How to Check Loss Rates in Solar Power Generation Simulations

When looking at solar power generation simulations, attention tends to go to annual generation and system capacity, but what you must always check in practice is the loss rate. The loss rate is the concept of the generation loss subtracted from the theoretically possible generated energy due to factors such as temperature, shading, wiring, conversion, soiling, snow, equipment downtime, and degradation over time. If the loss rate is underestimated, the simulated generation will appear larger, but the gap with actual post-installation generation may grow. This article explains, from a practical standpoint for practitioners who search for "solar power generation simulation," how to check loss rates.

Table of Contents

• The importance of checking loss rates in solar power generation simulations

• What the loss rate is intended to subtract

• Checking losses due to temperature

• Checking losses due to shading

• Checking losses due to wiring, conversion, and equipment configuration

• Checking losses due to soiling, snow, and surrounding environment

• Checking losses due to degradation over time and equipment downtime

• Beware of simulations with excessively low loss rates

• How to compare loss rates in vendor proposals

• How accurate on-site information increases the reliability of loss rates

• Summary

The importance of checking loss rates in solar power generation simulations

Solar power generation simulations show how much a planned system is expected to generate annually. Annual generation is a central figure in the decision to install, but how much you can trust that number depends greatly on how the loss rate is treated. Solar power systems cannot always generate at maximum under ideal irradiance conditions. In real sites, generation declines due to various factors such as temperature, shading, wiring, equipment, soiling, snow, equipment downtime, and degradation over time.

The loss rate is an important assumption used to reflect these generation reductions in the simulation. If the loss rate is set low, the annual generation will be displayed as higher. Conversely, if the loss rate is estimated realistically, the generation may look somewhat conservative. While proposals that show higher generation may look more attractive before installation, what matters for practitioners is grasping generation that is close to the post-installation reality.

When you receive proposals from multiple vendors, annual generation may differ even for the same building or land. That difference is not explained only by system capacity or irradiance. If the loss rate settings differ, simulation results will differ. If one proposal carefully accounts for shading and soiling, while another treats comprehensive losses as small, a difference in generation is natural.

Looking only at annual generation without checking loss rates can lead to excessive expectations. This is especially important if you are estimating self-consumption, electricity bill savings, profitability, or battery storage effects—if generation assumptions are overly optimistic, those effects will also be overstated. If you use solar power generation simulations in practice, you must always check not only the generation results but what losses and to what extent are being assumed.

Checking loss rates is not about distrusting vendors. It is to understand the assumptions of the proposal and to realistically adjust post-installation expectations. If the loss rate is clear, it is easier to explain the basis for generation figures and to compare vendors internally. Checking loss rates is a basic step to make solar power generation simulations a reliable basis for decision-making.

What the loss rate is intended to subtract

The loss rate is the concept of subtracting various real-world losses from the theoretical energy that a solar power system could generate under ideal conditions. Solar panels generate from incident solar irradiance, but the power produced by panels is not all available for use in the facility as-is. Panel temperature rise, shading, power conversion, wiring, soiling, snow, equipment downtime, degradation over time, and other factors reduce the actual energy obtained compared to the theoretical value.

In simulations, these losses may be set individually or treated as a combined overall loss rate. In either method, the important thing is to confirm what is included in that loss rate. If a document simply states "loss rate," the meaning differs depending on whether it covers only temperature loss, also wiring and conversion losses, or even shading and soiling.

When checking loss rates, first understand the difference between ideal generation and effective generation. Ideal generation refers to theoretical generation based on irradiance conditions and system capacity. Effective generation refers to generation closer to actual operation, considering on-site shading, temperature, wiring, equipment, soiling, and downtime. For practical installation decisions, you should rely on simulations closer to the effective value, not the ideal one.

Also, understand that the loss rate is not a fixed number but varies with site conditions. Even with the same system capacity, rooftop installations in poorly ventilated areas may experience higher temperature losses. Sites with many surrounding buildings or trees will have greater shading losses. Places with much dust or falling leaves may need to assume higher soiling losses. Regions with snow must consider winter generation reductions.

When confirming loss rates, it is important not only to look at the annual total loss but also monthly and factor-specific views. Shading can increase in winter and at sunrise or sunset. Temperature loss may be noticeable in summer. Snow concentrates its impact in winter. Soiling varies with the surrounding environment and cleaning practices. A single comprehensive loss rate may make it hard to see when generation declines.

When reviewing loss rates in solar power generation simulations, check not just "what percentage" but "what losses are included," "are they appropriate for site conditions," and "are they reflected in the monthly generation." This perspective helps you interpret generation figures more accurately.

Checking losses due to temperature

One representative loss to check in solar power generation simulations is temperature loss. Solar panels generate from incoming irradiance, but their output decreases as panel temperature rises. While higher irradiance tends to increase generation, in summer or high-temperature environments temperature rise can prevent generation from increasing as much as expected.

Temperature loss requires special attention in rooftop projects. Rooftops heat up easily under sunlight, and panel temperature varies with the distance between the roof surface and panels, ventilation conditions, roof material, and the arrangement of surrounding equipment. Poor ventilation causes heat to accumulate, increasing the potential for output reduction due to temperature.

Temperature loss is not irrelevant for ground-mounted projects either. Ground-mounted installations can often ensure better ventilation, but the effect of temperature depends on installation angle, ground conditions, surrounding environment, and airflow. If a simulation shows very high summer generation, you should check whether temperature-related decreases have been considered in addition to irradiance.

When examining temperature loss, pay attention to monthly generation. Summer has high irradiance and tends to increase generation, but output reduction due to high temperature is more likely then. If summer generation appears overly optimistic, confirm that temperature loss is adequately reflected. Conversely, simulations that appropriately account for temperature loss may show adjustments to generation that cannot be explained by irradiance alone.

When reviewing vendor proposals, check whether temperature loss is included in the overall loss rate. Even if temperature loss is not shown as an individual item, ask whether it is included in the total loss and how it was calculated. If temperature loss is not considered or the explanation is vague, the proposal may be overestimating summer generation.

Temperature loss is not as visible as physical obstructions, but it is a factor that certainly affects generation. To avoid overestimating summer generation in solar power generation simulations, confirm that temperature loss is realistically reflected.

Checking losses due to shading

Shading loss is one of the losses you should pay most attention to in solar power generation simulations. Solar panels generate from sunlight, so shading reduces generation. Simulations that do not adequately reflect shading may overstate annual generation.

Sources of shading vary by site. In rooftop projects, shading can come from surrounding buildings, roof penthouses, HVAC equipment, exhaust devices, railings, piping, antennas, signs, and upstands around skylights. In ground-mounted projects, trees, utility poles, surrounding structures, embankments, terrain elevation differences, and neighboring buildings cause shading. It is important to confirm whether the positions and heights of these items are correctly reflected in the simulation.

Shading changes with time of day and season. Shadows extend from the east in the morning and lengthen to the west in the evening. In winter, the sun's altitude is lower, causing shadows to extend where there was no problem in summer. If on-site checks before construction or during proposal stage were done only in summer, winter shading may be overlooked. If winter generation looks unnaturally high in the monthly generation, confirm whether shading assessment is sufficient.

You cannot judge shading loss simply by the shaded area. The impact on generation depends on the time of day the shading occurs, the panel interconnection configuration, and the shaded position. Even a small shadow can have a large impact if it occurs during periods of high generation. If the facility's demand time overlaps with shaded periods, it will affect self-consumption and electricity bill savings.

When confirming shading loss in simulations, it is helpful to compare generation with and without shading. Further, if you can check per-surface shading loss, monthly generation, and time-of-day generation curves, it becomes easier to judge where shading risks exist. A single overall loss rate may not make clear how much shading impact is included.

Simulations that carefully account for shading may show more conservative generation. However, that can be closer to reality. The higher the generation shown in a proposal, the more important it is to check how shading losses are accounted for.

Checking losses due to wiring, conversion, and equipment configuration

You should also check losses due to wiring, power conversion, and equipment configuration in solar power generation simulations. Power generated by solar panels is not directly usable by the facility; it passes through wiring and power conversion equipment before supply. Losses occur during this process.

Wiring losses depend on wiring length, conductor size, routing, and equipment placement. If panel layouts on rooftops or land are dispersed, wiring distances can increase. Wiring conditions change depending on where equipment is installed. If the initial proposal’s layout differs from the final design, the assumptions for wiring losses may also change.

Losses from power conversion are also important. Power generated by panels must be converted into a form usable by the facility, and losses occur in that conversion. Confirm whether the generation figures in the simulation are close to the panel-side generation or to the usable energy after conversion. Misunderstanding the meaning of the numbers can lead to overestimating the actual usable energy.

Losses due to equipment configuration should not be overlooked. How panels with different orientations or shading conditions are grouped can affect generation efficiency. If east-facing, west-facing, shaded, and less-shaded arrays are treated the same, the worse-performing parts may affect the overall generation. The actual impact varies with equipment configuration, but you should at least confirm how it is handled in the simulation.

Also check the relationship between panel capacity and the capacity of power conversion equipment. The combination of panel capacity and converter capacity can cause output to be capped during certain periods. If simulation does not consider output limits or conversion losses, generation may appear higher.

Losses from wiring, conversion, and equipment configuration are less visible than shading or soiling, but they are important for usable energy. In vendor proposals they may be included within the overall loss rate, so confirm the breakdown and the underlying assumptions.

Checking losses due to soiling, snow, and surrounding environment

Losses due to soiling, snow, and the surrounding environment are also important items to check in solar power generation simulations. If the panel surface receives less irradiance due to soiling, generation decreases. Because the impact of soiling and snow varies greatly with site environment, a general loss rate alone may not be sufficient.

Causes of soiling include dust, pollen, fallen leaves, bird droppings, exhaust-related dirt, and particulate matter. Sites with many nearby trees, unpaved land, nearby factories or high-traffic roads, or places that attract birds are more prone to generation declines from soiling. Low roof pitch can reduce natural washing by rain and allow soiling to accumulate.

Soiling losses are also related to cleaning and inspection policies. If the layout allows for easy cleaning, it is easier to manage generation loss from soiling. Conversely, if inspection walkways are lacking or rooftop work is unsafe, it is harder to deal with soiling. In generation simulations, confirm how soiling losses are treated and also consider maintainability.

In snowy regions, snow losses are important. When snow sits on panels, there are periods when they cannot generate. The impact varies with snowfall amount, the angle at which snow slides off, available snow storage areas around the site, temperature, and roof shape. If winter generation appears high, confirm that snow effects have been sufficiently accounted for.

Losses due to the surrounding environment can change over the long term. Trees may grow, increasing leaves and shading; new buildings may appear nearby; adjacent land use may change—these can increase loss factors in the future. While predicting all changes accurately is difficult, if any environmental changes are already known, they should ideally be reflected in simulations and operational plans.

Soiling, snow, and surrounding environment losses vary significantly by site. When checking loss rates, confirm that the site-specific risks for soiling and snow are reflected rather than relying only on generic assumptions.

Checking losses due to degradation over time and equipment downtime

When checking loss rates in solar power generation simulations, do not overlook losses due to degradation over time and equipment downtime. Solar power systems are assets operated over long periods. Judging only by the first-year generation can lead to incorrect evaluations of long-term generation and profitability.

Degradation over time refers to gradual changes in generation performance with prolonged use. Panels, equipment, wiring, and connection points change condition during long-term operation. Check whether the simulation shows only the first-year generation or includes long-term changes in generation.

Simulations that do not account for degradation at all can make long-term returns or project economics look better than they will be. Especially when assessing profitability or annual balances, you need to consider how generation will change over many years, not just in the first year. If long-term changes are not shown in the simulation, ask for separate confirmation.

Losses from equipment downtime can also occur in reality. Periods when generation is temporarily unavailable may arise from regular inspections, equipment replacements, abnormal event responses, power outages, communication failures, inspections of connection equipment, surrounding construction, and so on. It is difficult to predict all such events in advance, but if you evaluate assuming continuous ideal operation year-round, you may see differences with actual operation.

Downtime risk is related to maintainability. If equipment is placed for easy access, inspection walkways are secured, and monitoring makes abnormalities easy to detect, you can keep downtime short. Conversely, layouts that are hard to maintain may delay troubleshooting and prolong generation declines.

Degradation and downtime are items that tend to be overlooked in initial simulations. For long-term operation of solar power systems, it is important to account for these. When checking loss rates, confirm not only the first year but also long-term generation outlook and operational downtime risks.

Beware of simulations with excessively low loss rates

Be cautious of proposals where the loss rate in the solar power generation simulation is too low. The lower the loss rate, the higher the displayed annual generation. This can make a proposal look attractive at first glance. However, if real-world losses are not adequately considered, the gap between simulated and actual post-installation generation may be large.

Judge whether a loss rate is too low by comparing it with site conditions. If there are surrounding buildings, trees, or rooftop equipment causing shading yet the shading loss is hardly considered, be careful. If the installation is a rooftop prone to high temperatures but there is no explanation of temperature loss, check further. In environments at risk of dust, falling leaves, or snow, if soiling or snow losses are not considered at all, the proposal may be overestimating generation.

Also be wary of simulations that only show a total loss rate without a breakdown. If you do not confirm whether the total loss rate includes temperature, wiring, conversion, shading, soiling, and degradation, or which items are calculated separately, you cannot judge the basis for generation. It is important whether the vendor can explain which losses are included when you ask.

A low loss rate is not necessarily bad by itself. If shading is minimal, ventilation is good, wiring plans are rational, and soiling and snow impacts are small at the site, losses can be reduced. But if those reasons cannot be explained, the assumptions may simply be optimistic.

Checking monthly and time-of-day generation is also effective. If a site with shading shows excessively high winter generation, if a site likely to have temperature losses shows overly optimistic summer generation, or if there is no drop in the time-of-day profile, investigate how losses were handled. The higher the generation in a proposal, the more carefully you should review the loss rate breakdown.

Simulations with loss rates that are too low can lead to overly optimistic installation decisions. In practice, proposals that show conservative generation but have clear loss justifications may be closer to realistic long-term operation.

How to compare loss rates in vendor proposals

When you receive solar power generation simulations from multiple vendors, comparing loss rates is extremely important. If annual generation differs for the same system capacity and the same installation site, the difference may be due to how loss rates are treated as well as irradiance or layout. To compare proposals fairly, you need to check the breakdown of loss rates.

First confirm what is included in the total loss rate. Does it include temperature, shading, wiring, power conversion, soiling, snow, equipment downtime, and degradation, or only some of these? The same term "loss rate" cannot be compared if the included items differ.

Next, compare how shading is handled. Some vendors may reflect on-site obstructions in detail while others may use a rough estimate. Proposals that carefully account for shading may show more conservative generation but may be closer to reality. For sites where shading is significant, comparing generation with and without shading can make differences clear.

Also compare treatments of temperature, soiling, and snow. Confirm whether site-specific characteristics—such as high rooftop temperatures, surrounding-environment-driven soiling, and winter generation in snowy regions—are reflected. Proposals that rely on generic loss rates versus those that reflect on-site conditions will differ in reliability.

Generation per unit capacity is also useful. Proposals with low loss rate settings may show high generation per capacity. If excessively high, check irradiance and loss assumptions. Conversely, conservative generation may indicate that shading and losses are being prudently estimated. Rather than judging lower numbers as bad, it’s important to understand why those numbers were chosen.

When comparing vendor proposals, do not simply choose the one with the highest generation; prioritize proposals whose loss assumptions are clear and explainable. If you can see a breakdown of loss rates, it becomes easier to judge whether differences in generation stem from design differences or from differing assumptions.

How accurate on-site information increases the reliability of loss rates

Accurate on-site information is essential to improve the reliability of loss rates. Solar power generation simulations are calculated based on site conditions. If you do not correctly grasp sources of shading, roof or land orientation and slope, obstructions, surrounding environment, soiling susceptibility, and snow likelihood, the loss rate will deviate from reality.

For rooftop projects, you need accurate measurements of roof surface dimensions, orientation, pitch, rooftop equipment, railings, roof penthouses, piping, drains, access hatches, and the relative positions to surrounding buildings. Equipment not shown on drawings or piping added later can change shading and the assessable installation area. Conducting on-site checks and recording the positions and heights of obstructions lets you evaluate shading losses more accurately.

For ground-mounted projects, lot boundaries, trees, utility poles, surrounding structures, embankments, elevation differences, drainage ditches, access roads, and potential connection points are important. Shadows from trees and terrain, soiling from unpaved areas or nearby environment, and drainage conditions affecting maintainability can impact loss rates. Instead of treating the entire site uniformly, understand conditions by section.

When on-site information is accurate, you can set realistic loss rates. That also enables design measures to reduce losses—avoid shaded areas, plan maintenance for soiling-prone locations, rationalize wiring routes, and secure inspection walkways. Treat loss rates not merely as calculation figures but as items to improve based on site conditions.

Moreover, gathering accurate on-site information makes it easier to compare vendor proposals. If you can share the same on-site information with each vendor, you can fairly compare loss rates and generation. Conversely, if each vendor interprets site conditions differently, it becomes hard to tell whether differences in loss rates are due to design capability or input-condition differences.

Checking loss rates by looking only at numbers on the desk is insufficient. Accurately grasp the site reality and reflect that information in simulations to increase the reliability of generation forecasts.

Summary

To check loss rates in solar power generation simulations, it is important to examine not only the overall loss rate number but also its breakdown and how well it reflects site conditions. Multiple factors reduce generation—temperature, shading, wiring, power conversion, equipment configuration, soiling, snow, degradation over time, and equipment downtime. How far these are accounted for strongly affects annual generation, self-consumption estimates, and perceived profitability.

First, understand what the loss rate is intended to subtract. There is a gap between ideal generation and usable generation. Simulations that reflect that gap realistically are closer to post-installation reality. For temperature losses, check especially summer and rooftop high-temperature environments. For shading losses, check whether surrounding buildings, rooftop equipment, trees, and terrain are reflected by season and time of day.

Losses from wiring, conversion, and equipment configuration matter too: power generated by panels is not directly usable and incurs losses through wiring and equipment. Soiling, snow, and surrounding environment losses vary widely by site; confirm site-specific factors rather than relying on generic assumptions. For degradation and downtime, confirm whether the simulation assumes long-term operation.

Be wary of simulations with loss rates that are too low. Low loss rates make generation appear high, but if shading, temperature, soiling, degradation, etc., are not sufficiently considered, the gap with actual generation after installation may be large. For proposals showing high generation, carefully review the loss breakdown.

When comparing vendor proposals, confirm what the loss rate includes. The same "loss rate" can mean only temperature, or include shading and soiling, or treat degradation separately. Knowing the breakdown makes it easier to judge whether differences in generation are due to site conditions or to assumptions.

The foundation for reliable loss rates is accurate on-site information. If you can precisely grasp the candidate installation range, rooftop equipment, obstructions, trees, lot boundaries, inspection walkways, and surrounding structures, you can realistically evaluate losses from shading, soiling, wiring, and maintainability.

If you want to increase the accuracy of recording the installation candidate range, rooftop equipment, obstructions, trees, lot boundaries, inspection walkways, and surrounding structures on site and thereby improve the precision of loss rate checks in solar power generation simulations, using the LRTK, an iPhone-mounted GNSS high-precision positioning device, is effective. High-precision on-site position information makes it easier to identify shadows and obstructions, confirm installable areas, compare vendor proposals, perform pre-construction checks, and manage maintenance consistently. To correctly verify loss rates in solar power generation simulations, it is important to establish a system to accurately capture the site, not rely solely on desk calculations.