installations, accurately reflecting shading and topography increases simulation reliability. At the stage of deciding whether to install, prioritize results that realistically account for losses from shading, azimuth, and tilt rather than optimistic generation figures.

# Criterion 4: Does the balance between self-consumption and selling electricity match the installation policy?

In deciding on solar installation, confirming the balance between self-consumption and selling electricity is indispensable. Whether generated power is used on-site or excess power is sent outside greatly changes how installation effects are considered. In simulations, you need to look not only at total generation but also at how much of that can be self-consumed and how much will be surplus.

When prioritizing self-consumption, the most important factor is the overlap between generation and usage times. Facilities that consistently use electricity during the day can more easily consume generated power on-site. Self-consumption rates vary by operation hours and weekdays for factories, warehouses, offices, stores, and public facilities. For facilities with high demand on weekdays but low on holidays, surplus on holidays may increase. In simulations, it is desirable to reflect differences between weekdays and holidays, operating vs non-operating days, and seasonal demand differences as much as possible.

Even when considering selling electricity, it is premature to think “excess can simply be sold, so no problem.” Selling conditions depend on制度 (policies) and contract and connection conditions, so initial assumptions may not remain long-term. Therefore, when deciding, check whether the design does not overly depend on selling power, whether it can still work centered on self-consumption, and whether surplus is not excessive. Especially for projects aimed at self-consumption, oversized capacity can increase surplus and dilute expected effects.

The balance between self-consumption and selling electricity also directly influences equipment capacity decisions. Loading as much as possible on the roof or site is not always optimal. If generation increases but cannot be used during many hours, part of the installation may become excessive relative to the purpose. Conversely, if you expect future power demand increases, designing too small to match only current demand may lead to shortages later. Consider an appropriate scale based on current use and future operation plans.

Whether to combine energy storage also relates to the balance between self-consumption and selling. If generated power cannot be used up during the day, storage may enable use at night or during emergencies. However, adding storage complicates equipment configuration and operation. In deciding whether to install, consider not only increasing generation but also the necessity of storage, emergency use, load leveling, and reviewing electricity use.

When using simulation results for decisions, check self-consumption rate and surplus rate to ensure they do not conflict with the installation policy. The optimal balance varies depending on whether the purpose is to reduce electricity consumption, advance environmental response, strengthen emergency preparedness, or utilize vacant space. More important than large total generation is whether the generated power is used in line with the installation purpose.

# Criterion 5: Is the equipment capacity and installation space feasible?

The fifth criterion is whether the equipment capacity and installation space are feasible. In simulations, as equipment capacity increases, annual generation tends to increase. However, in reality there are many constraints such as available location, roof or ground strength, maintenance space, evacuation routes, legal restrictions, and interference with surrounding equipment. Filling the area with panels is not always optimal.

For rooftop installations, first confirm the usable area. Even if the entire roof looks available, parts may actually be unusable. Rooftop equipment, inspection walkways, drainage paths, space required for evacuation, safety distances at edges, and areas prone to shading must be excluded from installation. Also, the condition of roofing materials and waterproof layers can impose constraints on construction methods and load conditions. Confirm whether the capacity shown in the simulation can truly be achieved under actual construction conditions.

For ground installations, consider not only site area but slope, ground condition, drainage, surrounding roads, fences, maintenance vehicle access, and vegetation management. On sloped land, mounting height and layout become more complex and shading behavior changes. If part of the site is planned for other uses in the future, installing solar equipment can reduce land-use flexibility. In deciding, check compatibility between generation and post-installation operation and land use.

When deciding equipment capacity, also check the relationship between panel capacity and peripheral equipment capacity. Generating equipment is not just panels; conversion equipment, connection devices, protection devices, wiring, monitoring equipment, etc., are necessary. If these components are not appropriately configured, simulated generation may not be reflected in actual operation. Also, if capacity is too large, connection conditions or intake equipment may need to be revised.

Proposals that overfill installation space can make maintenance and inspection difficult. Narrow inspection routes, insufficient spacing between equipment, complex layouts to avoid shading, or installations too close to roof edges can create safety and workability issues in long-term operation. Solar installations are not “install and finish” but equipment used over a long period. Therefore, prioritize layouts that are easy to operate, inspect, and respond to problems over simply maximizing initial generation.

When deciding, confirm whether the capacity shown in the simulation is realistic given site and construction conditions. Check whether the plan is overly compact, whether it ignores shading or maintenance space, and whether it will impede future repairs or inspections. Equipment capacity should be decided based on a balance of generation, purpose, installation conditions, and maintenance—not simply “bigger is better.”

# Criterion 6: Can you make a decision that includes maintenance and long-term changes?

The sixth criterion is whether you can decide including maintenance and long-term changes. Solar power systems are long-term installations, and relying only on simulation results at installation time can lead to unexpected issues later. You need to consider not only first-year generation but how generation will change over the long term.

A representative factor of long-term change is aging degradation. Panels and peripheral equipment change performance slightly over years of use. If you look only at first-year generation in the simulation, you may overlook long-term generation decline. When deciding, estimate generation after a certain period as well as the first year, and confirm whether sufficient effect remains relative to the purpose.

Impacts from maintenance are also important. Panel surface soiling, fallen leaves, bird damage, snow cover, poor drainage, growth of surrounding vegetation, equipment failures, and poor monitoring can lead to generation declines. These are often treated in simulations as loss rates, but in reality they vary greatly depending on site management. Without deciding how to perform regular inspections, cleaning, anomaly detection, and vegetation management, it will be hard to maintain assumed generation.

Also consider changes in building or site usage after installation. Adding new rooftop equipment, construction on adjacent land, increased air-conditioning equipment, changes in operating hours, or changes in business content can affect generation and self-consumption rate. Since simulation results are based on assumptions at the time of installation, confirm how much margin there is for future changes.

Performance monitoring is essential for long-term operation. After installation, compare simulation values with actual generation and identify causes if differences occur. Distinguish whether differences are temporary due to weather, persistent due to shading or soiling, or caused by equipment failures so you can take appropriate actions. Deciding in advance how generation will be recorded, who will check it, and how to respond to anomalies smooths post-installation management.

When deciding whether to install, adopt realistic generation assumptions that include maintenance. Confirm not just ideal first-year generation but whether the installation purpose will still be met after considering dirt, degradation, inspection regimes, and changes in the surrounding environment. To obtain long-term stable effects, use simulation values not merely as “expected values at installation” but as baseline values for operational management.

# Points to note when using simulation results for installation decisions

When using solar generation simulations for installation decisions, checking the input assumptions as well as the numbers is essential. Results vary even for the same installation location if input conditions change slightly. You must understand the conditions used for calculation—solar radiation data, installation azimuth, tilt angle, equipment capacity, loss rates, shading treatment, temperature conditions, and equipment configuration—to judge the numbers’ validity.

Be especially wary of overestimation. To make generation look good, some may underestimate shading, set losses low, overstate available installation area, or idealize operating conditions. Since simulation is a forecast, some differences are inevitable. However, if the assumptions are too optimistic, the installation decision itself becomes risky.

Therefore, in practice it is effective to compare multiple conditions, not a single result. Viewing standard conditions, conservative shading assumptions, reduced equipment capacity, and self-consumption-focused scenarios makes it easier to judge which conditions best match the installation purpose. The proposal with the highest generation is not always optimal. Rather, a comprehensive judgment including constructability, maintainability, demand compatibility, and long-term stability is important.

When using results for internal approvals or customer explanations, clarify how to read the simulation. Organize annual generation, monthly generation, self-consumed energy, surplus energy, assumed losses, and assumptions, and share which numbers will be used as decision criteria. If only the person in charge understands the details, the decision-makers will not move forward. Explain in a way such as “under these conditions the installation meets the purpose,” “this condition produces excessive surplus,” or “this part requires on-site verification,” rather than listing technical terms.

Also adopt a mindset that anticipates differences between simulation and actual generation. Weather varies year to year and solar radiation is not constant. A single low-generation year is not necessarily an equipment problem. However, if differences remain large even accounting for weather, consider shading, soiling, equipment failures, construction conditions, or input errors. Thinking in terms of comparing simulation and actual results from before installation makes post-installation evaluation easier.

Simulation results are not only materials to support installation but also tools to identify conditions where installation should not proceed. If results show low generation, poor alignment with demand, significant shading, infeasible installation conditions, or difficult maintenance, you may need to revise the installation scale, change the installation location, adjust timing, or combine other measures. Use simulations to visualize risks rather than searching only for good outcomes—this is the correct practical use.

# Accurately understanding site conditions is the foundation of generation assessment

To improve solar generation simulation accuracy, the accuracy of input site conditions is indispensable. No matter how robust the calculation method, if roof dimensions, azimuth, tilt, surrounding structures, shading factors, terrain, and installable range are ambiguous, the reliability of results falls. When creating simulations for installation decisions, it is important to improve the quality of site surveys.

On-site surveys should capture not only the shape of the planned installation area but also surrounding obstacles and relative heights. Rooftop equipment, upstands, adjacent buildings, trees, utility poles, signs, and fences affect shading. For ground installations, site boundaries, slopes, level differences, drainage direction, and maintenance routes are also important. Conducting a simulation with insufficient information risks misjudging installable capacity and generation.

Do not overlook azimuth and position deviations. A site that appears regular on drawings may have actual azimuth, boundaries, or structure positions that differ from assumptions. Old drawings may not reflect renovations or extensions. Since azimuth and shading affect simulation results, use information close to current conditions.

Especially for sites with complex roof or land conditions, position and height data obtained on-site influence decision accuracy. Planar dimensions alone may not capture shading or installable range. Organizing on-site coordinates, elevation, and structure positions makes it easier to align understanding among design, construction, and decision-making stakeholders. As a result, simulation assumptions become clearer and the basis for installation decisions easier to explain.

As a means to streamline such site understanding, using LRTK (iPhone-mounted GNSS high-precision positioning device) is effective. In solar installation studies, grasping the site position, range, height, and surrounding structures is the foundation of simulation accuracy. If you can organize installable ranges and shading factors based on high-precision on-site position data, you can avoid overreliance on desk-based assumptions. In practice, when using generation simulations for installation decisions, pay attention not only to calculation results but also to the accuracy of the underlying site data.

# Summary

When making installation decisions using solar power generation simulations, you should not decide based solely on the size of annual generation. It is necessary to comprehensively confirm whether the annual generation is sufficient for the installation purpose, whether monthly generation and usage patterns align, whether losses from shading, azimuth, and tilt are realistically accounted for, whether the balance between self-consumption and selling electricity matches the policy, whether equipment capacity and installation space are feasible, and whether the plan holds up including maintenance and long-term changes.

Simulations are not for producing convenient numbers to push installation forward but for organizing conditions that warrant proceeding or must be revised. By checking not only favorable results but also major risks and uncertainties in assumptions, you can reduce gaps after installation. Practitioners should organize the basis for generation, assumptions, loss logic, and compatibility with demand clearly so they can explain them internally or to customers.

Solar power systems are long-term installations. By judging with an eye to future degradation, maintenance, surrounding environmental changes, and changes in electricity use as well as generation at installation, you can create a realistic installation plan. The foundation of that judgment is accurate site condition understanding. The more precisely you capture roof and site dimensions, azimuth, tilt, shading factors, and positions of surrounding structures, the more reliable generation simulations become. By using high-precision on-site positioning such as LRTK and carefully preparing the site data underpinning simulations, installation decisions for solar power become more certain.