7 Conditions to Compare in Solar Power Generation Simulations

When comparing solar power generation simulations, judging only by the annual generation figure can lead to results that diverge significantly from actual performance after installation. Even for the same building or the same site, simulation results change if system capacity, installable area, orientation, tilt, shading, generation losses, or assumptions about self-consumption differ. This article explains seven conditions you must always check when comparing multiple proposals or scenarios for "solar power generation simulation," from a practical, user-friendly perspective for practitioners.

Contents

• It is important to align conditions when comparing solar power generation simulations

• Condition 1: Compare system capacity and installable area

• Condition 2: Compare orientation and tilt angle

• Condition 3: Compare solar irradiance and regional conditions

• Condition 4: Compare how shading and obstacles are reflected

• Condition 5: Compare generation losses and loss rates

• Condition 6: Compare self-consumption and surplus electricity

• Condition 7: Compare constructability and maintainability

• Simulation assumptions that are easy to overlook in condition comparisons

• Practical way to read vendor proposals when comparing them

• Summary

It is important to align conditions when comparing solar power generation simulations

Solar power generation simulations are an important document for predicting output before installation. By checking how much power can be generated annually, how monthly generation varies, and how much can be self-consumed on site, it becomes easier to make installation decisions and to explain them internally. However, simulation results vary widely depending on the input assumptions.

When you receive proposals from multiple vendors, you may find that annual generation, self-consumption rate, and surplus electricity differ even though they target the same building or site. These differences are not just due to calculation methods. They arise from differences in system capacity settings, assumptions about installable area, orientation and tilt, solar irradiance data, shading assessment, generation losses, and assumptions about power consumption.

Therefore, when comparing solar power generation simulations, do not judge by generation figures alone. A proposal with a large annual generation may simply have a larger system capacity. It might also have failed to sufficiently account for shading. Or it may have assumed unrealistically low generation losses, producing results that differ from actual generation.

What matters in comparisons is aligning the conditions. Decide clearly whether you are comparing the same system capacity, maximum installable capacity, or capacity sized to match self-consumption. In addition to generation figures, check self-consumption, surplus electricity, monthly generation, generation per unit of capacity, and a breakdown of generation losses to see the true strength of each proposal.

Solar power generation simulations are not documents intended to make generation look large. They should reflect site and operational conditions and be used to judge how much usable power you can expect after installation. If you verify the conditions to compare, you can avoid being misled by surface numbers and make more realistic installation decisions.

Condition 1: Compare system capacity and installable area

The first conditions to compare are system capacity and installable area. A larger system capacity tends to result in a larger annual generation. Therefore, when comparing multiple simulations, looking only at total generation makes proposals with larger capacities appear advantageous. In practice, however, a larger capacity does not necessarily mean a more optimal plan.

For rooftop projects, you must distinguish between total roof area and installable area. Roofs have HVAC equipment, exhaust equipment, piping, railings, rooftop structures, access hatches, drains, waterproofing clearance requirements, and inspection paths. A roof that looks spacious on the drawings may have limited areas where panels can actually be installed. If the installable area is overestimated, both system capacity and annual generation can appear larger, but detailed design before construction may require revisions.

The same applies to ground-mounted projects. Even with a large site area, site boundaries, slopes, elevation differences, trees, drainage channels, access tracks, existing structures, and planned future use can limit the actual usable area. In ground installations, placing panel rows too close can create row-to-row shading in winter and reduce generation efficiency. A layout that does not provide maintenance access paths can hinder inspection, weeding, and cleaning.

When comparing system capacity, check not only the total capacity but also generation per unit of capacity. High generation per unit capacity may indicate efficient placement in favorable locations. Conversely, low generation per unit capacity may indicate that capacity has been increased to include shaded areas, unfavorably oriented areas, or locations that are difficult to maintain.

Also clarify whether comparisons are based on maximum capacity or an appropriate capacity. Proposals using maximum capacity tend to show larger generation, but may raise issues with surplus power or maintainability. If you prioritize self-consumption, compare capacities sized to match daytime facility demand. System capacity and installable area form the foundation for comparing simulations.

Condition 2: Compare orientation and tilt angle

The second conditions to compare are orientation and tilt angle. The direction panels face and the angle at which they are installed affect annual generation, monthly generation, and hourly generation profiles. Even with the same system capacity, differences in orientation and tilt can significantly change simulation results.

Generally, south-facing installations tend to yield higher annual generation. However, on rooftops there may not be enough area facing south. East- and west-facing surfaces may also be utilized. East-facing systems generate more in the morning, while west-facing systems generate more in the afternoon. Depending on a facility’s load pattern, east-west generation can be effective, not just south-facing generation.

Tilt angle is also important. On roofs, panels are often installed to match the existing roof pitch, so you may not be able to choose an ideal angle freely. On flat roofs or ground-mounted systems, racking can set the angle, but increasing tilt affects wind loads, row-to-row shading, spacing, and constructability. Do not decide the angle based solely on generation efficiency; consider the balance with capacity and maintainability.

When comparing orientation and tilt angle, it is important to review monthly generation as well as annual generation. Tilt angle changes the seasonal distribution of generation. For facilities with high winter demand, how much winter generation is secured is important. For facilities with large summer air-conditioning loads, consider summer generation together with output reductions due to high temperatures.

Also check hourly generation curves. For facilities with high morning demand, morning generation is important. For facilities with high afternoon demand, west-leaning generation can be effective. Even if annual generation is the same, whether the generation timing matches facility demand affects self-consumption and electricity bill savings.

Orientation and tilt are conditions that are hard to judge from layout appearance alone. In simulations, compare each installation surface’s orientation, tilt, generation, and hourly generation tendencies to confirm they match site conditions and facility usage.

Condition 3: Compare solar irradiance and regional conditions

The third conditions to compare are solar irradiance and regional conditions. Solar power generation depends on solar irradiance, so regional solar conditions form the basis of generation. Even with the same capacity, orientation, and tilt, annual generation varies if regional irradiance or weather conditions differ.

It is important to confirm what regional solar irradiance data the simulation uses. Using conditions near the installation site or broader regional averages affects the reliability of generation estimates. Mountainous areas, coastal areas, urban areas, basins, snowy regions, and fog-prone regions can have different irradiance even within nearby areas.

Also check monthly irradiance conditions. Annual average irradiance alone does not reveal seasonal generation variability. Rainy seasons, typhoons, periods with many cloudy days, short winter daylight hours, and snow cover should be reflected in the monthly generation. Simulations that do not reflect regional characteristics may make annual generation look better than reality.

Temperature conditions also affect generation. Even in summers with high irradiance, panel temperatures rise and output can decline. If summer generation is estimated to be large, verify that high-temperature generation losses are reflected. In snowy regions, ensure that snow losses and reduced irradiance are taken into account.

When comparing solar irradiance and regional conditions, confirm that multiple proposals use the same weather assumptions. If one proposal uses conservative irradiance and another uses optimistic irradiance, you cannot directly compare annual generation. Separate differences due to design from those due to irradiance assumptions.

Solar power generation is strongly influenced by regional conditions. By confirming solar irradiance, temperature, snow, cloudiness, and seasonal variability as comparison conditions, you can make generation estimates that are closer to reality.

Condition 4: Compare how shading and obstacles are reflected

The fourth condition to compare is how shading and obstacles are reflected. Shading on panels reduces generation. Simulations that do not sufficiently account for shading tend to overestimate generation and may differ greatly from post-installation performance.

Sources of shading vary by site. On roofs, rooftop equipment, rooftop structures, railings, piping, exhaust equipment, antennas, signage, and adjacent buildings create shading. On land, trees, utility poles, surrounding structures, terrain elevation differences, slopes, and neighboring buildings cause shading. Check how thoroughly these factors are reflected in the simulation.

Shading changes with time of day and season. Shadows that are short in summer can be long in winter because of lower solar altitude. Morning and evening shadows are often overlooked but can affect self-consumption if they overlap peak demand periods. For shading assessment, it is useful to check not only annual shading losses but also monthly and hourly generation.

When comparing shading reflection, observing the difference between generation with and without shading makes it clear. Proposals that show conservative generation after accounting for shading may be closer to reality. Conversely, a proposal that shows high generation despite many shading factors likely underestimates shading effects.

Obstacles affect not only shading but also installable area and maintainability. Placing panels close to rooftop equipment can hinder inspection and repair, not only cause shading. On land, placing panels near trees or structures can create problems with fallen leaves, soiling, weeding, and maintenance access.

When reviewing vendor proposals, confirm the extent of shading considered, whether a site survey was conducted, and whether rooftop equipment and surrounding buildings were reflected. The clarity of shading assumptions, not just generation figures, determines the reliability of the simulation.

Condition 5: Compare generation losses and loss rates

The fifth conditions to compare are generation losses and loss rates. Solar power systems do not always generate at theoretical maximum under ideal conditions. Actual generation is reduced by temperature rise, wiring losses, power conversion losses, shading, soiling, snow, equipment downtime, and aging.

In simulations, check how these generation losses are accounted for. If loss rates are set low, annual generation will be displayed as larger. Conversely, realistically reflecting loss rates based on site conditions can produce more conservative generation estimates. The important point is not whether generation is large but whether the loss rate assumptions can be justified.

Temperature losses are especially important for rooftop installations. Roof surfaces tend to heat up, and panel temperature increases lower output. If summer generation is estimated to be large, confirm whether high-temperature generation losses have been included.

Also check wiring and conversion losses. Power generated at the panels passes through wiring and equipment before being used on site, incurring losses. Understand whether the figures refer to panel-side generation or usable energy after conversion.

Soiling, snow, and aging are also comparison factors. In areas with heavy dust, many falling leaves, bird soiling, or snow, typical loss assumptions may be insufficient. For long-term operation, consider aging effects and possible equipment downtime.

Generation losses and loss rates can be presented differently among proposals. If only an overall loss rate is shown, verify what it includes. Whether temperature, wiring, conversion, shading, soiling, snow, and aging are included—or treated separately—must be clear to compare proposals correctly.

Condition 6: Compare self-consumption and surplus electricity

The sixth conditions to compare are self-consumption and surplus electricity. Simulations often show large annual generation, but what matters in practice is how much of that generated energy can actually be used on site. You cannot judge installation benefits correctly without separating self-consumption and surplus electricity.

Self-consumption is the amount of generated power actually used within the facility. This portion tends to reduce purchased electricity and directly affects electricity cost savings and profitability. Surplus electricity is the generated power that the facility cannot consume at the time of generation. Whether surplus is exported, stored in batteries, or curtailed affects installation benefits.

Relying on self-consumption rate alone is risky. With small system capacity, it is easier to use all generated power, producing a high self-consumption rate, but the absolute self-consumption amount may be small. With large capacity, self-consumption rate may drop while self-consumption amount increases. In comparisons, view self-consumption rate and self-consumption amount together.

Hourly load profiles are also a comparison factor. Proposals that calculate self-consumption using only annual consumption differ in accuracy from those that reflect hourly or monthly consumption data. Since solar generation occurs mainly during daytime, overlap with daytime demand is important. Confirm whether proposals reflect differences between weekdays and holidays and seasonal operating variations.

If a proposal shows a large amount of surplus electricity, the system capacity may be too large for the facility demand. Without a clear plan to utilize surplus, high generation may have limited practical benefit. When batteries are included, separate comparisons should be made between surplus without batteries and the increase in self-consumption with batteries, accounting for charge/discharge losses.

Comparing self-consumption and surplus electricity tells you whether generated power will be used or remain unused. Focusing on usable energy rather than generation alone is the practical way to read solar power generation simulations.

Condition 7: Compare constructability and maintainability

The seventh conditions to compare are constructability and maintainability. Simulations typically emphasize generation and system capacity, so constructability and maintainability can be overlooked. However, these factors strongly affect long-term operation.

For rooftop projects, structure, waterproofing, load capacity, inspection paths, and access to existing equipment are important. Filling the roof entirely with panels increases capacity but may make rooftop equipment inspection or waterproofing repairs difficult. Placing panels around drains or access hatches can interfere with building management.

For ground-mounted projects, check maintenance access, drainage, weeding, ground conditions, slopes, site boundaries, and surrounding environment. Filling the entire site with panels increases capacity but may hinder weeding, inspection, and cleaning. Poor drainage can cause mud or standing water during rain, affecting equipment and maintenance activities.

A layout with poor maintainability can affect long-term generation. Soiled areas that are hard to clean accumulate dirt and reduce generation. Difficult-to-inspect layouts delay the detection and response to equipment faults. These elements are hard to see in generation simulations but critical in actual operations.

When comparing constructability and maintainability, cross-check layout drawings with site conditions. Verify whether inspection paths are secured, whether there are awkward placements near obstacles, and whether future equipment replacement or roof repairs are feasible. Even a proposal with large generation can be risky long-term if maintenance is difficult.

Solar installations are not a one-time effort but equipment to be operated over many years. Including constructability and maintainability in comparison conditions helps you determine whether a plan is not only high-yield but also one you can operate stably over time.

Simulation assumptions that are easy to overlook in condition comparisons

When comparing solar power generation simulations, it is easy to overlook differences in assumptions. When generation and self-consumption rate figures are presented, you may be tempted to compare results directly. However, if the underlying assumptions differ, the comparison is invalid.

A commonly overlooked difference is between an initial proposal and the final design. An initial proposal may assume a wide installable area, but after a site survey, the layout may change to reflect inspection paths, obstacles, and waterproofing constraints. Using initial simulation figures as-is can lead to large differences from post-installation performance.

Next is the assumption about power consumption data. If self-consumption is estimated using only annual consumption versus using monthly or hourly data, results change. Simulations that do not consider holidays or seasonal variations may overestimate actual self-consumption.

Assumptions about batteries and emergency usage are also easy to miss. Looking only at the battery-included effect can obscure the standalone PV self-consumption and surplus issues. You need to separate battery vs. no-battery cases, account for charge/discharge losses, and reserve capacity for emergency use.

Also confirm what stage of energy is shown in the generation figures. Are the figures panel-side generation, usable energy after conversion, or effective values reflecting losses? The meaning of the numbers changes, and terminology may be used differently across proposals.

In condition comparisons, verifying assumptions is more important than the numbers themselves. If assumptions are not aligned, a proposal showing higher generation or a higher self-consumption rate is not necessarily better. Organize comparison conditions and view proposals on the same footing to improve practical decision accuracy.

Practical way to read vendor proposals when comparing them

When comparing multiple vendor proposals, do not simply rank them by annual generation; instead, read them against aligned conditions. First confirm system capacity and installable area. Proposals with larger capacity tend to show greater generation, so evaluating only total generation can mislead. Check generation per unit capacity and which installation surfaces contribute to generation.

Next confirm how shading and loss rates are treated. If a site has shading factors but a proposal shows high generation, it may have not sufficiently accounted for shading. Proposals with low loss rates also appear to generate more. Check how temperature, wiring, conversion, soiling, snow, and aging are included.

Assumptions about self-consumption are also important. Confirm the granularity of power consumption data used: whether daily/weekly/monthly/hourly patterns are reflected, and whether weekdays and holidays are separated. Even with high generation, if it does not match facility demand, surplus will increase. Proposals that leave surplus handling vague are harder to evaluate.

When reviewing vendor proposals, have a clear list of questions to ask. Why that system capacity? Which installation surfaces are used? How did you evaluate shading? What is included in the loss rate? From which power consumption data was self-consumption calculated? Proposals that can answer these questions concretely are more likely to have well-organized assumptions.

Also confirm each proposal’s objective. Is it optimized for maximum generation, for self-consumption, or for maintainability? Different objectives change what an optimal condition looks like. Comparing proposals with different objectives as-is can lead to incorrect decisions.

In practice, choose the proposal that fits site conditions and operational objectives, not the one with the highest generation. Solar power generation simulations become useful decision documents only when you read them with aligned comparison conditions.

Summary

The seven conditions to compare in solar power generation simulations are system capacity and installable area; orientation and tilt angle; solar irradiance and regional conditions; shading and obstacles; generation losses and loss rates; self-consumption and surplus electricity; and constructability and maintainability. You cannot judge a proposal’s merit by annual generation alone. It is essential to check the conditions under which figures were calculated.

For system capacity and installable area, confirm not only total capacity but also the realistically installable area considering inspection paths, obstacles, waterproofing, drainage, and maintenance access—not just total roof or land area. For orientation and tilt, check monthly and hourly generation as well as annual totals.

For irradiance and regional conditions, verify that weather data appropriate to the installation site is used. For shading and obstacles, ensure that winter and morning/evening shadows and rooftop equipment or tree impacts are reflected. For generation losses and loss rates, compare how temperature, wiring, conversion, soiling, snow, and aging are accounted for.

Self-consumption and surplus electricity are particularly important for judging installation benefits. High generation is of limited use if it cannot be consumed on site. Check both self-consumption rate and absolute self-consumption amount and separate surplus electricity. For constructability and maintainability, confirm that a layout maximizing generation does not create infeasible long-term operation.

Also pay attention to simulation assumptions: differences between initial proposal and final design, the granularity of power consumption data, the presence of batteries, and the stage of energy represented in generation figures can all change the meaning of “generation.” When comparing vendor proposals, align conditions and verify why each result was produced.

Accurate site information forms the foundation for reliable comparison conditions. If you can precisely capture installable areas, rooftop equipment, obstacles, trees, site boundaries, inspection paths, surrounding structures, and candidate connection points, the simulation assumptions become clear and vendor comparisons become easier.

If you want to accurately record installable areas, rooftop equipment, obstacles, site boundaries, inspection paths, and candidate connection points on site to improve the comparison accuracy of solar power generation simulations, using LRTK, an iPhone-mounted GNSS high-precision positioning device, is effective. High-precision location data from the field makes it easier to identify shading and obstacles, confirm installable areas, compare vendor proposals, perform pre-construction checks, and manage maintenance consistently. To correctly compare conditions in solar power generation simulations, it is important to complement desk calculations with a system for accurately understanding the site.