7 checks to distinguish the causes of low power output by season

When managing a solar power plant, there are times when you notice "low power output" in the daily reports or on the monitoring screen. However, a drop in power generation should not be immediately assumed to be a fault. This is because multiple factors often coincide, such as weather, solar irradiance, temperature, seasonal sun angle, shading, soiling, vegetation, snow accumulation, and the condition of equipment.

In practical work, if you judge only by day-over-day or year-over-year comparisons, it's easy to confuse natural seasonal fluctuations with anomalies that need to be addressed on site. What's important is not just to look at the result that power generation is low, but to hypothesize causes that are likely in each season and then systematically isolate them one by one while cross-checking site conditions against the data.

In this article, we present seven checks for operations personnel to distinguish seasonal causes of low power generation at solar power plants.

Table of Contents

• If you feel the power output is low, first suspect seasonal variation.

• Check 1: In spring, check for dirt and shadows caused by pollen, yellow sand, and new buds

• Check 2: During the rainy season, check separately for insufficient sunlight and poor drainage

• Check 3 In summer, check for output reduction due to high temperatures and verify equipment temperature

• Check 4 After typhoons and heavy rain, inspect for re-deposition of dirt and changes to equipment

• Check 5: Confirm the decrease in autumn due to lengthening shadows and falling leaves.

• Check 6: In winter, verify the solar altitude, frost, snow accumulation, and long shadows

• Check 7: Confirm decreases that cannot be explained by seasonal factors as equipment anomalies.

• A system for recording and comparison is important to quickly identify the causes of low power generation.

If you feel your power generation is low, first consider seasonal variation

When you notice low power generation, the first thing to check is whether the decrease falls within the range that can naturally occur due to seasonal changes, or whether it is an abnormality caused by equipment or site conditions. Because solar power generation produces electricity by receiving sunlight, even the same equipment will generate different amounts depending on the season. During periods with many sunny days, stretches of cloudy or rainy weather, and times when the sun's altitude is low and shadows become long, the same solar plant will show different output characteristics.

In practice, you should be careful not to judge solely by simple comparisons like "lower than last month" or "lower than yesterday." For example, during the rainy season, if cloudy or rainy weather persists, energy output tends to decrease. In winter, because daylight hours are shorter and the sun’s altitude is lower, even clear days are unlikely to produce the same output as in summer. However, since low temperatures can also have a favorable effect on the output of photovoltaic modules, it is necessary to consider the region, snowfall, panel tilt angle, and solar irradiance conditions together.

Therefore, when checking whether power generation is low, it is necessary to examine past data from the same season, local weather, comparisons with neighboring sections, output differences among power conditioners, and variations at the string level. If the entire system is uniformly lower, weather or seasonal factors are more likely. On the other hand, if only specific sections, specific circuits, or specific time periods show a decline, individual factors such as shading, soiling, disconnection, poor connections, or equipment shutdowns should be investigated.

To distinguish causes by season, it is important to judge by combining not only the power generation itself but also the period when the decline occurred, the time of day, the weather, the appearance of the site, alarm history, and past inspection records. Rather than viewing low power generation as a single number, treating it as an entry point for estimating what is happening on site makes it easier to approach the cause while reducing unnecessary inspections.

Check 1: Confirm dirt and shadows caused by pollen, yellow dust, and new shoots in spring

If power generation is low in spring, first check the panel surface for dirt and any changes in the surrounding vegetation. Spring is a season when temperatures begin to rise and sunlight conditions improve, so it is easier to expect a recovery in power generation. Nevertheless, if generation does not increase as much as expected on sunny days, you need to check whether pollen, yellow sand, soil dust, bird droppings, or dust from nearby roads have adhered to the panel surface.

Performance losses from soiling may be washed away naturally by rain, but depending on the type of soiling and how it adheres, they can persist. In particular, when streaks of dirt accumulate along the lower edge of panels or when only certain rows show noticeable soiling, this can appear as variability in power generation. If the entire surface is lightly soiled, the overall output of the plant may appear slightly reduced, making it difficult to detect from the monitoring screen alone.

Spring is also the season when surrounding vegetation begins to grow. Low shrubs and slope weeds that were not a problem during winter can cast shadows in front of the panels as new shoots develop. Because the effects of shading appear and disappear depending on the time of day, it is difficult to determine the cause by looking only at daily power generation. If you observe patterns such as low output only in the morning, low output only in the evening, or a distorted generation curve in a specific section, it is necessary to check the position of the shadows on site.

During spring checks, we look at power generation curves on sunny days to see whether the daytime peak is naturally extending and whether there are drops only during specific time periods. We then cross-check with site photos and inspection records to check for dirt on panel surfaces, surrounding vegetation, weeds under the racking, and changes in shadows from adjacent land. Because spring is a season when power generation tends to increase, if a decline is noticeable it is important to identify on-site causes early.

Check 2 During the rainy season, check for insufficient sunlight and poor drainage separately

When power generation is low during the rainy season, a common cause is insufficient solar radiation. If cloudy or rainy weather continues, generation will naturally decrease. What’s important here is to distinguish whether the decline is due to the weather or to a deterioration in site conditions. During periods of heavy rain, not only is there less sunlight, but poor drainage, muddiness, debris inflow, rapid weed growth, and increased humidity around equipment also tend to occur simultaneously.

First, check whether the entire plant’s output is decreasing in the same way in response to the weather. If generation is dropping by a similar amount across all sections, it is likely that insufficient solar irradiance is a major factor. On the other hand, if only certain sections within the same plant show a larger decrease, you should check for puddles, mud splashes, clogged drains, sediment inflow from slopes, changes in the environment around junction boxes, and so on.

During the rainy season, light dirt on the panel surface may be washed away by rain, but water containing splashed mud or sediment can dry and instead leave deposits. Panels installed at low positions, panels along walkways, and rows at the bottom of slopes are more likely to have dirt adhere to their lower edges due to ground splashback. If this dirt remains unevenly on certain cells or modules, it can not only reduce overall power generation but also appear as differences in output between sections.

Also, if power generation does not recover after rainy weather, it may not be simply a weather-related issue but rather that some change remains at the site. Possible situations include blocked drainage paths, grass that has fallen over and is casting shade, sediment washed in and covering cables and racking, and access routes becoming difficult to pass, delaying inspections. During the rainy season, when checking, it is important not only to look at weather data together with generation data but also to confirm the site conditions after the rain has stopped.

To correctly distinguish a drop in power generation during the rainy season, rather than viewing low output on rainy days themselves as a problem, it is effective to check whether generation also recovers when sunny periods return. If output remains lower than before even on clear or lightly cloudy days, including dirt, drainage, vegetation, and the condition of the area around the equipment in inspection targets will make it easier to narrow down the cause.

Check 3 Confirm output reductions due to high temperatures and verify equipment temperatures in summer

When power generation is low in summer, you may notice it because the output does not rise as much as expected despite strong sunlight. While solar power generation is generally easier with more sunlight, typical photovoltaic modules tend to lose output as temperatures increase. Therefore, even if generation does not rise as much as expected on a clear midsummer day, you should not immediately assume a fault; instead, check for the effects of high temperatures.

Especially in summer, the temperature of the panel surface and the area around equipment tends to rise during the day. When you look at the power generation curve, it may ramp up smoothly in the morning but start to slow around noon. This is because, even when solar irradiance is strong, output can be suppressed by rising temperatures. Furthermore, if equipment such as power conditioners (inverters) or junction boxes are exposed to a high-temperature environment, protective actions or behavior close to output limitation can also occur.

In summer inspections, we check not only power generation but also temperature, solar irradiance, the equipment's installation environment, ventilation conditions, and alarm history. On site, it is important to see whether grass has overgrown around the equipment and impaired airflow, whether the equipment is installed in a location receiving strong direct sunlight, and whether there is dust or obstacles around the intake and exhaust vents. Even if no alarms have been issued, if the output unnaturally plateaus only during certain time periods, it is worth suspecting an effect from temperature.

Also, in summer grass grows rapidly and can lead to shading and work obstructions in a short period of time. Even if there are no problems immediately after vegetation clearing, within a few weeks grass can grow under and in front of the panels and block low-angle morning and evening light. A drop in summer power generation can sometimes be explained solely by high temperatures, but because grass, dirt, equipment temperature, and localized shutdowns often coincide, it is necessary to check from multiple perspectives before narrowing down to a single cause.

One point to watch when making judgments in summer is that anomalies can be hidden even on days with high generation. When solar irradiance is strong, total generation can still look acceptable despite some decline, so you may not notice anomalies unless you compare with the same period in the previous year or with adjacent sections. If sunny days continue yet output is clearly lower than past data under the same conditions, you should check the equipment condition as well as the temperature.

Check 4: After typhoons and heavy rain, check for re-accumulation of dirt and changes to equipment

After a typhoon or heavy rain, if power generation is low, check whether the reduction persists even after the weather has cleared. Immediately after strong rain or wind, low generation is easily attributed to weather, but if output does not return after a sunny period, there may have been changes at the site. Typhoons and heavy rain can wash dirt from panel surfaces while also relocating mud, leaves, branches, airborne debris, and sediment to other areas.

First, check for debris settling on the panel surface and around the racking. Leaves, pieces of plastic, soil, bird droppings, and lightweight materials blown in from the surroundings can remain on some panels and cause power output to drop in that section. Localized shading is more likely than uniform soiling to appear as differences at the string or power conditioner level, so checking monitoring data for variability is effective.

Next, check for changes around the mounting structures, fences, drainage routes, slopes, and cables. If heavy rain has scoured the ground, clogged drainage channels, or caused sediment to accumulate, this can lead not only to reduced power generation but also to future equipment failures. Even if it appears to be generating power, if cables are exposed, soil around support parts has washed away, or inspection paths have collapsed, prompt action is required.

After a typhoon, if power generation is low, also check whether equipment has been shut down and whether there are any alarms. Power outages, grid-side impacts, protective shutdowns of equipment, temporary communication suspensions, and so on can make generation data appear low. It is also important to distinguish whether the system is not generating or is generating but monitoring data are not being collected. By cross-checking local equipment displays, communication status, missing data on monitoring screens, and alarm histories, you can separate an actual drop in generation from how it appears in the data.

When inspecting after a typhoon or heavy rainfall, it is important to record any abnormalities along with photographs and location information. The cause of reduced power generation can leave only faint traces by the time the site is visited later. Leaves and branches may have been removed, sediments may have dried, and rainwater may have receded, so preserving the condition as discovered makes it easier to prevent recurrence and to explain the situation to stakeholders.

Check 5 Confirm the decrease in autumn due to lengthening shadows and falling leaves

If power generation is low in autumn, check for lengthening shadows and falling leaves. In autumn, temperatures drop, and there are days when conditions for equipment and panels are better than in summer. On the other hand, daylight hours gradually shorten and the sun’s altitude decreases. Shadows from buildings, trees, slopes, fences, utility poles, and nearby equipment that were not a problem in summer can begin to fall on the panels in autumn.

Reductions caused by shading are often reflected in the shape of the power generation curve. If you observe patterns such as a delayed morning ramp-up, an early evening drop-off, or output that steps down only during specific time periods, suspect shading. In autumn, because the position of shadows shifts daily as the season progresses, production is more likely to become progressively harder to increase rather than to fall sharply on a single day.

Fallen leaves are also a factor to be cautious of in autumn. At power plants with trees nearby, fallen leaves can accumulate on panel surfaces, under the mounting racks, in drainage gutters, and along fences. Leaves left on panels cause local shading, and if leaves clog the drainage gutters the flow of rainwater worsens, which can lead to mud splashing and puddles. Even if the fallen leaves themselves are temporary, if left unaddressed they can lead to soiling and poor drainage, and prolonged decreases in power generation.

In autumn inspections, it is important not to rely solely on summer inspection records, but to check where shadows appear at the current solar elevation. If inspections are carried out only around noon, shadows in the morning and evening can be overlooked. Visiting the site at the times when power generation data shows a decline makes it easier to identify the causes of shading. This is especially effective for power plants near forests, slopes, or adjacent buildings, since shading effects increase from autumn into winter, so checking ahead of the season is useful.

In autumn, equipment changes remaining after the typhoon season may persist. If there were strong winds or heavy rains from summer into autumn, panel soiling, sediment, branches and leaves, and changes around mounting structures can directly lead to reduced power generation. Rather than treating autumn declines simply as shorter sunlight hours, checking shading, fallen leaves, and residual post-typhoon effects together makes it easier to distinguish the causes.

Check 6: In winter, check solar altitude, frost, snow accumulation, and long shadows

If power generation is low in winter, check for decreased solar altitude, shorter sunshine duration, frost, snowfall, and long shadows. Winter is a season when sunshine duration is shorter and solar altitude is lower, so depending on the region and equipment conditions, power output tends to decrease. However, because low temperatures can improve module output conditions, do not assume that generation on clear days is uniformly low; you need to assess solar irradiance, snowfall, shading, and installation angle together.

A characteristic of winter is that shadows tend to be longer. Fences, trees, slopes, adjacent facilities, and nearby equipment that had no impact in summer can cast shadows on the panels in winter. Shadows can remain not only in the morning and evening but, depending on site conditions, also during the daytime. If the power generation curve is not only generally lower but also shows an unnatural drop during specific time periods, check for shading caused by the decrease in solar altitude.

Frost and ice can also lead to reduced power output in winter. If frost has formed on the surface of the solar panels in the morning, it may take some time for output to increase even after the sun rises. If it melts during the day, generation will recover, but on cold days or in places where shade remains, recovery can be delayed. If the morning power ramp-up is slow, checking for frost or ice makes it easier to determine the cause.

In snowy regions, it is necessary to check for shading caused by snow. Power generation drops significantly while snow remains on panel surfaces. Even if only part of a panel is covered, this can appear as differences in output between sections. Also, if snow accumulates under the panels or in walkways, it can affect the safety of inspections and removal work. When observing power recovery, it is important to confirm that the snow has completely fallen off, that no snow or ice remains at the lower edge, and that refreezing has not occurred.

What to watch for with winter declines is that seasonal factors and equipment abnormalities can overlap and become difficult to distinguish. Because this is a period when generation naturally tends to appear low, minor abnormalities can easily be masked. You need to compare with sunny days in the same month of the previous year, with other sections within the same power plant, and with arrays of the same orientation to see whether performance is outside the natural range for winter. If you assume low output is normal in winter too readily, you risk overlooking shading, frost, snow accumulation, and equipment outages.

Check 7 Confirm declines that cannot be explained by seasonal factors as equipment anomalies

If checking seasonal factors does not explain the low power generation, suspect equipment malfunction or electrical faults. The causes of low power generation are not limited to issues that are easily visible, such as dirt or shading. The condition of components such as panels, junction boxes, cables, connectors, switches, power conditioners, and communication equipment can also reduce power output.

First, what I want to check is the scope of the drop. I look to see whether the entire plant is low, only some power conditioners are low, or only specific strings are low. If the whole plant is similarly low, candidates include the weather, output curtailment, grid-side effects, overall soiling, and so on. On the other hand, if only certain circuits are clearly low, you need to check for open circuits, poor connections, blown fuses, the status of breakers, localized panel faults, cable damage, and the like.

Alarm history is also important. On days when power generation is low, check whether alarms such as overvoltage, insulation faults, communication anomalies, shutdowns, or temperature rises have been issued. However, the absence of alarms does not necessarily mean there is no abnormality. Minor connection issues or partial circuit degradation may not appear clearly as alarms. Therefore, it is necessary to make a judgment by combining comparisons of monitoring data, on-site verification, and inspection records.

If you suspect an equipment abnormality, prioritize safety and do not attempt unsafe inspections. Internal inspection or measurement of electrical equipment requires specialized knowledge. What the person responsible for operations should do first is organize when the decline began, over what range it is occurring, whether it can be explained by weather or seasonal factors, and whether there are any alarm or shutdown records. Having that information makes it easier to narrow the scope of investigation when handing the case over to specialist contractors or electrical management personnel.

Also check whether the low power output ends in a short period or continues over multiple days. If the drop is temporary, it may be due to weather or transient shading, but if the same section continues to show low output across consecutive clear days, you should suspect a problem with the equipment. By excluding seasonal factors one by one, you are less likely to overlook the possibility of an equipment malfunction.

A system for recording and comparison is essential to quickly identify the causes of low power generation

To distinguish the seasonal causes of low power output, a system for daily recording and comparison is indispensable. A drop in power output can sometimes be identified from a single site inspection, but in many cases it is determined by comparing the data with on-site conditions. By recording when, in which season, in which section, at what time of day, and to what extent output declined, you can make faster judgments when a similar decline occurs again.

The items to record should not be limited to power output. Recording the weather, impressions of sunlight, temperature, rainfall, snowfall, wind strength, condition of vegetation, whether there is dirt, position of shadows, inspection photos, alarm history, dates of cleaning or weed removal, and the results of checks after typhoons or heavy rain, among other things, makes it easier to isolate causes. In particular, seasonal characteristics repeat every year, so being able to check what happened in the same period the previous year improves the accuracy of determining whether something is abnormal.

When making comparisons, it is important not to rely solely on the total amount of power generated but to compare like with like. Comparing sunny days with sunny days, the same season, the same section, and the same time of day reduces the influence of seasonal and weather differences. If a power plant has multiple sections, comparing sections with similar conditions makes it easier to identify whether only a specific location is underperforming. Abnormalities that are not apparent from the overall generation can often be detected by examining generation by section or by equipment.

Using site photos is also effective. On days with low power generation, record the panel surfaces, walkways, drains, slopes, fence lines, nearby trees, and areas around equipment so you can compare them with the data later. Because the position of shadows and the growth of vegetation change with the seasons, taking photos regularly from the same spots makes it easier to notice changes. This is especially important at large solar farms, where it is difficult to grasp everything by visual inspection alone, so keep records in a way that clearly shows locations.

The causes of low power generation vary by season: in spring, pollen and yellow dust; during the rainy season, insufficient sunlight and poor drainage; in summer, high temperatures and overgrown vegetation; after a typhoon, debris and changes to equipment; in autumn, lengthening shadows and falling leaves; and in winter, frost and snow accumulation. However, what is common across all seasons is that the power generation numbers alone cannot fully determine the cause. By combining data, on-site inspections, and historical records, you can separate natural seasonal variations from anomalies that need to be addressed.

When you feel that power generation is low, first check seasonal factors, and then, in order, inspect soiling, shading, vegetation, drainage, equipment condition, and alarm history. If you can identify the cause quickly, you can reduce unnecessary on-site dispatches and concentrate on the actions that are needed. Continuously monitoring the power plant’s condition and putting in place operations that do not overlook signs of decline will lead to long-term stability in power generation.

If you want to clearly link daily power generation data with on-site conditions and make it easier to manage seasonal changes and abnormal locations, you should establish a system that can centrally organize inspection records, generation data, on-site photos, location information, and response history. Finding the causes of low power generation quickly and streamlining the process from on-site verification to reporting will make it easier for operational staff to make decisions and take action.