Five items to check for snow accumulation, frost, and insufficient sunlight in winters with low power output

In winter, you will notice more days when solar power generation feels low. Compared with spring and summer, daylight hours are shorter and the sun’s elevation is lower, so even with the same equipment daily generation tends to decrease. However, if you attribute all winter generation declines to seasonal factors, you may overlook snow cover, frost, shading, soiling, or equipment malfunctions. Operations staff should, before concluding that it’s unavoidable because it’s winter, check site conditions and generation data separately.

Table of Contents

• Summarize the reasons why power generation is perceived to be lower in winter.

• Check for stoppage or partial reduction in power generation due to snow accumulation.

• Check for morning delays in power generation caused by frost or icing.

• Correlate insufficient sunlight and weather changes with power generation data

• Check for degradation caused by shadows and dirt specific to winter.

• Continuously monitor low power output and use the results to drive improvements.

• Summary

Organizing the reasons why power generation feels low in winter

One of the main reasons people feel power generation is low in winter is that solar irradiance conditions differ from those in spring and summer. Solar power generation produces electricity when sunlight reaches the panel surface. Therefore, in winter, when daylight hours are shorter and the sun’s altitude is lower, the total daily generation tends to remain low even on clear days. Rather than looking only at the result of low output, it is important to first distinguish between the natural declines that commonly occur in winter and the abnormal declines that should be addressed on site.

In winter, the start of power generation in the morning is delayed and the end of generation in the evening comes earlier. The daytime peak period is also shorter, and the generation curve can appear lower overall. Furthermore, because the sun’s altitude is lower, shadows from buildings, trees, mounting racks, fences, and surrounding equipment are more likely to reach the panels. Obstacles that had little effect in summer can cast long shadows in winter, causing reduced output during specific times of day.

Also, winter is a season when changes in weather are readily reflected in power generation. Overcast skies, rain, snow, fog, or widespread cloud cover associated with cold air reduce solar irradiance and lower power output. Even if there are days with low generation, if that day is cloudy or snowy the drop may be a natural occurrence. On the other hand, if output is clearly lower than that of nearby facilities of similar scale or than past days under the same conditions, you should check for snow accumulation, frost, shading, soiling, or equipment abnormalities.

In practice, we do not judge solely by power generation output; we verify by combining weather, solar irradiance, temperature, snow cover, site photographs, and records from monitoring equipment. For example, if it is sunny but generation struggles to ramp up only in the morning, frost or ice may be affecting performance. If generation hardly increases by midday, check for snow cover, soiling on the panel surface, or equipment shutdowns. If only specific rows or sections are low, partial snow cover, shading, or anomalies in the wiring system may be suspected.

When checking in winter, it's important not to assume that low power generation is caused by a single factor. There may be days when snowfall and overcast skies coincide, and days when, after frost melts, a lack of sunshine prevents generation from increasing. There are also compound causes—for example, mornings affected by shading and afternoons when clouds spread. The decline in winter generation is easier to sort out if you examine seasonal factors, meteorological factors, site factors, and equipment factors, in that order.

What on-site staff searching for "low power generation" particularly want to know is whether the decline in generation is within a natural range or whether it requires on-site intervention. To make that judgment, it is useful to look not only at daily generation totals but also at generation trends by time of day. In winter, because the day's generation hours are shorter, the daily total alone can feel low. However, if there is steady generation around noon on sunny days, it may fall within the range of seasonal decline. Conversely, if the peak is extremely low even on sunny days, the generation curve drops unnaturally during the day, or it is substantially lower compared with days with the same weather, you should prioritize on-site inspection.

When checking winter power generation, it's also important to have a baseline for normal conditions. If you don't know past winter generation data, the weather in nearby areas, the panel orientation and tilt, the installation environment, and how shadows fall around the site, it becomes difficult to accurately assess the extent of any decline. If generation drops the same way every winter, it may be a seasonal trend, but if it has fallen significantly only this year, you should check the condition of the panel surfaces and any changes to the equipment.

Check for power generation stoppage or partial reduction caused by snow accumulation

As a cause of reduced power generation in winter, the first thing to check is snow accumulation. When snow builds up on the surface of solar panels, sunlight has difficulty reaching the cells and power output drops significantly. If snowfall is heavy, generation can fall to almost nothing. Even a small amount of snow can reduce output if it remains on part of the panel surface, so at the site it is necessary to check not only the overall snow cover but also the location and extent of any remaining snow.

Power generation reductions caused by snow are relatively clearly reflected in generation data. On the day it snows and the following day, output may not increase even if it is sunny. If output remains low despite the weather improving, snow may still be left on the panel surface. In particular, if the generation curve does not rise even though it has been sunny since morning, or if it continues to stay low past noon, check for shading caused by snow.

The way snow sheds depends on the panels’ tilt and installation conditions. On tilted panels, solar radiation and rising temperatures can cause snow to slide off, but it does not necessarily fall uniformly across all panels. If snow remains near the eaves or around the frames, it can cast shadows on lower-row cells and reduce power generation. In systems installed at low heights, shed snow can accumulate nearby and again affect the working environment around the panels and mounting structures.

When checking for snow, it is important not to judge based only on overall photos. Even if the snow looks like it has melted from a distance, there may be residual snow at the lower edges of panels or along the boundaries between rows. Partial residual snow can appear as reduced power generation. Especially in installations with multiple circuits or sections, checking not only the total power generation but also differences in output by section and by system makes it easier to infer which areas still have snow.

However, snow removal must prioritize safety. Work on roofs, slopes, or frozen walkways carries the risk of slipping or falling. You should also avoid forcibly removing snow in ways that could damage panels or place stress on wiring or mounting structures. Do not immediately resort to manual snow removal simply because power output is low; instead, confirm on-site safety, equipment specifications, management rules, and the workers’ organization before making a decision.

In regions with frequent snowfall, establishing winter operating rules in advance makes responses smoother. Clearly defining at what level of snowfall a site inspection will be conducted, whether to wait for natural melting, and who decides to carry out work helps reduce reliance on individual judgment when power generation drops. It is also effective to create a workflow to check generation data and weather information before going to the site to determine whether a decline is likely due to snow.

Power output reductions due to snowfall can be unavoidable in the short term. However, recording how recovery proceeds after snowfall will provide useful information for decision-making next winter. For example, knowing how many hours or days it takes for output to return after snowfall, which sections tend to retain snow, and which rows receive poor sunlight will allow you to identify the cause of low-output days more quickly. Keeping photos and inspection notes helps improve the accuracy of winter management.

Check for morning power generation delays caused by frost or icing

If power generation is low in winter despite no snowfall, frost and icing are factors that should also be checked. On winter mornings, the surface of solar panels can accumulate frost due to overnight cooling. Even if the frost looks thin, it can block sunlight, delaying the start of generation and reducing output during the morning. Because it is less noticeable than snow, it is easily overlooked on site, so attention is needed.

Power generation decreases caused by frost are reflected especially in the morning generation curve. If, despite clear skies, generation does not ramp up for a while after sunrise, morning output is sluggish, or output suddenly recovers as the temperature rises, frost or freezing may be the cause. Because generation may return to near-normal by before noon, it can be difficult to identify the cause if you only look at the daily cumulative generation.

The way frost forms varies depending on the humidity, ventilation, terrain, and surrounding environment of the installation site. Locations near rivers, rice paddies, wetlands, or low-lying areas can become more humid from night through early morning, making frost more likely to occur. In mountainous or valley terrain, cold air tends to stagnate, so even within the same area only some equipment may be affected by frost. On rooftops, the temperature of roofing materials and radiative cooling can cause frost to remain on panel surfaces.

Morning site photos and surveillance camera images are useful for checking frost. During periods of reduced power output, you can confirm the relationship with generation data by checking whether panel surfaces are white, whether frost remains differently from row to row, and whether it is melting from sunlit areas. If there are consecutive days when output is low only in the morning and returns to near-normal by midday, it becomes easier to determine that you should also check for the effects of frost and sunlight conditions, not just equipment failure.

Freezing can also contribute to reduced power generation in winter. When moisture remains on the panel surface and temperatures drop, a thin film of ice can form. Such a film can be nearly transparent and may be difficult to notice from a distance, but it can impede light transmission or alter surface reflection and thus potentially affect power generation. In particular, if there was rain or sleet the previous day and it becomes very cold the next morning, check for freezing as well as frost.

Even when frost or icing is suspected, you should avoid forcibly rubbing the surface. Scratching the panel surface or putting stress on the frame and wiring can lead to long-term malfunctions. As a rule, it is often better to wait for the frost to clear naturally through solar radiation and rising temperatures. However, at sites where frost occurs frequently and power output is significantly reduced, it is important to comprehensively review the installation environment, drainage, local humidity, and shading conditions.

In practice, it is necessary to decide whether to treat generation drops caused by frost as anomalies or to record them as seasonal trends. If generation consistently falls only in the morning at the same time each year, it can be managed as a winter-specific trend. Conversely, if frost has a greater impact than the previous year, only certain strings recover slowly, or low output persists into midday, other factors such as soiling, shading, or connection faults should also be suspected. Because frost-related visual changes disappear quickly, it is important to check generation data in conjunction with the time of day.

Correlating insufficient sunlight and weather changes with power generation data

When power generation is low in winter, always check for insufficient sunlight. Because solar power generation is strongly affected by solar irradiance, output decreases on days with frequent clouds, rain, snow, or fog. In winter there are regions where clear skies persist and regions with many cloudy or snowy days, so seasonal differences between areas are large. Even if low output continues for several days, if the same period also shows continued lack of sunlight, the decrease is likely due to weather conditions rather than an equipment fault.

When checking for insufficient sunlight, it is important to view power generation and weather on the same time axis. Because looking only at daily weather leads to a coarse judgment, check the generation curve by time of day if possible. Solar irradiance conditions can change within the same day—for example, a day that was sunny in the morning but became cloudy from midday, or a day with snow clouds in the morning that was sunny only in the afternoon. If periods of low generation coincide with times of cloudiness or snowfall, it becomes easier to determine that the drop is due to insufficient sunlight.

What to watch for with winter sunlight deficiency is that the drop in power generation often appears gradual. Rather than suddenly falling to zero as with equipment shutdowns, it often manifests as an overall lack of output increase. Therefore, to avoid overlooking abnormalities, you need to compare the generation curves for sunny days and cloudy days separately. It's natural for output to be low on cloudy days, but if it's similarly low on sunny days, check for causes other than insufficient sunlight.

Also, because the sun’s altitude is lower in winter, even on clear days the solar irradiance conditions are not the same as in summer. Even if you feel the energy output is low, comparisons that do not take seasonal differences in solar radiation into account can lead to incorrect conclusions. For example, if you simply compare a clear winter day with a clear summer day, it is natural for the winter value to be lower. When making comparisons, it is important to match conditions such as the same month of the previous year, days with similar weather conditions, systems with the same orientation and tilt, or systems in the same area.

When checking for insufficient sunlight, it is useful not only to look at actual power generation but also to compare it with the expected generation. However, when using expected values, you must verify that they reflect on-site conditions. Expected values that ignore installation angle, orientation, surrounding shading, the presence or absence of snow, and equipment operating status can lead to incorrect assessments. In winter management, it is important not just to look at the difference between predicted and actual values, but to check the on-site factors on the days the discrepancies occurred.

When insufficient sunlight persists, operations personnel can become uncertain about how to respond to reduced power generation. Even if a site visit finds no equipment abnormalities, the inspection workload can still increase. Therefore, it is effective to first use the data to sort out possible meteorological causes and narrow down the days that require on-site confirmation. For example, treat cloudy or snowy days as continued monitoring, and prioritize focused checks on days that are low despite clear weather, days that are lower than surrounding areas, and days when only a specific section is low.

On the other hand, it is also risky to postpone everything just because of insufficient sunlight. In winter, power output can easily drop temporarily not only because of snowfall and frost but also when panel soiling, bird damage, fallen leaves, wiring faults, and equipment shutdowns overlap. During periods with many cloudy days, differences in power generation are hard to see and detection of abnormalities can be delayed. It is reassuring to establish a procedure to check whether power output recovers when sunny weather returns, and to move to an on-site inspection if it does not.

Check for declines caused by winter-specific shadows and dirt

One cause of reduced winter power generation that should be checked alongside snow and frost is shadows. In winter, because the sun’s altitude is lower, shadows extend farther. Shadows from buildings, trees, utility poles, fences, adjacent equipment, or mounting racks that didn’t reach the panels in summer can fall onto the generating surface in winter. Even if a shadow appears on only some panels, it can affect the output of the entire generation circuit, so it should not be overlooked.

Shading effects vary with the time of day. If the generation curve shows low output only in the morning, low only in the evening, and near-normal around noon, shadows caused by the sun’s altitude and azimuth may be to blame. In winter, shadow positions change significantly between morning and afternoon, so a single on-site inspection can miss the cause. It is important to inspect the site during the low-generation periods or to take photos at different times of day.

Pay attention to shadows from trees. Deciduous trees may have less impact because their leaves fall, but trunks and branches can still cast fine shadows on the panels. Near evergreen trees or bamboo groves, shadows tend to remain even in winter. Also, if surrounding vegetation grows, shadows that were not a problem in the previous winter can become a cause of reduced power generation this year. Be aware that shadow conditions are not fixed and can change with alterations in the surrounding environment.

Checking for dirt in winter is also important. Snowfall or rain can wash away grime, but during cold periods when surfaces dry slowly, splashed mud, bird droppings, fallen leaves, dust, and post-thaw residue can remain. When dirt accumulates at the bottom edge of panels, it can cause partial shading and reduce power output. While sliding snow can sometimes wash away dirt, it can also cause debris to gather near the frames, so inspecting conditions after snowfall is useful.

Fallen leaves and branches can also be factors in winter performance declines. From autumn to winter, leaves can accumulate on panels and around mounting racks, which can impede drainage and drying. When wet leaves adhere to the panel surface, they tend to remain until they dry and cause localized shading. It is advisable to check for remaining leaves and branches after strong winds, after rainfall, and before and after snowfall. Seasonal accumulations of debris can particularly affect power generation at plants located in forested or mountainous areas.

Checking only the overall power generation is insufficient to detect reductions caused by shadows or dirt. If possible, check generation differences by section, by circuit, and by row. Even if the overall output is only slightly low, a specific section may be showing a large drop. In that case, focus on that section during on-site inspection and check for shadows, dirt, remaining snow, frost, and the condition of connections. Using data to narrow down the area before performing on-site checks improves inspection efficiency.

Shadows and soiling shouldn't be treated as a one-time find; it's important to record locations that are prone to recurrence. If you note places where shadows appear at the same time each winter, rows where dirt tends to remain after the snow melts, and surrounding environments where fallen leaves accumulate, the next inspection will be quicker. Rather than starting from scratch every time generation is low, comparing the situation with past trends improves the accuracy of your assessments.

Continuously monitor low power generation and drive improvements

Ongoing monitoring is essential to properly assess decreases in power generation during winter. Rather than looking only at days with low generation, comparing with the days before and after, the same week, the same month of the previous year, and days with similar weather makes it easier to identify the causes of the decline. Especially in winter, when weather fluctuates greatly, judging an anomaly based only on a single day's generation can lead to unnecessary on-site responses. Conversely, if a decline continues and is overlooked, the loss of generation opportunities can be significant.

In continuous monitoring, it is important to look not only at the numerical values of power generation but also at the shape of the generation curve. When there is snow cover, power generation may hardly ramp up at all, or it may suddenly recover from the period when the snow melts. In the case of frost, the morning ramp-up tends to be slow and it generally recovers as temperatures rise. With insufficient sunlight, output is low throughout the day and may fluctuate with cloud movement. In the case of shading, there may be regular drops at specific times of day.

In this way, how generation data appears varies depending on the cause. Operations personnel form hypotheses from the data about which of snow accumulation, frost, insufficient sunlight, shading, soiling, or equipment malfunction best explains an instance of low power generation. They then confirm with site photos and inspection records as necessary. By operating in a manner that links data and the field, judgments about winter generation declines become more consistent.

In winter monitoring, the criteria for anomaly detection should also be reviewed to match the season. If you evaluate power generation using the same criteria as in summer, you are likely to judge the natural winter decline as an anomaly. Conversely, if you assume too strongly that lower output in winter is normal, you will overlook real anomalies. It is desirable to have criteria based on seasonal generation trends and to make judgments by combining recovery on sunny days, comparisons under the same conditions, and differences between sections.

How records are kept is also important. On days when power generation is low, record the date, weather, temperature, whether there was snowfall, whether there was frost, the results of on-site inspections, the actions taken, and the time until recovery. With these records, when a similar decline occurs in winters in subsequent years, it will be easier to determine whether it is a natural trend or an abnormality that requires countermeasures. Without records, every time personnel change you end up repeating the same checks, reducing operational efficiency.

Also, it is important not to let verification of decreased power generation be completed solely by on-site personnel. If managers, maintenance staff, and those who are familiar with the installation history can share information, isolating the cause will be faster. For example, if you share sections that previously had shading problems, circuits where wiring repairs were performed, and locations where snow tends to remain, it becomes clear which places should be prioritized for inspection on days with low power generation.

When addressing reduced power generation in winter, the appropriate countermeasures vary by site. If snowfall is the primary cause, establishing a safe inspection system and confirming recovery after natural snowmelt are important. If frost is the primary cause, grasp the seasonal tendency for delayed morning generation while checking locations where it persists for long periods. If shading is the primary cause, consider managing the surrounding environment and reviewing installation conditions. If soiling is the primary cause, determine the necessity and timing of cleaning. If insufficient sunlight is the primary cause, it is important to prepare comparative data to distinguish it from equipment faults.

Ultimately, the goal is to be able to explain why power generation is low. Simply feeling that it is low does not allow you to prioritize responses. If you can clarify whether it is temporarily low due to snow, low only in the morning due to frost, low overall because of insufficient sunlight, or low in particular sections because of shading or dirt, you will move closer to an operation that reduces unnecessary inspections while ensuring necessary measures are not missed.

Summary

When power generation is low in winter, it is important first to understand the natural seasonal decline and then check, in order, snow accumulation, frost, insufficient sunlight, shading, soiling, and equipment condition. Winter has shorter daylight hours and a lower sun angle, so generation tends to decrease; however, if you simply conclude that everything is unavoidable because it is winter, you may overlook causes of reduced generation that should be addressed on-site.

If panels are covered by snow, power generation drops significantly. Even after the snow melts, if snow remains at the lower edge or on some rows, localized decreases may persist. Frost or icing can delay the start of generation in the morning. Because recovery may occur by midday, it is important to look not only at daily cumulative values but also at generation curves by time of day. Insufficient sunlight is a major factor in reduced winter generation, but if low output continues even during clear weather, other causes should be checked.

Shading and soiling also have a greater impact in winter. Because the sun's elevation is lower, shadows from surrounding objects that were not a problem in summer can fall on the panels. Fallen leaves, mud splatter, and dirt remaining after snowmelt can also stay on the panel surface. Since these factors are difficult to detect from total power output alone, it is effective to verify them by combining per-section and time-of-day data with on-site photographs.

To address the drop in winter power generation, it is essential to manage by linking power generation data, weather, site conditions, and historical records. Rather than looking at a low-output day in isolation, comparing it with the days before and after, the same month of the previous year, and days with the same conditions makes it easier to distinguish between natural declines and abnormal drops. Furthermore, recording the cause of the decline, the verification results, and the steps to recovery will speed up decision-making in future cases.

When you feel that power generation is low in winter, before rushing to take on-site action it is important to check, in the order of snow accumulation, frost, insufficient sunlight, shading, and dirt, and compare these factors with the generation curve and on-site conditions. By continuously performing such checks, it becomes easier to explain the causes of decreased generation and to clarify the priorities for inspection and maintenance. To accumulate changes in generation as on-site data and improve the accuracy of winter management, it is essential to establish an operational system that allows continuous review of inspection records and generation status.