10 Items to Confirm in Calculation Conditions for Solar Power Generation Simulations

When reviewing a solar power generation simulation, judging solely by the annual generation figure can lead to discrepancies with post-installation actuals. Generation depends on multiple calculation conditions such as system capacity, installable area, insolation, azimuth, tilt, shading, temperature, soiling, snowfall, wiring, power conversion, and facility power usage. In other words, to read a simulation result correctly, it is important to check the assumptions that produced the numbers, not just the numbers themselves. This article explains 10 items that practitioners searching for "solar power generation simulation" should always confirm as calculation conditions.

Table of contents

• The meaning of checking calculation conditions in solar power generation simulations

• Item 1: Confirm assumptions about system capacity and number of panels

• Item 2: Confirm installable area and layout conditions

• Item 3: Confirm assumptions about insolation and regional conditions

• Item 4: Confirm azimuth and tilt angle settings

• Item 5: Confirm how shading and obstacles are reflected

• Item 6: Confirm loss rates such as temperature, soiling, and snow

• Item 7: Confirm wiring losses, power conversion losses, and output limits

• Item 8: Confirm monthly generation and time-of-day generation curves

• Item 9: Confirm assumptions about power usage, self-consumption, and surplus

• Item 10: Confirm assumptions about degradation, maintenance, and performance management

• How to read calculation conditions when comparing vendor proposals

• Summary

The meaning of checking calculation conditions in solar power generation simulations

A solar power generation simulation is a document to predict how much power a planned roof or land installation will generate. Because annual generation and monthly generation are presented as numbers, the simulation is easy to use for investment decisions, internal approvals, financial review, and comparing vendor proposals. However, simulation results are predictions based on input conditions, and if conditions change, the generation will change.

Even for the same building or land, different proposals can show different generation. Reasons include differences in system capacity, how installable area is interpreted, the extent to which shading is modeled, the loss rate settings, or how facility power usage is treated—i.e., differences in calculation conditions. A proposal that shows higher generation is not necessarily better. Proposals that carefully reflect on-site conditions may appear to forecast more conservative generation.

The purpose of checking calculation conditions is not to distrust the numbers, but to understand their meaning correctly and reduce gaps after installation. If you can distinguish whether a high annual generation comes from larger system capacity, better insolation assumptions, or optimistic loss rates, it becomes easier to judge the proposal's validity.

Also, organizing calculation conditions helps with post-installation performance management. If actual generation is lower than expected, the recorded calculation conditions provide clues as to whether the cause is weather, shading, soiling, snow, equipment downtime, or changes in facility demand. Without recorded calculation conditions, it becomes difficult to analyze discrepancies between expectations and actual results.

A solar power generation simulation is a document to read not only for its results but also for its assumptions. For use in investment decisions, it is important to check each calculation assumption that underlies the generation figures and to enable comparisons under the same assumptions.

Item 1: Confirm assumptions about system capacity and number of panels

The first calculation conditions to check are system capacity and the number of panels. Generally, generated energy increases with system capacity. Therefore, when comparing multiple simulations, proposals with larger system capacity may appear more favorable if you only look at annual generation. That does not mean they are more efficient; they may simply have more panels installed.

When confirming system capacity, look not only at total capacity but also at how much is allocated to each roof surface or land area. For roof projects, split capacity by installation surfaces such as south-facing, east-facing, west-facing, and flat roof zones. For land projects, confirm how many panels are allocated to each parcel within the site. Because conditions differ by installation area, total capacity alone does not justify the generation estimate.

Panel count is also important. More panels generally increase generation, but if panels are forced into shaded or hard-to-maintain locations, generation per capacity can decrease. A plan that installs the maximum possible number of panels and a plan that restricts panels to better conditions will differ in total generation and in practical effectiveness.

Also check annual generation per unit capacity to better assess proposal reasonableness. Even if total generation is large, low generation per capacity may indicate poor conditions like shading, unfavorable azimuth or tilt, or high loss rates. Conversely, a proposal that keeps total capacity modest but has high generation per capacity may be efficiently using the best areas.

When checking capacity in a simulation, confirm whether the assumption is maximum feasible capacity or an appropriate capacity matched to facility self-consumption. If the focus is on self-consumption, filling every available spot on a roof or land may not be optimal. It is important to review system capacity together with generation, self-consumption, and surplus energy.

Item 2: Confirm installable area and layout conditions

The second calculation condition to check is installable area and layout. In simulations, system capacity is sometimes set based on the total area of a roof or land. However, total area and actually usable area are not the same. Overestimating installable area leads to overestimated generation.

For roof projects, consider rooftop equipment, piping, penthouses, handrails, drains, inspection hatches, waterproofing clearances, and inspection walkways. Areas that look free on drawings may need to be left open for inspection, repair, drain cleaning, or waterproofing work. Simulations that pack panels across the entire roof can show large generation but may require layout changes before construction.

For land projects, check site boundaries, slopes, elevation changes, trees, utility poles, drainage ditches, existing structures, maintenance paths, and potential connection points. Even a large site cannot necessarily be fully used for generation equipment. Paths and spaces are required for weeding, inspection, cleaning, equipment replacement, and drainage management. Layouts that ignore these needs can create maintenance problems after installation.

Layout conditions should also include panel orientation, row-to-row spacing, aisle width, equipment placement, and wiring routes. For flat roofs and land projects, row spacing is required to avoid inter-row shading. Tightening row spacing increases installable capacity but can reduce generation due to inter-row shading in winter.

Installable area and layout can change between initial study and field survey. Initial simulations may be based on drawings or aerial photos, but field surveys may reveal equipment positions, obstacles, inspection routes, drainage, boundaries, and elevation differences that require layout revision. For investment decisions, use simulations based on a final layout that reflects on-site conditions.

Item 3: Confirm assumptions about insolation and regional conditions

The third calculation condition is insolation and regional conditions. Because solar generation depends on received sunlight, regional insolation conditions form the foundation of generation forecasts. Even with the same system capacity, azimuth, and tilt, generation will differ if insolation or weather conditions vary by region.

When checking insolation assumptions, verify that conditions close to the installation site are used. If only broad regional averages are used, predictions may deviate in mountainous areas, coastal zones, basins, snowy regions, or frequently cloudy areas. Insolation can also vary within the same region due to topography and surrounding environment.

Check monthly insolation as well as annual totals. Annual totals alone do not show seasonal generation variability. Determine whether generation is expected mainly in summer or how much it drops in winter, and how much impact is assumed for the rainy season, typhoons, or prolonged cloudiness. If monthly generation is not provided, it is difficult to assess seasonal variability.

Temperature is also part of regional conditions. Even in sunny summers, high module temperature can reduce generation efficiency. If summer generation looks high, verify whether temperature losses are accounted for. Simulations based only on insolation risk overestimating summer generation.

In snowy regions, check for generation reductions due to snow. In addition to winter insolation, snow on modules and lingering snow can cause periods of no generation. If winter generation appears high, confirm how much snow impact is assumed.

Insolation and regional conditions are central to generation forecasts. When comparing proposals, check whether the same insolation conditions are used and whether regional characteristics are reflected. Comparing proposals with different insolation assumptions can lead to incorrect conclusions about generation differences.

Item 4: Confirm azimuth and tilt angle settings

The fourth calculation condition is azimuth and tilt angle. The direction panels face and the angle at which they are installed affect how much sunlight they receive and therefore annual, monthly, and intra-day generation. Even with identical system capacity, different azimuth and tilt settings change generation forecasts.

Regarding azimuth, south-facing surfaces tend to yield higher annual generation. However, south-facing is not always optimal in practice. East-facing panels generate more in the morning and west-facing more in the afternoon. If a facility’s demand peaks in the morning or afternoon, east- or west-facing panels can more effectively support self-consumption.

When checking azimuth in a simulation, ensure the building or site is not treated as a single orientation. For roof projects with multiple roof surfaces, orientations differ by face. For land projects, site shape and connection constraints may prevent ideal orientation. Checking generation per installation surface reveals which orientations contribute to generation.

Tilt angle is also important. For roof projects, modules are often installed according to existing roof pitch, so ideal tilt may not be freely chosen. For flat roofs and land projects, rack angle can be set, but increasing angle affects inter-row shading, wind load, spacing, and maintainability. Reducing angle can allow greater installable capacity but may affect seasonal generation balance and the way soiling accumulates or clears.

Azimuth and tilt affect monthly generation balance. Tilt can change the summer-winter generation ratio. Depending on whether facility demand is higher in summer or winter, the relevant generation metric may differ.

When checking azimuth and tilt as calculation conditions, confirm that the simulation uses conditions that can actually be constructed rather than idealized ones. If azimuth or tilt in the simulation differ from actual constructible conditions, the forecast may diverge from reality.

Item 5: Confirm how shading and obstacles are reflected

The fifth calculation condition is how shading and obstacles are reflected. Shading is a major cause of reduced solar generation. Simulations that do not sufficiently account for shading may overstate annual and monthly generation.

Sources of shading include surrounding buildings, rooftop equipment, penthouses, handrails, piping, HVAC equipment, exhaust equipment, trees, utility poles, signs, slopes, and terrain variations. For roof projects, rooftop equipment and surrounding buildings are important shading sources. For land projects, check trees, utility poles, neighboring buildings, slopes, and nearby structures.

Shading changes with time of day and season. Shadows that are short in summer lengthen in winter as solar altitude falls. Morning shadows are commonly cast by east-side obstacles, and evening by west-side obstacles. In simulations, confirm how much shading is assumed for winter and for morning/evening periods.

It is useful to compare generation with and without shading to see the impact. If generation is conservative when shading is included, the simulation is likely closer to reality. Conversely, if the site has many shading factors but the simulation shows high generation, shading may be underrepresented.

Obstacles affect not only shading but also maintainability and soiling. Trees can cause soiling from leaves or attract birds. Rooftop equipment affects inspection routes in addition to shading. Slopes and surrounding structures on land sites relate to drainage and maintenance access as well as shading.

Shading and obstacle conditions can change significantly before and after a field survey. Obstacles overlooked in an initial simulation based on drawings or aerial photos may be discovered during a field survey. For final decisions, use simulations that reflect shading and obstacles identified in the field.

Item 6: Confirm loss rates such as temperature, soiling, and snow

The sixth calculation condition is the loss rates for temperature, soiling, snow, and similar factors. Simulations estimate practical generation by subtracting various losses from ideal generation. Different loss rate settings produce different forecasts even for the same system capacity.

Temperature loss is the output reduction caused by elevated module temperature. This is especially relevant in summer and for roof-mounted installations. Even with high insolation, high module temperature can prevent generation from increasing as expected. If summer generation looks extremely high, check whether temperature losses are included.

Soiling loss is generation reduction caused by dust, pollen, leaves, bird droppings, exhaust-related grime, and particulates adhering to module surfaces. In areas with many trees, nearby unpaved areas, dust sources, or conditions attracting birds, soiling should be factored in. Ease of cleaning and inspection also affects the assumed soiling loss.

In snowy regions, account for generation reductions due to snow. Snow on modules causes periods without generation. The angle at which snow slides off, snow storage space, ease of snow removal and inspection, and the system’s snow-load tolerance all influence winter generation. If winter generation looks high, verify whether snowfall and residual snow are sufficiently considered.

Loss rates may be presented as aggregate numbers, but comprehensive loss rates can obscure what is included. Confirm the extent to which temperature, shading, wiring, conversion, soiling, snow, and degradation over time are included. Proposals with low loss rates show higher generation but may be optimistic relative to site conditions.

Loss rates greatly influence forecast reliability. As a calculation condition, verify that each loss aligns with the site environment.

Item 7: Confirm wiring losses, power conversion losses, and output limits

The seventh calculation condition is wiring losses, power conversion losses, and output limits. Power generated by panels is not directly usable by the facility; it passes through wiring and conversion equipment to be made usable, and losses occur along the way.

Wiring losses occur when transmitting power from panels to equipment and from equipment to facility systems. Long wiring distances or complex routing can increase losses and affect constructability. Check the assumed wiring losses in the simulation. If equipment positions or wiring routes change from the initial layout, wiring loss assumptions should be revisited.

Power conversion losses occur during conversion of generated power into a form usable by the facility. The usable energy after conversion depends on converter/inverter capacity, installation conditions, and operating state. Confirm whether the reported generation is the panel-side generation or the usable energy after conversion.

Also check output limits or clipping. Even with larger panel capacity, converter capacity or connection conditions may cap output during some periods. This is not necessarily bad, but you should confirm how much generation loss occurs and whether it affects self-consumption.

Handling of surplus energy is also relevant. If facility demand cannot absorb all generation, determine whether surplus can be exported, stored in batteries, or curtailed—each option changes the actually usable energy. Simulations showing only total generation can obscure output limits and surplus handling.

Wiring losses, conversion losses, and output limits directly affect simulated generation. Confirm not only how much the panels can generate but how much energy is expected to be usable by the facility.

Item 8: Confirm monthly generation and time-of-day generation curves

The eighth calculation condition is monthly generation and time-of-day generation curves. Annual generation alone does not show in which seasons or at what times of day generation occurs. To assess self-consumption and surplus, you must check the temporal distribution of generation.

Monthly generation shows seasonal differences. Summer may have high insolation but could see temperature losses. Winter has shorter daylight, lower solar altitude, and greater susceptibility to shading and snow. Generation may be suppressed during rainy seasons, typhoons, or cloudy periods. If monthly generation is not provided, seasonal variability is hard to assess.

Time-of-day generation curves indicate when generation occurs. Near-south-facing surfaces tend to peak around midday; east-facing peaks in the morning, west-facing in the afternoon. For the same annual generation, self-consumption varies depending on when facility demand occurs.

Generation curves are also useful for judging surplus. If generation around midday far exceeds facility demand, surplus is likely. Combining east and west faces can smooth generation peaks. However, check balance with total generation and installed capacity.

Time-of-day generation also offers clues about shading. A lack of morning generation may indicate east-side shading; a rapid falloff in the evening may indicate west-side shading; unusual midday dips may point to rooftop equipment or nearby structures.

By checking monthly generation and time-of-day curves, you can see practical usability that annual totals omit. Confirm not only how much generation occurs but whether it occurs at times when it can be used.

Item 9: Confirm assumptions about power usage, self-consumption, and surplus

The ninth calculation condition is power usage and assumptions about self-consumption and surplus. Simulations should cover not only how much is generated but how much of that generated energy the facility can use. This condition is particularly decisive for self-consumption-oriented projects.

When confirming power usage, annual consumption alone is insufficient. Solar generation mainly occurs during daytime, so assess daytime demand. Facilities that operate mainly at night may find self-consumption difficult despite high annual consumption. If a facility has high weekday demand but low weekend demand, surplus may increase on weekends.

Monthly power usage is also important. Facilities with high cooling loads in summer find it easier to consume summer generation. Facilities where heating or production equipment drives winter demand may be disadvantaged if winter generation is insufficient. Check the seasonal overlap between generation and usage.

Self-consumed energy is the portion of generated power used within the facility during generation. Surplus energy is the portion generated but not used at the same time. Simulations should separate these two. Large total generation with small self-consumption can limit the benefits of installation.

Do not judge only by self-consumption rate. Small system capacity often yields a higher self-consumption rate but low absolute self-consumed energy. Large capacity may lower the self-consumption rate but increase absolute self-consumed energy. Review self-consumption rate, self-consumed energy, and surplus together.

If batteries are included, compare results with and without storage. Batteries can shift surplus to other times but have charge/discharge losses and capacity limits. Results that include batteries alone may obscure the appropriate standalone solar capacity and surplus risk.

Confirming power usage and assumptions about self-consumption and surplus lets you link simulation results to actual project value.

Item 10: Confirm assumptions about degradation, maintenance, and performance management

The tenth calculation condition is assumptions about degradation, maintenance, and performance management. Solar equipment is intended for long-term use, and first-year generation does not continue unchanged. For long-term evaluation, include equipment degradation, generation loss, maintenance, and performance management in assumptions.

Degradation refers to changes over time in panels, equipment, wiring, connections, and racking. Confirm whether the simulation shows only first-year generation or accounts for long-term generation changes. Estimates used for long-term revenue or financial review that do not include degradation tend to be optimistic.

Maintenance assumptions are also important. Check whether inspection routes are provided, whether cleaning is feasible, whether equipment is accessible, and whether weed or snow removal is feasible. Poor maintainability can delay detection of soiling, shading, or equipment failures and prolong generation decline. In practice, a layout that maintains generation longer is more valuable than one that maximizes short-term generation.

Performance management assumptions should be checked. Whether post-installation monthly generation, time-of-day generation, generation per installation surface, self-consumed energy, and surplus can be monitored affects long-term operations. If simulation assumptions are recorded, comparing them with actuals makes it easier to identify causes of generation decline.

Changes in the surrounding environment are also long-term calculation factors. Trees may grow and increase shading, new buildings may be constructed nearby, rooftop equipment may be added, or dust may increase from adjacent unpaved areas. While you cannot predict everything, you should catalog foreseeable risks during the field survey.

By confirming degradation, maintenance, and performance management assumptions, simulations move from a first-year forecast to a long-term operational decision tool. If you plan to use solar generation simulations in practice, verify assumptions up to what can be managed after installation.

How to read calculation conditions when comparing vendor proposals

When comparing vendor proposals, lining up generation figures side-by-side is insufficient. If calculation conditions differ between proposals, differences in annual generation or self-consumption may stem from differing assumptions rather than proposal quality. First, confirm whether the proposals are comparable under the same conditions.

For proposals with different system capacities, check generation per unit capacity rather than total generation. For differing interpretations of installable area, verify how rooftop equipment, inspection routes, drainage, site boundaries, and maintenance paths are reflected. Proposals that do not reflect site conditions tend to show larger generation but may require corrections before construction.

Also compare insolation and regional assumptions. If proposals use different insolation premises, do not evaluate generation differences directly. Check whether monthly generation, snowfall, temperature, and cloud cover are treated similarly.

Loss rate breakdowns are also important. Compare how much of temperature, shading, soiling, snow, wiring, conversion, and degradation are included. Low loss rate proposals show higher generation but may be optimistic for the site.

For self-consumption estimates, confirm whether calculations use annual consumption only or reflect monthly and time-of-day usage. If weekday/weekend differences, seasonal changes, or daytime demand are not reflected, self-consumption may be overestimated.

When comparing vendor proposals, favor not the proposal with the highest generation but the one with clear calculation conditions that match on-site conditions and facility operation. Simulations with clear assumptions are also easier to use for post-installation performance management.

Summary

To confirm calculation conditions in solar power generation simulations, comprehensively review system capacity, installable area, insolation, regional conditions, azimuth, tilt, shading, loss rates, wiring, power conversion, output limits, monthly generation, time-of-day generation curves, power usage, self-consumption, surplus, degradation, maintenance, and performance management. Generation figures are the outcome of these assumptions.

Item 1 covers system capacity and panel count. Check not only total generation but generation per capacity and capacity by installation surface. Item 2 covers installable area and layout conditions—confirm that simulations use usable area rather than total roof or land area. Item 3 covers insolation and regional conditions—assess monthly insolation, temperature, snowfall, and cloudiness.

Item 4 covers azimuth and tilt angle settings—ensure calculations use constructible rather than ideal conditions. Item 5 covers how shading and obstacles are modeled—confirm winter and morning/evening shading and rooftop/vegetation impacts. Item 6 covers loss rates for temperature, soiling, and snow—understand the composition of aggregate loss rates and whether they fit site conditions.

Item 7 covers wiring losses, conversion losses, and output limits—verify how much panel generation is actually expected to be usable by the facility. Item 8 covers monthly and time-of-day generation curves—know when generation occurs, not just how much. Item 9 covers power usage and assumptions about self-consumption and surplus—separate what can be generated from what can be used. Item 10 covers degradation, maintenance, and performance management—ensure the simulation is usable for long-term operation, not only the first year.

When comparing vendor proposals, check that calculation conditions are aligned rather than choosing the largest generation figure. If conditions are vague, you cannot determine the cause of generation differences. Simulations that reflect site conditions and clearly state loss rates and self-consumption assumptions are more useful for investment decisions.

Accurate field information is the foundation for improving calculation condition accuracy. If you can precisely identify installable ranges, rooftop equipment, obstacles, trees, site boundaries, azimuth, tilt, inspection routes, and connection candidate points, the assumptions behind the solar power generation simulation become clearer and the forecast more reliable.

If you want to improve the accuracy of recording installable ranges, rooftop equipment, obstacles, trees, site boundaries, azimuth, tilt, inspection routes, and connection candidate points on site, using an iPhone-mounted GNSS high-precision positioning device called LRTK is effective. High-precision on-site location data makes it easier to organize shading and obstacles, installable ranges, wiring routes, and maintenance routes, and supports consistent work from vendor comparison to pre-construction checks and post-installation performance management. To correctly confirm calculation conditions in solar power generation simulations, do not rely on desk calculations alone—accurately survey the site and produce materials that allow comparison under the same assumptions.