6 Steps to Organize Causes of Low Power Generation by Generation Loss Rate

Table of Contents

• Key considerations to grasp before organizing causes by power generation loss rate

• Step 1 First, align the comparison criteria and define the state of low power generation

• Step 2 Check the power generation loss rate by day and by month to confirm the pattern of anomalies.

• Step 3 Separate the effects of weather, solar radiation, and air temperature

• Step 4: Break down equipment-side losses by location of occurrence

• Step 5: Bridge the gap between figures and reality through on-site verification

• Step 6 Track post-countermeasure loss rates to prevent recurrence

• Using the generation loss rate makes reporting and decisions about improvements easier.

• Summary: When power generation is low, organize the causes by loss rate rather than by intuition.

Key concepts to understand before organizing causes by power generation loss rate

When operating a solar power generation system, you may at some point notice anomalies such as consistently low output, a lack of growth compared with the previous year, or lower generation performance than systems of the same scale. What makes this difficult for operations staff is that it is hard to immediately determine whether the decline is truly abnormal, a natural fluctuation due to weather, or a fault in the equipment. If you look only at generation output, days with low insolation, days with shading, and days when the power conditioner is limiting output all appear the same: "low generation."

A useful approach is to to organize causes using the concept of a generation loss rate. The generation loss rate here is a practical metric that shows, as a percentage, how much generation fell short of the expected output. For example, you set the expected generation based on insolation conditions and equipment capacity, and treat the difference between that assumed value and the actual result as the loss. Instead of simply viewing generation as low, checking under which conditions and by what proportion it was lost makes it easier to prioritize the causes.

However, the power generation loss rate should not be used to draw conclusions from a single number. There are many factors that can cause low generation, such as weather, solar irradiance, temperature, snowfall, dirt, shading, weeds, fallen leaves, bird damage, wiring, connection points, power conditioners, voltage rise suppression, communication errors, and missing measurement values. If you suspect all of these at once, the verification work becomes scattered and it is difficult to align understanding among personnel. Therefore, when using the power generation loss rate, it is important first to make the comparison conditions consistent, then examine trends over time, and finally distinguish between external factors and equipment-related factors.

When you feel the power output is low, instead of immediately considering replacement or major repairs, identifying at which stage and by how much loss is occurring will clarify the priority for on-site inspections. If the cause is weather or temporary output control, you can choose to monitor the situation, and if large losses occur only in specific circuits or sections, you can narrow the inspection scope. The generation loss rate is a practical metric for converting on-site impressions into numbers and making it easier to explain to stakeholders.

Step 1: First, standardize the comparison criteria and define what constitutes a low power output

The first step in organizing the causes of low power generation by generation loss rate is to clarify what it is being compared against. If you begin investigating causes while this is unclear, confusion arises—for example, it may be lower than the same month last year but reasonable when solar irradiance is considered; low in monthly results but largely affected by a specific day; or being compared to a plant with a different installed capacity. A determination that generation is low must always be treated together with the comparison baseline.

Commonly used comparison benchmarks include the design-stage power generation forecast, past actuals for the same month, performance of nearby or similarly conditioned facilities, expected generation reflecting solar irradiance, and generation per unit of installed capacity. Rather than looking at just one of these, choose among them according to the purpose. For example, if you want to monitor progress against an annual plan, comparison with the generation forecast is helpful. On the other hand, if a day with suddenly low generation occurs, comparing daily actuals and solar irradiance becomes important.

When considering the generation loss rate, it is important to align the expected generation with the actual generation. If the expected generation is overestimated, the loss rate will appear larger than necessary. Conversely, if the expected generation is set too low, abnormalities can become difficult to detect as losses. Even when using design values as-is, you need to confirm whether they are appropriate for the current equipment by taking into account actual equipment conditions, orientation, tilt, aging, surrounding environment, and seasonal factors.

Also, the period used for comparison is important. Since solar power generation varies greatly day to day, judging an anomaly based on a single day's generation can easily lead to misunderstanding. During periods of consecutive cloudy or rainy days, even properly functioning equipment will produce less power. Conversely, looking only at sunny days can cause you to overlook morning and evening shadows and daytime output curtailment. By looking at multiple time units—daily, weekly, monthly—and separating short-term fluctuations from sustained declines, it becomes easier to clarify the meaning of the loss rate.

At this stage, define a low generation state numerically rather than by intuition. For example, if you define it as: consecutive days when actual output is lower than expected by a certain percentage; lower than the same month of the previous year even after accounting for solar irradiance conditions; or only a specific section of the installation being clearly lower, it becomes easier to proceed with subsequent checks. The purpose here is not to determine the cause. As a starting point for the investigation, decide which figures to use as the baseline for assessing losses.

Step 2: Check the pattern of anomalies in power generation loss rates by day and by month

Once you've aligned the comparison criteria, next examine the power generation loss rate over time. The causes of low generation can be ones that occur the same way every day, or ones that appear only during specific times of day or particular seasons. If you look at the generation loss rate only as a single monthly value, it becomes difficult to understand the patterns in which losses occur. Breaking the data down by day, time of day, and month makes it easier to narrow down the possible causes.

Looking at daily loss rates makes it easier to distinguish between temporary anomalies and persistent abnormalities. For example, if the loss rate is high on just one day, possible causes include weather, communication outages, temporary shutdowns, maintenance work, and impacts from the power grid. If the loss rate is high for several consecutive days, you should suspect ongoing causes such as dirt, shading, weeds, equipment stoppages, poor connections, or configuration changes. If the loss rate is gradually increasing month by month, also check for aging degradation, changes in the surrounding environment, lack of cleaning, and the growth of trees or weeds.

Time-of-day generation loss rates are also important. If generation is low only in the morning, factors such as shadows on the east side, frost, remaining snow, delayed startup, or an offset in the measurement start time may be involved. If losses are pronounced around midday, candidates include output reduction due to high temperatures, temperature rise of the power conditioner, voltage-rise suppression, and output limitations associated with overloading design. If output is low only in the evening, check for shadows on the west side and shadows from surrounding structures. Even with the same decrease in generation, the suspected causes vary depending on the time of occurrence.

Viewing loss rates by month makes it easier to identify causes with seasonality. If loss rates tend to increase in summer, check for high temperatures, inadequate ventilation, vegetation growth, bird damage, and the accumulation or adhesion of dirt. If loss rates increase in winter, factors include snowfall, frost, shadows from low solar altitude, shorter hours of sunlight, and any dirt or damage remaining after snow removal. Some installations are also prone to specific shading in spring or autumn. Considering seasonal solar altitude and the surrounding environment reveals the background of generation loss rates, not just low power generation.

In this step, what matters is not only the magnitude of the loss rate but also the pattern of the loss. Check whether it suddenly increased, has been gradually worsening, stands out only on sunny days, appears similarly on cloudy days, or occurs predominantly during certain time periods. Once you understand the pattern of the loss rate, you can form hypotheses to verify before visiting the site. Compared with going to the site without hypotheses, this shortens investigation time and makes it easier to reduce oversights.

Step 3 Isolate the effects of weather, solar radiation, and air temperature

When organizing the causes of low power generation, the first thing to separate out is the effect of weather and solar irradiance. Solar power generation is greatly influenced by sunlight conditions, so a day with low output alone does not necessarily indicate an equipment fault. Cloudy skies, rain, snowfall, yellow dust, heavy haze, or the passage of localized clouds can reduce output even with properly functioning equipment. To correctly assess the generation loss rate, it is essential to check not only the actual generated power but also the external conditions required for generation on that day.

If solar irradiation is being measured, looking at the relationship between generation output and irradiation makes it easier to determine whether a decline is due to weather or to equipment. If days with low irradiation also have low generation, then even if the loss rate appears high it may be within the range of natural variation. On the other hand, if irradiation is sufficient but generation does not increase, there is a stronger reason to suspect equipment-side losses. In particular, if generation on clear days is lower than expected, checks for shading, soiling, curtailment, equipment shutdowns, and the like are necessary.

The effect of ambient temperature should not be overlooked. Solar panels tend to generate more power when solar irradiance is high, but their output decreases as panel temperature rises. For that reason, on a clear summer day the amount of power generated may not increase as much as expected. This is not necessarily a malfunction, but if conditions such as poor ventilation, heat buildup on the back of the panels, and insufficient heat dissipation around the power conditioner coincide, losses can become significant. When checking the power generation loss rate, confirm not only the solar irradiance but also the ambient temperature and the equipment’s heat-dissipation conditions.

Snow and frost also have a large impact on power loss rates. Areas where snow remains cannot receive solar irradiance, reducing power output. If the panels are completely covered with snow, overall generation falls, but if only part of a panel or array retains snow, attention is needed for output differences at the string level and the possibility of local temperature rises. Even after snow removal, issues can remain such as soiling on the panel surface, changes around wiring and mounting caused by sliding snow, and altered shading from snow piled up nearby. When assessing winter loss rates, it is important not to assume generation drops simply because temperatures are low, but to check how snow and shadows persist.

Also, it is necessary to verify the reliability of the measured solar irradiance and temperature data themselves. Dirt on sensors, misalignment of the installation angle, communication failures, or differences in recording intervals can cause the expected power output to be calculated incorrectly. If the pyranometer is dirty and reads low, the generation system may be operating normally yet the apparent loss rate can look small. Conversely, if the solar irradiance is recorded as excessively high, the power output may appear low. When using generation loss rates, the validity of the environmental reference data is as important as the generation data.

Step 4 Break down equipment-side losses by each location where they occur

After confirming the effects of weather and solar irradiance, if low power output persists, break down the equipment-side losses by location. In a photovoltaic power system, the DC power generated by the panels passes through junction boxes and wiring, is converted to AC by the power conditioner, and is sent to the grid. If losses occur at any point in this flow, the final power output will be reduced. Rather than looking only at the total generation, it is important to check at which stage the losses are greatest.

First, what you should check is the losses occurring on the panel surface. Typical factors that reduce power generation include panel dirt, bird droppings, fallen leaves, dust, pollen, yellow sand, shadows from vegetation, shadows from nearby structures, panel damage, and surface degradation. Partial soiling or shading can affect power generation more than they appear. In particular, if only certain rows or sections have lower power output, suspect shading or soiling common to that area. When power loss rates can be viewed by section, it is important to focus on the low-performing areas rather than the overall average.

Next, check for losses in the DC-side wiring and connections. If there are loose connections, corrosion, cable damage, poor connector contact, or degradation of the wiring route, the generated power may not be transmitted sufficiently. Because these issues can be hard to detect by appearance alone, use clues such as variations in current and voltage readings, differences between strings, and abnormal temperature rises. When low power output is caused by a DC-side problem, it can appear as only some inputs connected to the same power conditioner showing low values.

Losses around the power conditioner are also important. Reduced conversion efficiency, output limitations due to temperature rise, malfunctions of cooling fans or ventilation paths, clogged filters or air inlets/outlets, internal abnormalities, operation stoppages, and setting changes all affect the amount of power generated. When checking days with low generation, you need to look not only at the generated energy but also at the operating status, alarm history, stop history, whether output was limited, and temperature-related information. If the generation curve plateaus during midday high irradiance, you should also check for output limitation associated with oversizing design and for voltage-rise suppression.

The effects around the AC side and grid interconnection cannot be overlooked. When voltage rise suppression occurs, generation equipment may reduce its output. This is not necessarily equipment failure; it can occur as a control action in response to grid-side voltage conditions. However, if it happens frequently or for long periods, it will affect the amount of electricity sold and generation performance. If the generation loss rate is high around late morning to early afternoon on sunny days, checking the voltage trend and suppression history can make the cause easier to identify. If you do not separate equipment issues from grid-side conditions, you may end up replacing parts unnecessarily or taking incorrect countermeasures.

Finally, issues with data acquisition or communications can also appear as losses on the equipment side. Even if power is actually being generated, if generation is not being recorded due to communication errors or instrument malfunctions, it will appear as low generation. If there is a day when the generation loss rate suddenly increases substantially, cross-check the on-site meter, the monitoring screen, stored data, measurement intervals, and missing time periods. By separating whether the loss is in the generation itself or a loss in the recording, you can avoid going down the wrong path in your investigation.

Step 5: Fill the gap between figures and reality through on-site verification

When you form hypotheses about causes from the generation loss rate, on-site verification is used to bridge the gap between the figures and the actual situation. Even if the data shows large losses, on site it may turn out to be a simple communication failure. Conversely, even when the discrepancy in the data is small, on site you may find that shading, dirt, or deteriorating wiring has progressed. To correctly identify the causes of low power output, you must not rely solely on the numbers but corroborate them with the field conditions.

During on-site inspections, first correlate the days or time periods with high loss rates to situations that can actually occur at the site. If losses are large in the morning, check morning shadows, frost, shadows from surrounding buildings and trees, and delays in the generation start time. If losses are large at noon, check the power conditioner temperature, voltage rise suppression, intake and exhaust conditions, and output saturation. If losses are large in the evening, check shadows from the west and changes in the surrounding environment. If you inspect the site without considering the time of day, you may overlook the critical conditions that cause the losses.

When inspecting the panel surface, do not only assess the overall uniformity but also focus on sections with high loss rates. Check for dirt, bird droppings, fallen leaves, shadows from vegetation, residual snow, surface scratches, cracks, discoloration, and similar issues. For shadows, pay attention not only to the time of inspection but also to changes by season and time of day. Even if no shadows are present during the inspection, shadows may have occurred during periods of low power generation. If past photos or time-series records are available, it becomes easier to track changes in shading.

Around the equipment, check alarm indications, shutdown history, operating status, overheating, abnormal noises, ventilation paths, cable fastening, and corrosion or moisture ingress around junction boxes. Abnormalities in wiring and connection points can be difficult to assess by visual inspection alone, but discoloration, burn marks, looseness, damage to insulation, and poor fastening are important clues. For parts related to safety, do not touch them unnecessarily; establish a system in which personnel with the required qualifications or authority verify them as needed. Investigations into reduced power generation should proceed not only to determine the cause but also in conjunction with ensuring safety.

Always record the results of on-site inspections linked to the power generation loss rate data. Simply noting "it was dirty" or "there were shadows" makes it difficult to use for future decisions. By recording which section had what loss rate, when it occurred, and what was confirmed on site, comparisons when the issue recurs become easier. Keeping photos, the inspection date and time, weather, inspector, target section, and actions taken together will make reporting to stakeholders more concrete.

What's important in this step is not to use the generation loss rate as a substitute for on-site verification. The loss rate is a tool to narrow down causes and does not fully explain the actual on-site situation. By forming hypotheses from the numbers, verifying them on site, and, when necessary, returning to the data, you can organize the causes of low power generation in a way that closely reflects reality.

Step 6 Track post-countermeasure loss rates to prevent recurrence

After identifying the cause and implementing countermeasures, track the power generation loss rate to confirm the effectiveness of the improvements. Even if you discover the cause of low power generation, you cannot assess the validity of the work unless you verify that losses have actually decreased after the measures. When performing cleaning, mowing, repair of connections, review of equipment settings, ventilation improvement, snow removal, or addressing obstacles that cause shading, it is important to compare the loss rate before and after the measures under the same conditions.

When comparing results after countermeasures, do not judge solely by whether power generation increased. If clear weather continues after the measures, generation will naturally increase; conversely, if the weather is poor after the measures, generation may not increase as much as expected. Therefore, verify how the power generation loss rate has changed while examining solar irradiance, temperature, past performance in the same season, and differences between the treated section and other sections. Making comparisons under equalized conditions makes it easier to explain the effect of the measures.

Evaluate the effectiveness of improvements in both the short term and the medium-to-long term. Immediately after cleaning, the loss rate may decrease, but depending on the surrounding environment, dirt can accumulate again. Immediately after mowing, shadows may be eliminated, but weeds can grow back after a few weeks and create shadows again. Even if the intake and exhaust of the power conditioner are improved, if filters and vents are not continuously maintained, temperature rises can recur. By regularly tracking the power generation loss rate, you can determine whether measures are temporary or lead to sustained improvement.

Also, recording the loss rate after countermeasures makes it easier to compare in the event of future anomalies. For example, if you know how much the loss rate improved after cleaning, you can more easily judge whether dirt is likely when a similar drop occurs next time. If you record the periods and times when voltage rise suppression occurs, it becomes easier to understand changes in grid conditions and seasonality. It is important not to treat the cause of low power generation as a one‑off investigation, but to accumulate it as a history of operational improvements.

The power generation loss rate is also useful when deciding the priority of countermeasures. Because it is unrealistic to address all faults and degradations at once, we handle them in order starting with those that have large loss rates, those that are prone to recurrence, those that involve safety, and those that can spread to other equipment. Even issues with a small impact on generation output are given higher priority if they pose safety risks. The power generation loss rate is only one factor for judgment, but it serves as an effective axis for quantitatively explaining the effects of improvements.

Using a power generation loss rate makes reporting and improvement decisions easier

The advantage of organizing the causes of low power output by using a generation loss rate is that it helps not only with on-site inspections but also with reporting and decisions on improvements. Simply saying that power output is low can be interpreted differently by stakeholders. One person in charge may regard it as abnormal, while another may see it as within the range of weather conditions. By quantifying it as a generation loss rate, it becomes easier to explain how large the difference is, under what conditions it occurs, and which factors are most likely.

Especially when multiple stakeholders such as administrators, construction personnel, maintenance staff, and owners are involved, organizing information using power generation loss rates helps align understanding. For example, a report that monthly power generation is lower than planned alone can make it difficult to determine whether immediate on-site action is required or whether it is better to wait and see considering the weather. Therefore, if you present loss rates adjusted for solar irradiance conditions, time-of-day biases, differences by equipment section, and the results of on-site inspections together, the basis for the decision becomes clear.

In reports, it is also important not to use expressions that assert the cause too strongly. Low power generation can result from multiple overlapping factors. For example, shading and soiling may occur at the same time; a rise in the power conditioner’s temperature and voltage-rise suppression may appear during the same time period; communication failures and actual output declines can coexist. When using generation loss rates, rather than concluding “this factor alone is the cause,” organize your findings as “based on the loss rate for this time period, the factors that should be prioritized for inspection are these,” which reduces practical misunderstandings.

When judging improvements, the power generation loss rate can also be used to determine the priority of countermeasures. In practice, because implementing measures requires personnel and time, it is necessary to decide which actions to take first. Factors that have high loss rates, occur frequently, and are likely to recur are worth prioritizing for countermeasure consideration. Conversely, temporary losses with a low occurrence frequency may be monitored while a decision is made. Combining numerical data with on-site conditions makes it easier to avoid both excessive responses and delayed responses.

Furthermore, by continuously managing power generation loss rates, the characteristics of each power plant become apparent. Some facilities tend to accumulate dirt, some are prone to morning shadows, some are more susceptible to temperature effects in summer, and some experience prolonged impacts from snowfall in winter—loss patterns vary by plant. If you understand these characteristics, you can adjust inspection plans, cleaning schedules, grass-cutting timing, patrol frequency, and report content for each facility. Power generation loss rates can be used not just for anomaly detection but as an ongoing management metric to improve operations.

Summary: When power output is low, identify causes by loss rates rather than by intuition

When you feel the power generation is low, immediately assuming it’s an equipment failure or, conversely, overlooking it as merely the weather can delay identifying the cause. What’s important is to first align the comparison baseline and confirm how large the difference is in terms of generation loss rate. On that basis, examine the pattern of losses by day, by time of day, and by month, separate the influences of weather, solar irradiance, and temperature, and break down where on the equipment side the losses are occurring.

By using the generation loss rate, you can organize the causes of low power generation not by intuition but by combining numerical data with on-site conditions. You can systematically check whether it was simply due to low solar irradiance, whether generation didn’t increase even on sunny days, whether morning or evening shading is having an effect, whether daytime output is plateauing, whether only certain sections are low, or whether communication data loss is involved. By narrowing down candidate causes in order, you can reduce unnecessary on-site inspections and make reports more persuasive.

Also, by tracking the generation loss rate after implementing countermeasures, you can verify the effectiveness of the improvements and help prevent recurrence. After carrying out cleaning, grass cutting, equipment inspections, ventilation improvements, wiring checks, and so on, continuing to monitor the loss rate using the same criteria will allow you to determine whether the measures were appropriate. Rather than treating low power generation as an issue resolved by a single inspection, you can convert it into ongoing operational improvement.

In managing power generation facilities, accurately grasping on-site conditions and cross-referencing them with data is indispensable. In addition to organizing generation loss rates, being able to centrally handle on-site photographic records, equipment locations, inspection results, and changes in shading or soiling makes cause investigation and reporting much easier. To efficiently identify the reasons for low power output, it is important to establish operations that do not treat generation data, environmental data, and on-site inspection records separately but instead continuously review them according to the same standards.