7 Items to Reduce Investment Risk in Solar Power Generation Simulations

In introducing solar power generation, proposals that show large annual generation and expected returns often seem attractive. However, if you make a decision without checking the assumptions of the solar power generation simulation, gaps may arise after installation such as “it generates less than expected,” “self-consumption is not possible,” “there is a lot of surplus,” or “maintenance is burdensome.” To reduce investment risk, it is important not only to look at the generation numbers but also to verify site conditions, power usage, generation losses, constructability, and long-term operation. This article explains seven items to check to reduce investment risk for practitioners who search for “solar power generation simulation.”

Table of Contents

• The importance of assessing investment risk with solar power generation simulations

• Item 1: Verify the basis for annual generation

• Item 2: Check monthly generation and seasonal variation

• Item 3: Separate and confirm self-consumption and surplus electricity

• Item 4: Confirm generation reduction due to shading, azimuth, and tilt

• Item 5: Confirm loss rates such as temperature, soiling, and snow

• Item 6: Confirm constructability and maintainability

• Item 7: Confirm long-term revenue and changes in demand

• Points to note when comparing vendor proposals

• Summary

The importance of assessing investment risk with solar power generation simulations

Solar power generation simulations are important documents to understand expected generation, self-consumption, surplus electricity, and installation benefits before installation. Based on system capacity, installation location, insolation conditions, azimuth, tilt angle, shading, generation losses, and electricity consumption, you can quantify the expected generation after installation. However, simulations are forecasts based on input conditions and do not guarantee post-installation generation.

To reduce investment risk, you must not only look at the simulation numbers but also confirm the assumptions behind those numbers. Even proposals that show large annual generation may overstate results if they overestimate usable installation area, fail to fully account for shading, underestimate generation losses, or overestimate daytime demand.

Investment risk in solar is not limited to insufficient generation. Even if generation is large, surplus increases if the facility cannot consume it. Oversizing capacity can increase generation that does not translate into self-consumption. If roof or land constructability is impractical, layout changes may be required before construction, or inspection and cleaning may be difficult after installation.

When using simulations to reduce investment risk, read generation that can be produced, generation that can be used, surplus generation, and amounts that may be reduced separately. Also consider how generation and facility demand will change over long-term operation, not just the first year. Solar generation simulations should be used not to present optimistic installation benefits but to identify risks and bring decision-making closer to reality.

Item 1: Verify the basis for annual generation

The first item to reduce investment risk is to verify the basis for annual generation. Annual generation is the most straightforward figure for judging the benefits of installing solar power. However, unless you confirm the conditions used to calculate this number, it is insufficient as a basis for investment decisions.

Annual generation varies with system capacity, insolation, installation location, azimuth, tilt angle, shading, temperature, wiring, power conversion, soiling, snow, and aging. Even for the same roof or land, annual generation can differ among vendors. Those differences may arise not only from equipment performance but also from differences in assumptions.

First, check the system capacity. The larger the capacity, the easier it is for annual generation to increase. Therefore, when comparing multiple proposals, looking only at total generation can make higher-capacity proposals appear favorable. What matters is generation per unit capacity. You need to confirm whether the increased capacity is efficiently generating power or just increasing total generation by using poorly oriented or shaded areas.

Next, confirm the assumption for usable installation area. For roof projects, check whether rooftop equipment, piping, inspection hatches, drainage outlets, railings, waterproofing clearances, and inspection pathways are considered. For land projects, confirm whether site boundaries, slopes, elevation differences, drainage, maintenance paths, trees, and existing structures are reflected. Overestimating usable area leads to overestimation of both capacity and annual generation.

Insolation assumptions are also important. Confirm whether regional insolation conditions, monthly weather trends, snow, cloudy days, and temperature effects are reflected. Even if annual generation looks high, if regional characteristics and site conditions are not sufficiently reflected, actual results after installation may differ significantly.

Annual generation is the entry point for investment decisions but not the final judgment. Rather than focusing on whether the generation number is large, the first step to reduce investment risk is to confirm whether its basis matches site conditions.

Item 2: Check monthly generation and seasonal variation

The second item is monthly generation and seasonal variation. Even if annual generation looks sufficient, monthly figures may be heavily skewed. Solar generation changes month to month due to daylight hours, solar elevation, insolation, temperature, weather, snow, and the length of shadows.

By checking monthly generation, you can see which months have high generation and which have low. While generation tends to increase from spring to summer, it may drop during the rainy season, typhoons, or extended cloudy periods. In summer, although insolation is high, temperature-related losses can occur due to panel temperature rise. In winter, shorter daylight and lower solar elevation cause surrounding buildings, rooftop equipment, and trees to cast long shadows. In snowy regions, snow on panels or residual snow can cause periods when generation is not possible.

For investment risk, the important question is whether months with high generation align with months of high facility demand. For facilities with large cooling demand in summer, summer generation significantly affects installation benefits. For facilities with large heating or production equipment demand in winter, winter generation declines can impact payback and cash flow. Even if annual generation is sufficient, if generation is low in months when demand is high, expected benefits are difficult to achieve.

Monthly generation is also a clue to overlooked generation losses. If winter generation appears high despite expected shading or snow, shading or snow impacts may not be fully reflected. If summer generation looks overly optimistic, confirm whether temperature losses have been accounted for.

By checking monthly generation, you can identify months in which installation benefits are likely and months that require caution. To reduce investment risk, it is important to check not only the annual total but also seasonal generation and its relationship with facility demand.

Item 3: Separate and confirm self-consumption and surplus electricity

The third item is to separate and confirm self-consumption and surplus electricity. Electricity generated by solar does not all have the same value. Electricity used within the facility and electricity left unused as surplus affect investment returns differently.

Self-consumption refers to the amount of electricity generated by solar that is actually used within the facility. This portion tends to reduce externally purchased electricity and is central to installation benefits. Surplus electricity is the amount generated that the facility cannot use at the same time. Investment decisions vary depending on whether surplus is exported externally, stored in batteries, or curtailed.

Estimating self-consumption requires more than annual consumption figures. Since solar generates mainly during daytime, daytime facility demand is critical. Facilities that operate mainly at night may have high annual consumption but find self-consumption difficult. There may be surplus on holidays even if self-consumption is possible on weekdays when demand is lower on weekends.

Relying solely on self-consumption rate is risky. Small capacity systems tend to show high self-consumption rates but may still provide small absolute self-consumption. Large capacity systems may lower self-consumption rates but increase absolute self-consumption. To reduce investment risk, you should view self-consumption rate, self-consumption amount, and surplus electricity as a set.

If surplus electricity is large, the system capacity may be too large relative to facility demand. Even proposals with high generation may overestimate benefits if the handling of surplus is unclear. For proposals including battery storage, separate the surplus without batteries and the additional self-consumption gained with batteries. Batteries do not increase generation; they change the timing of electricity use, so charging/discharging losses and capacity constraints must be considered.

To reduce investment risk, focus on usable generation rather than producible generation. Separating self-consumption and surplus electricity makes it easier to judge the appropriateness of capacity and the realism of expected benefits.

Item 4: Confirm generation reduction due to shading, azimuth, and tilt

The fourth item is generation reduction due to shading, azimuth, and tilt. These factors vary greatly with site conditions and directly affect the generation figures in simulations. Simulations that do not sufficiently reflect shading, azimuth, and tilt can make the installation benefits look overly optimistic.

Sources of shading include surrounding buildings, rooftop equipment, railings, penthouses, piping, trees, utility poles, signs, and terrain elevation differences. Shading varies by time of day and season. Shadows may be short in summer but extend long in winter when solar elevation is low. In simulations, it is important to check the difference in generation with and without shading, monthly generation, and hourly generation.

Azimuth affects generation and self-consumption. South-facing surfaces tend to yield higher annual generation, but east- and west-facing surfaces can be effective depending on facility demand timing. Facilities with high morning demand benefit from east-facing arrays; those with high afternoon demand benefit from west-facing arrays. Optimal azimuth should be considered not only for annual generation but for compatibility with facility load patterns.

Tilt angle also affects generation. For roof projects, panels often follow the existing roof slope and cannot always be set at an ideal angle. For flat roofs and land projects, mounting angles can be set, but larger angles affect inter-row shading, wind loads, spacing, and constructability. Even if a larger angle slightly increases generation, if it makes construction or maintenance impractical, investment risk increases.

Shading, azimuth, and tilt are items easily overlooked in initial proposals. Reflecting site survey results in the assumptions may reduce generation. However, such corrections are not negative; they are necessary to reduce post-installation gaps. In investment decisions, use effective generation based on site-reflected conditions rather than ideal conditions.

Item 5: Confirm loss rates such as temperature, soiling, and snow

The fifth item is loss rates such as temperature, soiling, and snow. In simulations, various losses are subtracted from ideal generation to estimate effective generation. If these loss rates are unrealistic, investment benefits may be overestimated.

Temperature loss refers to output reduction due to panel temperature rise. This requires particular attention in summer and for roof-mounted systems. Even in high-insolation seasons, panel temperature rises can prevent generation from increasing as much as expected. For proposals showing high summer generation, confirm whether high-temperature output losses are reflected.

Soiling loss is also important. When dust, pollen, fallen leaves, bird droppings, exhaust-related dirt, or particulates adhere to panel surfaces, they reduce the received insolation and lower generation. Do not underestimate soiling losses in areas with many surrounding trees, nearby unpaved ground, dust-prone locations, or places where birds gather. Ease of cleaning and inspection also affects the loss rate.

In snowy regions, confirm generation reduction due to snow. When snow accumulates on panels, periods of no generation occur. Panel tilt for snow shedding, snow storage areas, snow load, and ease of snow removal and inspection affect winter generation and long-term operation. For proposals showing high winter generation, confirm how much snow and residual snow effects are accounted for.

Loss rates may be summarized comprehensively in proposals. In that case, check how much temperature, shading, wiring, conversion, soiling, snow, and aging are included. Proposals with low loss rates will show higher generation, but if they do not sufficiently reflect site conditions, the gap with actual performance after installation may be large.

To reduce investment risk, do not make generation look good by understating losses; realistically estimate losses suited to the local environment. Confirming loss rates for temperature, soiling, and snow increases the reliability of generation forecasts.

Item 6: Confirm constructability and maintainability

The sixth item is constructability and maintainability. Simulations tend to focus on generation and profitability, but unless you confirm whether the plan is actually buildable and maintainable over the long term, investment risk will not decrease. Even layouts showing high generation can cause problems after installation if construction or maintenance is impractical.

For roof projects, check structure, waterproofing, loads, inspection pathways, and access to existing equipment. Filling the roof with panels increases capacity but can make rooftop equipment inspection, drainage cleaning, waterproofing repairs, and emergency response difficult. A layout that maximizes generation can increase building management risks.

For land projects, check maintenance paths, drainage, ground conditions, weeding, slopes, site boundaries, potential connection points, and surrounding environment. Lining panels across the entire site increases capacity but can make weeding, inspections, cleaning, and equipment replacement difficult. Poor drainage requires attention for management during rainy seasons or snowmelt.

Maintainability directly affects sustained generation. Equipment that is hard to inspect may delay noticing soiling, shading issues, equipment faults, or wiring problems. Difficult-to-clean layouts can prolong soiling losses. If access to inspection points after high winds or snowfall is limited, it becomes harder to identify causes of generation declines.

Reflecting constructability and maintainability may reduce system capacity compared to initial simulations. Corrections such as securing inspection pathways, avoiding roof edges, leaving space around drainage outlets and equipment, and allocating snow storage areas may be necessary. Generation may decrease in such cases, but long-term investment risk can be reduced.

To reduce investment risk, confirm not only simulated generation but whether the system can be operated safely and maintained easily after construction. Proposals that downplay constructability and maintainability may carry long-term risks in generation and operational costs.

Item 7: Confirm long-term revenue and changes in demand

The seventh item is long-term revenue and changes in demand. Solar power systems are long-lived equipment, so to reduce investment risk you must look beyond the first year. Even if first-year generation and self-consumption look good, that state may not persist.

When assessing long-term revenue, confirm aging effects. Panels, equipment, wiring, and connections change over long-term operation. Check whether the simulation indicates only first-year generation or also considers long-term generation changes.

Facility electricity consumption may also change. Demand can change due to additions of production equipment, energy-saving measures, HVAC upgrades, changes in operating hours, changes in holiday operations, changes in building use, or introduction of electric equipment. Capacity optimized for current demand may not remain optimal in the future.

Over the long term, changes in the surrounding environment also become investment risks. Trees may grow and increase shading, buildings may be constructed nearby, rooftop equipment may be added, or land surroundings may change, affecting generation. While it is difficult to predict everything, known plans and risks should be included in simulation assumptions.

Assumptions about equipment replacement and maintenance also affect long-term revenue. Inspections, cleaning, equipment checks, repairs, and replacements may be required. Long-term projections that stack generation benefits without considering maintenance and upkeep tend to be optimistic.

To reduce investment risk, do not view multi-year revenue as a single optimistic line; instead assume cases where generation underperforms, demand decreases, surplus increases, or maintenance burden grows. It is important to check long-term revenue under assumptions that both generation and facility operation change.

Points to note when comparing vendor proposals

When comparing vendor proposals, do not judge solely by the size of annual generation or expected revenue. If generation or investment effects differ among proposals, confirm whether those differences stem from design differences or differences in assumptions.

First, compare system capacity and installation scope. Proposals with larger capacities appear to generate more. Confirm whether usable installation area is overestimated and whether rooftop equipment, inspection pathways, site boundaries, and maintenance paths are reflected. Looking at generation per unit capacity makes it easier to judge whether a proposal is efficient.

Next, compare the breakdown of loss rates. Check how much temperature, shading, wiring, conversion, soiling, snow, and aging are included. Proposals with low loss rates will show higher generation but may be optimistic relative to site conditions. Proposals that conservatively reflect losses may be closer to actual performance after installation.

Also compare assumptions about self-consumption. The accuracy differs greatly depending on whether only annual consumption was used or whether monthly and hourly usage is reflected. If weekday vs. weekend, seasonal variation, and daytime demand are not reflected, self-consumption may be overestimated.

For proposals including battery storage, separate the effects of solar only and the additional effects from batteries. Looking only at results with batteries can obscure surplus and self-consumption issues. Check charging/discharging losses, capacity constraints, and reserved capacity for emergencies.

When comparing vendor proposals, prioritize proposals with clear assumptions that fit site conditions and facility operations, rather than the proposal that shows the best numbers. To reduce investment risk, focus on looking at the basis behind the results.

Summary

To reduce investment risk with solar power generation simulations, it is necessary to comprehensively verify not only annual generation but also monthly generation, self-consumption, surplus electricity, shading, azimuth, tilt, temperature, soiling, snow, constructability, maintainability, long-term revenue, and changes in demand. Proposals with large generation figures appear attractive, but if their assumptions are optimistic, the gap with actual performance after installation may be large.

Item 1: Verify the basis for annual generation. Check whether system capacity, usable installation area, insolation, shading, and generation losses fit site conditions. Item 2: Check monthly generation and seasonal variation. Confirm whether generation is sufficient in months of high demand and whether downward risks in winter or summer are reflected.

Item 3: Separate and confirm self-consumption and surplus electricity. The core of investment decisions is to look at usable generation rather than producible generation. Item 4: Confirm generation reduction due to shading, azimuth, and tilt. Use effective generation based on site surveys rather than ideal conditions.

Item 5: Confirm loss rates such as temperature, soiling, and snow. Simulations with unrealistically low loss rates can overstate generation and benefits. Item 6: Confirm constructability and maintainability. If the system cannot be inspected or cleaned over the long term, it is difficult to maintain generation. Item 7: Confirm long-term revenue and changes in demand. First-year benefits may not continue over the long term, so consider aging and operational changes.

When comparing vendor proposals, do not simply choose the proposal that appears to have the largest generation or revenue; prioritize proposals with clear assumptions that fit site conditions and facility operations. To reduce investment risk, do not just accept simulation numbers—confirm why those numbers arise.

Accurate site information forms the foundation for reducing investment risk. If you can accurately grasp the candidate installation area, rooftop equipment, obstacles, trees, site boundaries, azimuth, tilt, inspection paths, and potential connection points, the simulation assumptions become clear and the accuracy of judgments about generation, self-consumption, surplus electricity, and long-term revenue improves.

If you want to record candidate installation areas, rooftop equipment, obstacles, trees, site boundaries, azimuth, tilt, inspection paths, and potential connection points accurately on site and improve the accuracy of investment risk reduction with solar power generation simulations, using LRTK, an iPhone-mounted high-precision GNSS positioning device, is effective. High-precision location data of the site makes it easier to organize shading and obstacles, installation ranges, wiring routes, and maintenance paths, and facilitates consistent progress from vendor proposal comparison and pre-construction confirmation to post-installation maintenance management. To correctly reduce investment risk in solar power generation simulations, it is important to establish a system to accurately understand the site as well as desktop revenue estimates.