How to Judge the Validity of Sales Proposals Using Solar Power Generation Simulations

When you receive a sales proposal for solar power generation, various numbers are presented: annual generation, self-consumption, surplus electricity, expected benefits, payback outlook, and so on. However, a proposal that shows a large generation value is not necessarily a valid proposal as-is. Because solar power generation simulations can vary greatly depending on input conditions, it is important to check whether the calculation assumptions match the site conditions rather than focusing on the magnitude of the numbers. This article explains how to judge the validity of sales proposals for practitioners who search for "solar power generation simulation."

Table of Contents

• Basics for reviewing sales proposals with solar power generation simulations

• Look at the basis, not just the size of the annual generation

• Check whether system capacity and installable area are realistic

• Verify assumptions about irradiance, orientation, and tilt

• Check losses from shading, temperature, soiling, and snow

• Judge self-consumption and surplus electricity separately

• Check the monthly generation and power usage compatibility

• Confirm constructability, maintainability, and long-term operation

• Points to note when comparing multiple proposals

• Summary

Basics for reviewing sales proposals with solar power generation simulations

In sales proposals for solar power generation, the results of generation simulations become major decision factors. They show how much annual generation can be expected, how much can be self-consumed, how much surplus electricity will be produced, and what level of benefit can be expected after installation. These figures are important for making an installation decision.

However, solar power generation simulations change depending on input conditions. Increasing system capacity tends to raise annual generation. Underestimating shading makes generation look higher. Setting a low loss rate makes generation and expected benefits look better. Overstating daytime facility demand makes self-consumption look larger. In other words, to judge the validity of a sales proposal, you need to look not only at the resulting numbers but also at the assumptions under which those numbers were produced.

What practitioners should pay attention to is not whether the proposal’s generation is high or low. Important is whether it fits the site conditions, whether the layout can be implemented after construction, whether the plan can be managed after installation, and whether downside risks are explained. A proposal that looks conservative in generation may be close to actual performance if it realistically reflects shading, loss rates, and maintainability. Conversely, a proposal showing very high generation may diverge from expectations after installation if it fails to reflect site conditions or generation losses adequately.

When reviewing a sales proposal, it is easier to follow a check order: system capacity, installable area, generation, monthly generation, self-consumption, surplus electricity, loss rate, and maintenance plan. The basic rule is to confirm the conditions under which the generation is produced before evaluating the generation itself.

Solar power generation simulations serve both as material to positively consider a sales proposal and as material to identify risks after installation. To judge whether a proposal is valid, you need an attitude of checking not only favorable numbers but also conditions that could make the results worse.

Look at the basis, not just the size of the annual generation

The annual generation is the figure that tends to catch the eye first in a sales proposal. How much can be generated annually is an important metric when deciding on solar power installation. However, judging a proposal by the size of the annual generation alone risks misinterpretation. Annual generation varies according to system capacity, irradiance, orientation, tilt, shading, loss rate, and other conditions.

When you see a proposal with high generation, check why that generation is achieved. Is the generation large simply because the system capacity is large? Is it because they efficiently use favorable roof surfaces or land areas? Or does it look large because shading and loss rates were underestimated? You need to distinguish among these reasons.

Particularly useful for comparison is generation per unit of installed capacity. Even if total generation is large, a larger capacity will naturally increase generation. Checking generation per unit of capacity makes it easier to judge how efficiently the same capacity produces electricity. If generation per unit capacity is low, the plan may include shaded areas or unfavorably oriented surfaces.

Also check whether the annual generation is a first-year forecast or a long-term projection. Solar equipment is used over long periods, and generation changes due to equipment degradation, soiling, changes in shading, and equipment downtime. Proposals that treat first-year generation as the long-term benefit may look optimistic.

To judge a sales proposal’s validity, do not accept the annual generation as a result without decomposition. Generation that can be explained by system capacity, local irradiance conditions, loss rate, shading, and the installation area is easier to use for internal explanations and for comparison with actual performance after installation. Generation with unclear basis should be treated cautiously even if the numbers are large.

Check whether system capacity and installable area are realistic

Checking system capacity and installable area is essential when judging the validity of a sales proposal. In solar simulations, larger capacity tends to increase annual generation, so it is necessary to verify whether the capacity setting is reasonable.

For rooftop projects, the total roof area and the area actually usable differ. Rooftops have air-conditioning equipment, piping, rooftop equipment rooms, railings, drain outlets, inspection hatches, waterproofing clearance, maintenance walkways, and so on. If a sales proposal treats the entire roof as an installable area, the initial simulation may show a large generation, but the number of panels may need to be reduced after a site survey or before construction.

The same applies to land projects. Even if the total land area is large, considering site boundaries, embankments, elevation differences, trees, utility poles, drainage channels, existing structures, maintenance paths, and candidate grid connection points limits the usable area. A plan that places panels across the whole site may look like it yields high generation but can make inspection, weed control, drainage, equipment replacement, and emergency response difficult.

Proposals with very large system capacities can be attractive, but you should also confirm whether they are excessive relative to facility demand. For self-consumption purposes, it is important whether the generated power can be used within the facility. If capacity is increased and generation exceeds daytime facility demand, surplus electricity will increase.

In sales proposals, it is helpful to separately check a maximum-capacity proposal and a realistic-capacity proposal. The maximum capacity helps you know the upper limit of generation, but considering constructability, maintainability, and compatibility with self-consumption, it is not always optimal. Confirm that simulations reflect site conditions and leave margins necessary for inspection and maintenance.

When system capacity and installable area are realistic, the reliability of generation forecasts increases. Conversely, proposals with a loose view of area may see large changes in generation before and after installation.

Verify assumptions about irradiance, orientation, and tilt

In solar power generation simulations, irradiance, orientation, and tilt are the basic conditions for generation. To judge a sales proposal’s validity, confirm that these assumptions match the site.

Irradiance is an important condition indicating how much sunlight the installation location can receive. Generation varies with regional irradiance conditions, monthly weather patterns, snowfall, cloudy days, and temperature. Check whether conditions close to the installation site were used or whether broad regional averages were applied. Generation conditions can vary within the same region in mountainous areas, coastal areas, basins, and snowy regions.

Orientation also greatly affects generation. Surfaces close to south-facing tend to yield higher annual generation, but east- and west-facing surfaces can be effective depending on the facility’s load timing. East-facing generation can help self-consumption for facilities with high morning demand, while west-facing generation can help in the afternoon for facilities with peak afternoon demand. In sales proposals, check the assumed distribution of equipment by orientation.

Tilt angle is also important. For rooftop projects, installations often follow the existing roof pitch, so the ideal angle cannot always be freely selected. For flat roofs or land projects, mounting angles can be set, but increasing the angle affects inter-row shading, wind, snowfall, required spacing, and maintainability. An angle chosen solely for maximizing generation may not be easy to construct on-site.

If a proposal calculates using ideal orientations or tilts, be cautious. If generation is calculated with assumptions different from actual roof pitch or land conditions, the results may deviate from post-installation performance. When using multiple roof surfaces or land parcels, check orientation, tilt, and generation for each surface.

If assumptions about irradiance, orientation, and tilt match site conditions, the proposal’s generation approaches reality. If assumptions are ambiguous, request a re-simulation after a site survey to confirm final generation.

Check losses from shading, temperature, soiling, and snow

Checking generation losses is extremely important when judging a sales proposal’s validity. Solar equipment does not always generate at maximum output under ideal conditions. In reality, generation decreases due to shading, temperature, soiling, snow, wiring, power conversion, and aging. The extent to which these losses are anticipated affects the reliability of generation forecasts.

Shading is a representative factor that reduces generation. Surrounding buildings, rooftop equipment, rooftop equipment rooms, railings, piping, trees, utility poles, embankments, and terrain elevation differences create shading. Shading changes with season and time of day. A site that is fine in summer can have long shadows in winter when solar altitude is low. If a sales proposal does not sufficiently reflect shading, annual generation may look high.

Temperature losses occur when panel temperature rises and output drops. This is especially important in summer and for rooftop installations. Even in periods of high irradiance, panel temperature increases can prevent generation from rising as expected. For proposals that show very high summer generation, check how much temperature loss was assumed.

Soiling losses are caused by dust, pollen, leaves, bird droppings, exhaust-related deposits, and particulates, which reduce generation. In locations with many trees nearby, unpaved land, or where dust easily occurs, do not underestimate soiling. Ease of cleaning and inspection also relates to long-term maintenance of generation.

In snowy regions, check generation reduction due to snow. When snow accumulates on panels, there will be periods with no generation. Check slope for snow to slide off, snow storage space, ease of snow removal and inspection, and resistance to snow loads. If winter generation looks high, confirm whether snow and residual snow were adequately reflected.

Loss rates are sometimes shown as a composite figure, but it is important to check the breakdown. You cannot judge generation validity without confirming how much temperature, shading, soiling, snow, wiring, conversion, and aging are included. Proposals with low loss rates make generation look high but may be optimistic relative to site conditions.

Judge self-consumption and surplus electricity separately

When judging the validity of a sales proposal, it is important to check self-consumption and surplus electricity separately, not just total generation. Generated power is divided into the portion used within the facility and the portion that remains unused. If you don’t check this difference, you may overestimate the installation benefits.

Self-consumption refers to the amount of generated power actually used within the facility. It ties closely to reductions in purchased electricity and is the core of installation benefits. Surplus electricity is the amount generated but not used within the facility at the same time. Whether surplus is exported externally, stored in batteries, or curtailed affects the evaluation.

Be careful if a sales proposal makes it look like all generation will be self-consumed. In reality, self-consumption depends on the facility’s load pattern. Facilities that mainly operate at night may have limited daytime self-consumption despite large annual usage. Facilities with high weekday demand but low weekend demand may see increased surplus.

Avoid judging by self-consumption rate alone. A small system capacity can make the self-consumption rate look high, even though the absolute self-consumed amount is small. A large system capacity may lower the self-consumption rate but increase absolute self-consumption. In proposals, check self-consumption rate, self-consumption amount, and surplus electricity together.

Also assess how much surplus increases if capacity is increased. Adopting the maximum capacity that fits on the roof or land may increase generation but could produce more generation than the facility demands. If surplus electricity becomes too large, reconsider capacity, batteries, load control, or operational methods.

For proposals that include batteries, compare results with and without batteries. Looking only at battery-included results makes it harder to understand the surplus risk of the PV system alone. Confirm how much self-consumption batteries are assumed to add and how much charge/discharge loss is assumed.

By judging self-consumption and surplus electricity separately, you can evaluate the proposal’s validity based on the amount of electricity actually usable after installation rather than on generation size alone.

Check the monthly generation and power usage compatibility

To judge a sales proposal’s validity, check the compatibility between monthly generation and the facility’s monthly power usage. Even if annual generation is sufficient, if months with high generation do not align with months of high facility demand, the self-consumption effect may be smaller than expected.

Solar generation varies by season. There may be higher generation in summer, but there is also temperature-related loss. In winter, daylight hours are shorter, shadows stretch longer, and snowy regions can have periods with no generation. Regions affected by rainy seasons or prolonged cloud cover may see suppressed generation in certain months.

Facility power usage also changes month to month. For facilities with heavy air-conditioning demand in summer, summer generation is important. For facilities with high heating or production equipment demand in winter, be cautious about winter generation declines. Facilities with busy and slack seasons may experience increased surplus in months when generation is high but demand is low.

Overlaying monthly generation and monthly usage reveals months where benefits are likely and months to watch. A proposal that looks good annually may have issues like excessive surplus in specific seasons or low generation in months with high demand. Sales proposals should include monthly self-consumption and surplus figures as well as annual generation to give a more realistic assessment.

Time-of-day generation curves are also useful. South-facing surfaces tend to generate more around midday, east-facing more in the morning, and west-facing more in the afternoon. Depending on when the facility’s demand peaks, self-consumption can vary even with the same annual generation.

If a proposal does not provide monthly or time-of-day information, it becomes difficult to judge post-installation benefits. To confirm validity, it is important to see when generation occurs and when the facility uses power.

Confirm constructability, maintainability, and long-term operation

A sales proposal’s validity cannot be judged by generation and self-consumption alone. You must confirm whether the system can actually be constructed, whether it can be maintained after installation, and whether it can be operated long-term. Solar equipment is used for extended periods, and maintainability is important to sustain generation.

For rooftop projects, check roof structure, waterproofing, load capacity, rooftop equipment, drain outlets, inspection hatches, and maintenance walkways. Filling the whole roof with panels may make generation look large but can complicate drain cleaning, waterproofing repairs, air-conditioning equipment inspection, and piping repairs. Layouts that interfere with building management can lead to long-term problems.

For land projects, check site boundaries, embankments, elevation differences, drainage, ground conditions, maintenance paths, weed control, equipment siting, and candidate grid connection points. Filling the whole site with panels may look like it yields high generation but can make weed control, inspection, cleaning, equipment replacement, and drainage management difficult. Layouts that cannot be maintained may delay responses when generation falls.

Equipment placement is also important. Confirm that inverters, junctions, and wiring routes are placed where they can be inspected and accessed in case of abnormalities. Hard-to-access equipment makes it difficult to identify causes when generation declines.

For long-term operation, consider equipment degradation and changes in the surrounding environment. Panels, equipment, wiring, and racking may require inspection or repair during extended operation. Tree growth, changes in surrounding buildings, additions of rooftop equipment, soiling, and snow all affect long-term generation.

Sales proposals should address not only first-year generation but whether generation can be maintained over the long term. Proposals that specify inspection routes, ease of cleaning, equipment access, and performance management methods are less likely to cause post-installation trouble. Proposals that only show high generation without explaining maintainability should be reviewed carefully.

Points to note when comparing multiple proposals

When comparing multiple sales proposals, lining up annual generation figures alone is insufficient. If proposals differ in system capacity, irradiance assumptions, loss rates, or self-consumption assumptions, differences in generation and expected benefits may stem from these differing assumptions.

First, align system capacity for comparison. Proposals with larger capacity tend to show larger generation, so check generation per unit of capacity as well as total generation. Next, compare installable area and layout. Confirm how each proposal accounts for rooftop equipment, maintenance walkways, site boundaries, and access paths.

You should also align assumptions about irradiance and regional conditions. If proposals use different irradiance assumptions, do not evaluate generation differences directly. Check monthly generation, temperature, snow, and cloudy-day handling.

Compare the breakdown of loss rates. Verify how much temperature, shading, soiling, snow, wiring, conversion, and aging are included. Proposals with low loss rates look like they produce more generation but may be optimistic relative to site conditions.

Self-consumption assumptions are also important. Check whether they are based only on annual usage or reflect monthly and hourly usage. Proposals that do not reflect weekday/weekend differences, seasonal variation, or daytime demand may overestimate self-consumption.

When comparing multiple proposals, prioritize those with clear assumptions that match site conditions and facility operations rather than those that simply show larger numbers. Validity should be judged by explanatory power of the assumptions and reproducibility after installation, not by the height of the generation figures.

Summary

To judge the validity of sales proposals using solar power generation simulations, it is necessary to check not only the size of annual generation but also the basis for the generation, system capacity, installable area, irradiance, orientation, tilt, shading, loss rates, self-consumption, surplus electricity, constructability, maintainability, and long-term operation. A proposal that shows large generation is not always good, and proposals that carefully reflect site conditions may appear conservative.

First, confirm the basis of the annual generation. Check why the generation is that value and whether system capacity, installation area, irradiance conditions, and loss rates can explain it. Next, verify whether the system capacity and installable area are realistic. What matters is not the total roof or land area but the area calculated based on what can actually be constructed and maintained.

Also confirm the assumptions about irradiance, orientation, and tilt. Check that they match actual roof pitches and land conditions, not ideal conditions. Verify how much shading, temperature, soiling, and snow losses are assumed. Proposals with loss rates that are too low may overstate generation.

Always judge self-consumption and surplus electricity separately. The amount that can be generated is not the same as the amount usable within the facility. Check monthly generation and power usage compatibility to see if generation occurs during periods and times when demand is high. Additionally, confirm constructability, maintainability, and long-term operation to judge whether the proposal can be managed after installation.

When comparing multiple proposals, align not only total generation but also generation per unit of capacity, calculation assumptions, loss rates, and self-consumption assumptions. Choose the proposal with clear assumptions that fit site conditions and facility operations rather than the one that shows the best numbers.

The foundation for judging the validity of a sales proposal is accurate site information. If you can precisely identify installable ranges, rooftop equipment, obstacles, trees, site boundaries, orientation, tilt, inspection routes, and candidate connection points, the simulation assumptions for solar power generation become clearer and it becomes easier to judge whether the proposal’s generation is realistic.

If you want to improve the accuracy of judgments using solar power generation simulations by accurately recording installable areas, rooftop equipment, obstacles, trees, site boundaries, orientation, tilt, inspection routes, and candidate connection points on site, using LRTK, an iPhone-mounted GNSS high-precision positioning device, is effective. High-precision position information from the site makes it easier to organize shading and obstacles, installable ranges, wiring routes, and maintenance routes, and to confirm whether the proposal’s generation matches site conditions. To correctly judge the validity of sales proposals using solar power generation simulations, it is important to go beyond desk-based numbers, accurately grasp the site, and set up a state where comparisons are made under the same assumptions.