Six Vendor-Comparison Items to Avoid Failing in Solar Power Generation Simulations

When receiving proposals from multiple vendors for solar power installation, you will see numbers such as annual generation, on-site consumption, surplus electricity, payback outlook, and system capacity. At first glance, you may be tempted to choose the proposal with the largest generation or the greatest apparent installation benefits, but solar power generation simulations can vary greatly depending on calculation assumptions. To avoid mistakes in comparison, it is important not to focus on the magnitude of the numbers but to confirm whether the assumptions are consistent, whether site conditions are reflected, and whether the plan can be reproduced after installation. This article explains six items that operational staff who search for "solar power generation simulation" should always check when comparing vendors.

Table of Contents

• How to think about vendor comparison for solar power generation simulations

• Item 1: Align assumptions for system capacity and installable area

• Item 2: Check irradiance, orientation, and tilt-angle conditions

• Item 3: Evaluate how shading, obstacles, and the surrounding environment are reflected

• Item 4: Compare loss rates and the breakdown of generation losses

• Item 5: Separate evaluation of on-site consumption and surplus electricity

• Item 6: Confirm constructability, maintainability, and post-installation management

• Judgments to avoid in vendor comparison

• Summary

How to think about vendor comparison for solar power generation simulations

When comparing vendors using solar power generation simulations, the worst mistake is judging proposals solely by the size of the annual generation. Annual generation is an important indicator, but it tends to increase if system capacity is made larger, and it looks higher if shading or loss rates are underestimated. In other words, generation numbers are the result of the proposal and do not directly reflect the superiority of the vendor.

Even for the same building or land, proposals from different vendors can show different generation figures. Those differences arise not only from differences in equipment used but also from assumptions about installable area, irradiance conditions, shading evaluation, loss rates, methods for calculating on-site consumption, and allowances for maintenance space. A proposal that appears to show large generation may not have adequately considered rooftop equipment or inspection walkways. Another proposal that looks conservative in generation may realistically reflect shading and maintenance access.

What matters in vendor comparison is comparing on the same assumptions. If you compare total generation among proposals with different system capacities, proposals with larger capacities will appear advantageous. If proposals assume different irradiance levels, differences in weather assumptions may appear as generation differences. If expected loss rates differ, the higher generation figure is not necessarily realistic. Before comparing, organize what is the same and what differs.

Also, the numbers you should compare depend on the purpose of the solar installation. If you prioritize electricity cost reduction or lowering purchased electricity, on-site consumption matters more than total generation. If you plan to use surplus power, check when surplus electricity occurs and how it relates to battery storage. Even if you want to make the most of land or roof area, installing the maximum capacity is not always optimal. Configurations that are hard to maintain or that produce too much surplus can cause problems after installation.

To avoid failure in vendor comparison, adopt an attitude of verifying the basis for the generation figures rather than just accepting the proposal numbers. Reviewing annual generation, on-site consumption, surplus electricity, monthly generation, loss rates, constructability, and maintainability in order makes differences between proposals easier to see. Below, we explain six items to check in practice.

Item 1: Align assumptions for system capacity and installable area

The first items to check in vendor comparison are system capacity and installable area. In solar power generation simulations, the larger the system capacity, the more annual generation tends to increase. Therefore, when comparing multiple proposals, looking only at total generation may lead you to mistakenly favor the proposal with the larger capacity.

For rooftop projects, separate the roof’s total area from the actually usable area. Roofs have air-conditioning equipment, piping, roof structures, handrails, drain outlets, inspection hatches, waterproofing clearances, inspection walkways, and so on. Treating the entire roof as installable area without excluding these elements makes both system capacity and annual generation appear larger. In actual construction, you may need to reduce the number of panels.

For land projects, total land area and actually usable area differ as well. Consider site boundaries, slopes, elevation differences, trees, drainage channels, existing structures, maintenance access, potential connection points, and snow storage space. Having a large site does not mean you can place panels across the entire area. Leaving space for management and maintenance is also an important condition for long-term operation.

When comparing system capacities, check not only total generation but generation per unit capacity. If total generation is large but generation per capacity is low, the proposal may be using areas with shading or poor conditions. Conversely, a slightly smaller total generation with high generation per capacity may indicate a proposal that efficiently uses the best areas.

It is also important to compare maximum-capacity plans and realistic-capacity plans separately. A maximum-capacity plan is useful for seeing how much of the roof or land can be used, but when considering constructability, maintainability, and compatibility with on-site consumption, it is not always optimal. Comparing realistic capacities that secure inspection walkways, drainage, and equipment access can reduce gaps after installation.

In vendor comparison, first align assumptions for system capacity and installable area. Confirm which proposal generates more efficiently for the same capacity and which is realistic for the same installable area. Comparing generation without this foundation makes it easy to make wrong judgments.

Item 2: Check irradiance, orientation, and tilt-angle conditions

The second item is irradiance, orientation, and tilt-angle conditions. Solar generation depends not only on system capacity but also on how much irradiance the installation can receive at the site. For the same capacity, differences in irradiance conditions, orientation, and tilt angle will change annual generation and monthly generation.

Assumptions about irradiance may differ by vendor. Check whether they use local conditions near the site, regional averages, and whether they reflect monthly irradiance variations. In mountainous areas, coastal zones, basins, snowy regions, or cloudy regions, irradiance can vary within the same region. Even a proposal that shows high generation can differ from actual post-installation performance if the irradiance assumptions are optimistic.

Orientation is also important. South-facing surfaces tend to obtain higher annual generation, but east- or west-facing surfaces can be useful depending on facility load timing. For facilities with high morning demand, east-facing generation may contribute to on-site consumption; for facilities with high afternoon demand, west-facing generation may help. Check the vendor proposals for assumptions about how much equipment is placed on which orientations.

For tilt angle, verify that calculations reflect site conditions rather than ideal conditions. For rooftops, tilt usually follows the existing roof slope and the angle cannot be freely chosen. For flat roofs or land projects, mounting angles can be set, but increasing angle affects row-to-row shading, wind loads, spacing, and maintainability. Calculating with an ideal tilt to inflate generation is useless if it cannot be constructed.

For proposals using multiple roof surfaces or land plots, check orientation, tilt, and generation by each installation surface. Total annual generation alone does not show which surfaces contribute. Separating generation from south-facing, east/west-facing, and flat-roof zones makes it easier to understand the proposal.

If irradiance, orientation, and tilt-angle assumptions match the site, the simulation’s reliability increases. Conversely, proposals with vague conditions or calculations based on ideal assumptions may change after site survey. In vendor comparison, confirm assumptions so you can compare generation under the same conditions.

Item 3: Evaluate how shading, obstacles, and the surrounding environment are reflected

The third item is how shading, obstacles, and the surrounding environment are reflected. In solar power, shading on panels reduces generation. Simulations that do not sufficiently reflect shading tend to overestimate annual generation and may diverge from post-installation performance.

Sources of shading include surrounding buildings, rooftop equipment, roof structures, handrails, piping, air-conditioning equipment, exhaust equipment, trees, utility poles, signs, slopes, and terrain elevation differences. On rooftops, rooftop equipment and adjacent buildings often cause problems, while on land sites, trees, utility poles, slopes, and neighboring buildings are common shading causes.

Shading changes by time of day and season. Shadows may be short in summer but extend in winter when solar altitude is low. In the morning, eastern obstacles create shadows; in the evening, western obstacles do. It is important to check how simulations reflect winter or morning/evening shading that is hard to see in site photos.

When comparing vendor proposals, check the difference between generation with and without shading. A proposal that shows conservative generation after considering shading is likely closer to reality. Conversely, if the site has many shading factors but a proposal still shows very high generation, shading is probably underrepresented.

The surrounding environment affects not only shading but also soiling and maintainability. Nearby trees can cause leaf fall and bird-related shading. Unpaved ground or heavily trafficked roads nearby can generate dust and affect generation. When rooftop equipment is abundant, pay attention to inspection walkways and cleanability in addition to shading.

Also consider that the surrounding environment can change in the future. Trees can grow, neighboring buildings can be constructed, rooftop equipment can be added, or land use can change, all of which affect long-term generation. While predicting everything precisely is difficult, check whether known plans or risks are reflected in the simulation.

How well shading, obstacles, and the surrounding environment are reflected becomes an important differentiator in vendor comparison. For proposals that show high generation, carefully confirm how much shading and surrounding conditions have been taken into account.

Item 4: Compare loss rates and the breakdown of generation losses

The fourth item is loss rates and the breakdown of generation losses. Solar power simulations estimate effective generation by subtracting various losses from ideal generation. If loss-rate settings differ, generation forecasts change even for the same system capacity.

Typical generation losses include temperature loss, wiring loss, power conversion loss, shading, soiling, snow, equipment downtime, and degradation over time. Vendors sometimes present these as a combined loss rate. However, looking only at the combined loss rate can make it unclear what is included and to what extent.

Temperature loss occurs when panel temperature rises and output decreases. This requires attention in summer and for rooftop installations. Even in seasons with high irradiance, high panel temperatures can prevent generation from increasing as expected. For proposals that show large summer generation, check whether temperature loss has been adequately considered.

Soiling loss is generation reduction due to dust, pollen, leaves, bird droppings, exhaust-related dirt, and particulate matter. Soiling risk varies with the surrounding environment. If trees are numerous, unpaved ground is nearby, or particulate matter is likely, a standard loss rate may be insufficient.

In snowy regions, check generation reductions from snow. For proposals that show high winter generation, confirm how snow cover and residual snow are reflected. Tilt angle for snow shedding, snow storage space, and ease of snow removal and inspection also matter.

Proposals with low loss rates tend to show higher generation, but low loss rates alone are not inherently good. Low loss rates that do not match site conditions increase the risk of underperformance after installation. In vendor comparison, prioritize proposals that can explain the basis for loss rates rather than those that simply show low loss rates.

If the breakdown of generation losses is clear, it is easier to identify causes if actual generation falls below expectations after installation. In vendor comparison, check not only the loss-rate numbers but also their contents and fit with site conditions.

Item 5: Separate evaluation of on-site consumption and surplus electricity

The fifth item is on-site consumption and surplus electricity. Sales proposals for solar often emphasize large annual generation. However, if the generated electricity cannot be used within the facility, the installation benefits may be smaller than expected. When comparing vendors, it is important to evaluate on-site consumption and surplus electricity separately rather than relying on total generation.

On-site consumption is the portion of generated electricity used within the facility. It directly reduces purchased electricity and is central to explaining installation benefits. Surplus electricity is the portion generated that cannot be consumed within the facility during the same time period. Whether surplus is exported, stored in batteries, or curtailed affects evaluation.

To compare on-site consumption, you need to look at the facility’s electricity use by time of day. A facility with high annual consumption but primarily at night has limited compatibility with solar. A facility with daytime demand on weekdays but low demand on holidays will see increased surplus. If a sales proposal estimates on-site consumption using only annual consumption, it may be overly optimistic.

Avoid comparing solely by self-consumption rate. A small system capacity tends to have a higher self-consumption rate but may still offer low absolute self-consumption. A large capacity can lower the self-consumption rate while increasing absolute self-consumption. Check self-consumption rate, on-site consumption, and surplus electricity together.

It is also important to see how surplus changes when capacity is increased. If generation increases but the additional amount just becomes surplus, installation benefits are limited. Vendor proposals that allow comparison of on-site consumption and surplus for multiple capacity patterns make it easier to determine the appropriate capacity.

For proposals including batteries, separate results for no-battery and with-battery cases. Results shown only for with-battery scenarios make it hard to understand how much surplus solar alone would produce. Check how much on-site consumption increases with a battery, and what charge/discharge losses and capacity constraints have been assumed.

In vendor comparison, prioritize the amount usable within the facility rather than just how much can be generated. Separating on-site consumption and surplus electricity helps judge whether the sales proposal’s benefits are realistic.

Item 6: Confirm constructability, maintainability, and post-installation management

The sixth item is constructability, maintainability, and post-installation management. Even if a proposal looks good in terms of generation or on-site consumption, configurations that cannot be practically constructed or are difficult to maintain can lead to long-term problems. In vendor comparison, you must confirm whether the simulated plan is feasible on site, not just its simulated generation.

For rooftop projects, check structure, waterproofing, load, rooftop equipment, drain outlets, inspection hatches, and inspection walkways. Filling the roof with panels may show large generation, but it can make cleaning drain outlets, waterproofing renovation, air-conditioning inspection, and pipe repair difficult. Proposals that impede building management lead to post-installation trouble.

For land projects, confirm site boundaries, slopes, elevation differences, drainage, ground conditions, maintenance access, weed control, equipment locations, and connection candidate points. Filling the land with panels increases capacity but may make weed control, inspection, cleaning, and equipment replacement difficult. Layouts that cannot be maintained will delay responses when generation falls.

Also check equipment placement. Ensure access to inverters, connection points, and cable routes; that inspections can be performed during abnormalities; and that working space exists. Hard-to-reach equipment makes it difficult to identify causes when generation falls. Power generation equipment is not a one-time installation but requires long-term management.

It is also important whether proposals provide post-installation performance monitoring. Proposals that enable comparison of monthly generation, hourly generation, generation by installation surface, on-site consumption, and surplus with actual performance make it easier to identify causes when generation differs from expectations. Proposals without monitoring standards make it harder to determine reasons for generation decline.

Consider equipment degradation and changes in the surrounding environment over the long term. Panels, equipment, cabling, and mounting systems may require inspection or replacement. Tree growth, neighboring buildings, added rooftop equipment, soiling, and snow also affect generation. If a sales proposal shows only first-year generation, confirm assumptions for long-term operation and maintenance.

Confirming constructability, maintainability, and post-installation management reveals a proposal’s feasibility. Judging whether a proposal can be operated stably over the long term, not just by high generation, is key to avoiding mistakes in vendor comparison.

Precautions when comparing multiple proposals

When comparing multiple sales proposals, avoid lining up numbers whose assumptions are not aligned. If system capacity, installable area, irradiance, loss rates, and on-site consumption assumptions differ, differences in annual generation and installation benefits may arise from differing assumptions rather than the superiority of proposals.

First, align system capacity for comparison. Larger-capacity proposals tend to show larger generation, so check generation per capacity as well. Looking at generation per capacity allows you to compare efficiency when installing the same capacity.

Next, compare installable area and layout. Check how much rooftop equipment, inspection walkways, and drain outlets, site boundaries, and maintenance access are reflected. Proposals that do not reflect site conditions tend to overstate generation and may require revision prior to construction.

Align irradiance and regional conditions as well. If proposals assume different irradiance conditions, do not evaluate generation differences as-is. Confirm that monthly generation, snow, temperature, and cloudiness are treated the same.

Also compare the breakdown of loss rates. Confirm how far temperature, shading, soiling, snow, wiring, power conversion, and degradation are included. Proposals with low loss rates appear to show higher generation but may be optimistic relative to site conditions.

Assumptions for on-site consumption are also important. Check whether proposals use annual consumption only or reflect monthly and hourly consumption. If weekday/holiday differences, seasonal variations, and daytime demand are not reflected, on-site consumption may be overestimated.

When comparing multiple proposals, prioritize those with clear assumptions that fit site conditions and facility operations rather than the proposal with the best numbers. Judge by reproducibility after installation, not by the size of the numbers.

Summary

To avoid failing at vendor comparison using solar power generation simulations, do not judge solely by the magnitude of annual generation. Instead, comprehensively confirm system capacity, installable area, irradiance, orientation, tilt, shading, loss rates, on-site consumption, surplus electricity, constructability, maintainability, and post-installation management. Proposals that show large generation are not always better, and proposals that carefully reflect site conditions may appear conservative in their numbers.

Item 1 aligns assumptions for system capacity and installable area: check generation per capacity and realistic installation range as well as total generation. Item 2 checks irradiance, orientation, and tilt-angle conditions: ensure calculations use site-appropriate irradiance and constructible orientations/angles.

Item 3 evaluates how shading, obstacles, and the surrounding environment are reflected: generation that ignores shading tends to overstate results. Item 4 compares loss rates and the breakdown of generation losses: confirm how temperature, soiling, snow, wiring, power conversion, and degradation are accounted for.

Item 5 separates on-site consumption and surplus electricity: prioritize the amount usable within the facility rather than just how much can be generated. Item 6 confirms constructability, maintainability, and post-installation management: judge whether the plan can actually be constructed, inspected, cleaned, and monitored.

What to avoid in vendor comparison: lining up annual generation only, ignoring differences in system capacity or loss rates, confusing on-site consumption with surplus electricity, and using pre-survey estimates as the final judgment. What matters is not the size of the numbers but whether the conditions that produce those numbers are clear.

Accurate site information provides the foundation for improving vendor-comparison accuracy. If you can precisely capture installable ranges, rooftop equipment, obstacles, trees, site boundaries, orientation, tilt, inspection routes, and connection candidate points, it is easier to align each vendor’s simulation assumptions and compare the plausibility of generation forecasts.

If you want to accurately record installable ranges, rooftop equipment, obstacles, trees, site boundaries, orientation, tilt, inspection routes, and connection candidate points on site and improve the accuracy of vendor comparison using solar power generation simulations, using LRTK, an iPhone-mounted high-precision GNSS positioning device, is effective. If you can acquire high-precision positional information on site, it becomes easier to organize shading and obstacles, installable ranges, cable routes, and maintenance routes, and to compare each vendor’s proposals under the same assumptions. To avoid failure in vendor comparison using solar power generation simulations, it is important not to rely only on desk-based numbers but to accurately grasp the site and translate that into evidence-based comparison materials.