Four Criteria to Distinguish Seasonal Low Solar Power Generation from Anomalies

When operating a solar power system, it is not uncommon to have days or months when you feel the "generation is low." Especially in winter, during the rainy season, after typhoons, or during prolonged periods of rain, the generation graph can drop sharply, causing concern that there may be an abnormality in the equipment. However, solar power generation fluctuates due to multiple factors such as weather, season, temperature, solar irradiance, shading, dirt, and equipment condition. Therefore, it is premature to judge a fault solely because the numbers are low.

The important thing is to distinguish whether a decline is a natural seasonal decrease or an abnormality that should prompt inspection or repair. For operations personnel, when they detect a drop in power generation, it is important to have a clear procedure for the order of checks, which conditions to monitor, and which conditions require on-site verification or professional inspection.

Table of Contents

• Why seasons with low power generation are easily mistaken for abnormalities

• Basic concepts for seasonal reductions in power generation

• Criterion 1 Check whether the reduction corresponds to changes in solar irradiance and weather

• Criterion 2 Check whether it deviates from past performance and seasonal generation curves

• Criterion 3 Check whether only part is lower compared with equipment or circuits under the same conditions

• Criterion 4 Check for signs of soiling, shading, equipment stoppage, or wiring faults

• Procedure that field operators should follow when power generation is low

• Information to record to distinguish seasonal factors from anomalies

• Summary Visualizing on-site information is important for quickly identifying the causes of low power generation

Reasons why seasons with low power generation are easily mistaken for anomalies

The amount of electricity generated by a solar power system is not determined solely by its installed capacity. Even at the same plant, there can be large differences in output between periods of prolonged clear weather and periods of extended cloudy or rainy weather. Many instances in which generation appears low can be explained not by equipment failure but by reduced solar irradiance, shorter hours of sunlight, or unsettled weather.

One situation that often confuses operations staff is when a decline in power generation appears sudden. Even if power generation had been steady through the previous month, when output falls in a particular month they tend to suspect a problem with the solar panels or peripheral equipment. However, month-to-month comparisons are heavily influenced by seasonal variation, and simply looking at the change from the previous month can lead to incorrect conclusions.

For example, in seasons when the hours of sunlight are shorter, the time during which power can be generated is reduced even when it is sunny. When the sun’s elevation is low in the morning and evening, shadows from surrounding buildings, trees, slopes, mounting structures, and adjacent rows of panels tend to lengthen. Shadows that were not noticeable in summer can fall on the generation period in winter. Under such conditions, power output decreases even if the equipment is functioning normally.

On the other hand, there are declines that are difficult to explain by seasonal factors alone. If power output fails to increase significantly on sunny days, if only some circuits within the same power plant show low output, if output is clearly lower compared with the same season in past years, or if the generation graph shows unnatural steps or periods of stoppage, you should consider the possibility of an anomaly.

In other words, when judging whether power generation is low, it is essential to look at a combination of factors — not a single number — including the season, weather, historical performance, comparisons within the facility, and on-site conditions. What is important in a situation of low power output is not to attribute the cause to a single factor, but to have verification criteria to distinguish between natural variation and an anomaly.

Basic Concept of Seasonal Decline in Power Generation

In solar power generation, in general the more solar irradiance and the longer the hours of sunlight, the more power generation tends to increase. However, summer is not always the peak. Although summer has long hours of sunlight, high temperatures tend to reduce the output of solar panels. Conversely, in spring and autumn the balance between solar irradiance and temperature is often better, and power generation can be higher.

Winter is often perceived as a season of low power generation. Because the sun's altitude is low and daylight hours are shorter, even on clear days the time available for generation is limited. In snowy regions, accumulation of snow on panel surfaces and effects from the surrounding environment can further reduce generation. Even without snow, extended morning and evening shadows can make the rise and fall of the generation curve more gradual.

The rainy season and periods with frequent prolonged rain are also times when power generation tends to appear low. When cloudy or rainy days continue, monthly power generation tends to decline. Although power is generated even on overcast days, it does not reach the same levels as on sunny days. Therefore, even if power generation is lower compared with the previous month during the rainy season, that alone cannot be taken as evidence of an abnormality.

Be vigilant during typhoon season and the autumn rainy season. While unstable weather can easily cause decreases in power generation, physical impacts can also occur from strong winds, flying debris, fallen leaves, mud splatter, and changes around the mounting structures. Because seasonal factors and on-site abnormalities often coincide, when you detect a drop in power generation, do not attribute it solely to the weather; carry out an on-site inspection as needed.

Also, when judging seasons of reduced power generation, regional differences must be taken into account. In areas with heavy snowfall, coastal areas, mountainous regions, urban areas, and around farmland, the factors that affect power generation differ. Snow accumulation, salt, dust, fallen leaves, bird droppings, yellow sand, and the growth of surrounding trees — even in the same season, the points to watch vary for each power plant.

Seasonal declines in power generation can occur even at normally operating plants. However, if the decline exceeds what can be explained by seasonal factors, it should be treated as a sign of an anomaly. Therefore, when you feel that power generation is low, it is effective to first check whether the decline is natural for that season, and then check whether there are any abnormal differences under the same conditions.

Criterion 1: Confirm whether the decrease corresponds to changes in solar irradiance and weather

The first criterion is whether the decrease in power generation corresponds to changes in solar irradiance or weather conditions. Because solar power generation relies on sunlight, prolonged cloudy or rainy conditions will reduce output. Therefore, you should not judge a decrease as an anomaly based only on generation figures; you need to check the weather during that period.

For example, even if power generation drops sharply on a single day, if that day was rainy or covered by thick clouds all day, it is likely not an anomaly. Conversely, if conditions were nearly clear yet generation was significantly lower than on a typical sunny day, that would be grounds to suspect some kind of abnormality.

In practice, it is important to look not only at daily data but also at power generation graphs by time of day. If the decline is due to weather, generation will fluctuate with the passing of clouds or the graph will show a gently lower level throughout the day.

On the other hand, equipment-side abnormalities can produce unnatural patterns such as generation suddenly stopping at a specific time, not rising above a certain output, or only some circuits being low despite sunny periods.

Also, when checking the weather, broad descriptors like "clear", "cloudy", or "rain" may be insufficient. Even thin cloud cover reduces power generation, and generation fluctuates depending on the thickness of clouds and how frequently they pass. Judging solely by weather information from locations far from the site can differ from the actual weather around the power plant. If possible, it is desirable to combine meteorological information close to the site, solar radiation data, and records from monitoring equipment when making a judgment.

Depending on the season, power generation may not increase as much as expected even on sunny days. In winter, daylight hours are shorter and the sun’s altitude is lower, so even on clear days it is difficult to achieve the same generation as in summer or spring. Lower temperatures can be advantageous for conversion efficiency, but because solar irradiance and the hours available for generation have a larger impact, monthly power generation can appear low.

When power generation is low, the first thing you should do is not simply suspect equipment failure but compare the generation with the weather. By checking whether the days with reduced output were mostly rainy or cloudy, whether output was low even on sunny days, whether it was low only in the mornings and evenings, or whether daytime peaks were also reduced, you can more easily distinguish a natural decline from one that may indicate an abnormality.

Criterion 2: Check whether it deviates from past performance and seasonal generation curves

The second criterion is whether the current power generation deviates significantly from past performance and seasonal generation curves. Because solar power output varies by season, relying solely on day-over-day or month-over-month comparisons can easily lead to mistakes. In particular, around seasonal transitions the generation trend can change readily, and short-term comparisons alone can either miss actual anomalies or mistakenly identify normal declines as abnormalities.

An effective approach is to compare with the same month of the previous year or the same season in past years. For example, if you feel this winter’s power generation is low, instead of comparing it to summer or autumn, compare it to last winter, the winters of the past few years, and sunny days in the same month. Of course, weather conditions differ from year to year, so it will not exactly match past performance. Even so, checking whether it deviates significantly from the normal range for the same season makes it easier to assess the possibility of an anomaly.

Seasonal generation curves are also important. At a power plant operating normally, the generation graph on clear days generally ramps up in the morning, peaks around midday, and then declines toward the evening. The peak height and generation duration vary by season, but there is a consistent pattern to the shape on clear days. If the shape deviates significantly from that pattern, it needs to be checked.

For example, if only the power generation around midday is unnaturally low, there are multiple possible causes such as output curtailment, equipment limitations, temperature effects, or problems on the facility side. If generation is extremely low only in the morning or only in the evening, seasonal shadow lengthening or changes in the surrounding environment may be involved. If overall power generation steps down from a certain day, it may indicate that some equipment has stopped or that a circuit fault is present.

When comparing against past performance, it is easier to make a judgment by comparing sunny days rather than simply looking at monthly power generation totals. Monthly figures that include rainy or cloudy days are strongly influenced by the weather, so the information becomes too coarse to determine whether an anomaly exists. By comparing power generation and power output curves only on sunny days, differences in equipment condition become easier to see.

However, when using past performance data, you should also confirm whether equipment conditions have changed. The reference for power generation can be affected by panel additions, nearby construction, tree growth, terrain changes after land development, newly constructed surrounding buildings, changes in system settings, or whether cleaning or inspections have been carried out. If past and current conditions differ, any comparison must take those differences into account.

Even in seasons when power generation is low, if the output falls within roughly the same range as the same season in past years, it is easier to regard it as a natural seasonal variation. On the other hand, if the output is clearly lower than past performance even after accounting for weather, it is necessary to further check for possible anomalies according to the following criteria.

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Criterion 3: Verify whether only certain parts are lower compared with equipment or circuits under the same conditions

The third criterion is to check whether only part of the generated output is lower when compared with other equipment under the same conditions or elsewhere in the same power plant. If the decline is due to seasonality or weather, it will generally affect the entire plant similarly. Of course, orientation, tilt, and shading conditions can cause differences, but if you see an extreme discrepancy in circuits or sections with similar conditions, you should suspect an equipment-side abnormality.

In a solar power plant, multiple components such as solar panels, junction boxes, power conditioners, wiring, and monitoring devices work together to generate electricity.

Therefore, if you only look at the total output of the entire plant, it can be difficult to notice some faults. Even if the plant as a whole is producing power, if only specific circuits or sections are underperforming, the difference can appear small in the aggregate value.

When comparing under the same conditions, you should compare installations that have the same orientation, the same tilt, the same installation conditions, and similar shading conditions. Simply comparing installations with significantly different conditions will not lead to a correct judgment. This is because east-facing, south-facing, and west-facing systems, as well as shaded plots versus open plots, exhibit different power generation trends. By aligning the comparison targets, it becomes easier to detect anomalies where only some parts are underperforming.

When only part of the system is underperforming, possible causes include dirt on the panel surfaces, fallen leaves, bird droppings, partial shading, poor wiring contact, equipment shutdown, activation of protective functions, broken wires, faulty connections, and failures to acquire monitoring values. Because these occur independently of seasonal factors, they can be easily overlooked when they coincide with seasons of low power generation.

For example, even during the rainy season when overall power generation is low, if a single circuit is significantly lower than the others, the weather alone may not be enough to explain it. In winter, even when overall generation is low, if a particular section is extremely low from morning through midday, shadows, soiling, or equipment problems may be involved. It is risky to attribute everything to seasonal factors simply because the overall level is low.

Also, if you manage multiple power plants, it can be useful to compare plants in the same region or with similar conditions. If, despite similar weather in the same area, only a particular plant is significantly lower, there may be causes specific to that plant. Conversely, if multiple plants in the same region are similarly low, it's more likely to be due to wide-area weather factors.

Comparisons under the same conditions help with early detection of anomalies. If you feel the power generation is low, checking not only the total for the entire plant but also the data broken down by section, by circuit, by equipment, and by time period will make it easier to narrow down the scope and cause of the anomaly.

Criterion 4 Check for signs of dirt, shadows, equipment stoppage, and wiring faults

The fourth criterion is to check on-site and in monitoring data for signs of soiling, shading, equipment outages, wiring faults, and the like. Low power generation is not caused only by weather or the season. Small on-site changes can also affect power generation.

Soiling is one of the common causes of reduced power generation. The types of soiling vary with the installation environment, including dust, yellow sand, pollen, bird droppings, fallen leaves, mud splashes, dust from farmland and roads, and salt deposits in coastal areas. Rain may wash some of it away to some extent, but depending on how it accumulates some residues can remain and affect power generation. In particular, if heavy soiling remains on part of a panel, it can lead to a decrease in power generation.

Shadows are also important. Because the sun's altitude changes with the seasons, shadows tend to be longer in winter. Causes of shading vary: growth of surrounding trees, weeds, neighboring buildings, utility poles, fences, signs, mounting racks, adjacent rows, and so on. Shadows that were not a problem in summer can fall on the power-generation hours in winter. Even during seasons when generation is low, if output is reduced only during certain times of day, it's worth checking for shading effects.

Do not overlook equipment shutdowns. If the entire plant has stopped, it is easy to notice, but if only some equipment has stopped, it may appear as only a slight decrease in total power generation. You need to check the monitoring screen for alarms or shutdown logs, whether output drops to zero during the daytime, and whether there are periods when only specific equipment is not generating.

Wiring faults and poor connections can be difficult to detect just by looking from the outside. They may present as reduced power generation, intermittent shutdowns, abnormal graphs, or protective actions by equipment. Because they can pose safety risks such as electric shock or fire, if you suspect a problem it is important not to attempt to handle it on site and to follow the operator’s maintenance procedures or have a qualified professional perform an inspection.

Also, you need to pay attention to faults in the monitoring data itself. There are cases where power is actually being generated, but the generated power is shown as low due to communication or measurement issues. If the generated power is extremely low, data are missing, values have not been updated for a period of time, or multiple items show abnormal values simultaneously, check not only the generation equipment but also the measurement and communication status.

When conducting on-site inspections, it is important to correlate the timing of the decline in power generation with the timing of changes at the site. Organizing the timeline—such as whether the decline occurred after a typhoon, during the leaf-fall season, after nearby construction increased shading, or whether it recovered after cleaning—makes it easier to identify the cause.

On-site indicators are crucial for distinguishing seasonal declines in power generation from actual anomalies. Even when generation figures alone are insufficient to make a determination, examining soiling, shading, shutdown history, abnormalities around wiring, and communication status together enables a more accurate assessment of whether an anomaly is present.

Steps for operational staff to check when power generation is low

When you notice low power generation, it's important to decide the order of checks before rushing into onsite work. If the sequence isn't organized, you risk misidentifying a normal weather-related decline as a fault, or conversely dismissing a clear abnormality as a seasonal factor.

First, check the period during which power generation is low. Determine whether it is a single day, several consecutive days, low over a month, or low only during specific time periods. If the drop occurs only on a single day and that day had bad weather, the cause can be considered weather-related. On the other hand, if generation is continuously low even on sunny days, or if similar drops repeat during particular time periods, additional checks are necessary.

Next, check the weather and solar radiation conditions. Look for any situations that might affect power generation, such as rain, cloudy skies, snowfall, fog, yellow sand, soiling after strong winds, or debris blown in after typhoons. Consider not only the daily weather but also hour-by-hour cloud movements and the conditions around the site.

Next, compare with past performance. Look at the power generation curves for the same month in the previous year, the same season in past years, and for sunny days. If the decline is due to seasonal effects, it often does not deviate significantly from the same period in the past. Conversely, if it is clearly lower despite similar weather, there may have been changes to the equipment or site conditions.

Furthermore, comparisons within the facility are performed. We check power generation for the entire power plant, by section, by circuit, and by equipment to see if there are any areas that are lower only in certain parts. If only a specific area is lower, we suspect problems unique to that area rather than just weather or seasonal factors. The narrower the area showing a decline, the easier it is to narrow down the possible causes.

Finally, determine whether on-site verification or inspection is necessary. If dirt or shadows are suspected, confirm with photographs or on-site records. If equipment stoppage or alarms are present, check inspection histories and operating conditions. If wiring or equipment abnormalities are suspected, a specialist inspection is required after ensuring safety.

Standardizing this workflow reduces variability in decision-making by staff. When there is an inquiry or internal report that power output is low, it also clarifies what information to collect, making it easier to shorten the time needed to identify the cause.

Information to record to distinguish seasonal factors from anomalies

To correctly determine the cause of low power generation, it is important to keep regular records. If you try to collect information only after an anomaly occurs, it will be difficult to make a judgment unless past weather, on-site conditions, cleaning history, and inspection history have been recorded.

The first thing you should record is power generation by day, by month, and by time of day. Monthly generation alone does not allow a detailed analysis of the causes of any decline. With time-of-day data, you can check whether output is only low in the morning, whether the midday peak is reduced, or whether it falls off quickly in the evening. This makes it easier to distinguish among factors such as shading, weather, and equipment outages.

Next are records concerning weather and solar irradiance. Even simple entries like "sunny," "cloudy," or "rainy" can be helpful when examining the relationship with power generation. If possible, keeping solar irradiance information and weather notes from near the site will make it easier to explain decreases in power output.

On-site photographs are also useful. Regularly photographing panel surface soiling, the way shadows fall, surrounding trees, weeds, fallen leaves, areas around the mounting structure, and drainage conditions will help you monitor seasonal changes. In particular, winter shadows, dirt after the rainy season or typhoons, autumn leaf fall, and pollen or dust in early spring can affect power generation.

Cleaning and inspection histories should also be recorded. By keeping records of when cleaning was carried out, which areas were cleaned, what was checked during inspections, whether any abnormalities were found, and whether repairs or parts replacements were performed, it becomes easier to trace the relationship with changes in power generation. If power generation recovers after cleaning, it may indicate that soiling was the cause.

The history of equipment alarms, shutdowns, and recoveries is also important. If periods of low power generation coincide with equipment shutdowns, that can help identify the cause. Conversely, if power generation is low but no alarms were triggered, it is necessary to consider other factors such as soiling, shading, or measurement issues.

The purpose of keeping records is not simply to create an audit trail. It is to enable a rapid determination of whether low power generation is due to natural seasonal variation or an on-site anomaly. When records are well maintained, comparisons with past data remain possible even if personnel change, improving the accuracy of reporting and inspection decisions.

Summary: Visualizing on-site information is essential to quickly identify the causes of low power generation

When you feel that solar power generation is low, you don't need to immediately assume there's a fault. During winter, the rainy season, prolonged rain, typhoon season, and periods of snowfall, power generation can be low even when equipment is operating normally. Generation naturally fluctuates depending on sunshine hours, solar altitude, cloud cover, temperature, shadow length, and regional characteristics.

On the other hand, there are declines that cannot be explained by seasonal factors alone. If power generation fails to increase even on sunny days, is significantly lower than in the same season in the past, is lower only in part compared with equipment under the same conditions, the generation graph shows unnatural steps or stoppages, or there are signs of dirt, shading, or equipment shutdown, it is necessary to consider the possibility of an anomaly.

The criteria for distinguishing are four: whether the drop is consistent with solar irradiance and weather; whether it deviates from past performance or the seasonal generation curve; whether only part of the system or a circuit is lower compared with equipment under the same conditions; and whether there are signs of dirt, shading, equipment shutdowns, or wiring faults. By checking these four in order, it becomes easier to organize the causes of low power generation.

In practice, it is important to look not only at the power generation figures but also at on-site photos, inspection records, cleaning history, weather information, and circuit-level data together. If site information is lacking, it becomes difficult to distinguish between natural seasonal declines and abnormalities, which can lead to unnecessary site visits and missed anomalies.

To quickly identify the causes of low power output and make appropriate decisions, it is effective to manage the site's status and generation data together. When a decline in power output is confirmed, gather information in the following order: weather, past performance, comparisons among equipment, and on-site conditions, and establish a system that allows consulting specialist contractors or maintenance personnel as necessary.