6 Precautions When Calculating Solar Power Generation in Snowy Regions

When calculating solar power generation in snowy regions, estimating annual generation based only on typical solar irradiance and system capacity can result in large differences from actual generation. In areas where it snows, multiple factors interact: the period during which panel surfaces are covered by snow, the angle of roofs and mounting structures, shading from nearby snowdrifts, equipment characteristics under low temperatures, and operational conditions for snow removal and inspections. Rather than simply thinking that "winter produces less," it is important to separate and organize which factors affect generation during which periods.

This article is aimed at practitioners who search for "solar power generation calculation" and explains six points to check when estimating or revising power generation in snowy regions. It is organized around on-site perspectives that are easy to get confused about, so it is convenient for rough estimates before installation, verifying the actual performance of existing equipment, adjusting monthly forecasts, and organizing calculation conditions for internal explanations.

Table of Contents

• In snowy regions, the assumptions for power generation calculations should be considered separately from those for typical regions

• Note 1: Estimate the monthly shading periods caused by snow

• Note 2: Check panel angle and the likelihood of snow shedding separately

• Note 3: Consider both reflected light and shading after snowfall

• Note 4: Verify power generation characteristics at low temperatures and equipment limitations

• Note 5: Include snow removal, inspections, and safety management in the calculation conditions

• Note 6: Continuously review winter corrections using measured data

• In snowy regions, the accuracy of defining and organizing conditions determines the results of power generation calculations

In snowy regions, treat the assumptions for power generation calculations separately from those for non-snowy regions

When calculating photovoltaic (solar) power generation, it is common to estimate output by combining the installed capacity, solar irradiance, installation orientation, tilt angle, system losses, inverter conversion efficiency, wiring losses, and temperature effects. In areas with many clear days and little shading from snow, using monthly irradiance data and the system parameters alone can make it easy to grasp general generation trends. However, in snowy regions, even with the same installed capacity the winter generation can vary greatly, so applying the same calculation formula as for non-snowy areas to produce an annual value can result in an overestimate.

Particular attention should be paid to the fact that the impact of snow is not simply a "reduction in solar irradiance." Not only do overcast skies or falling snow reduce the irradiance arriving from the sky, but snow accumulating on the panel surface also makes it harder for light to reach the cells. If an entire panel is covered in snow, there will be periods when, even if sunlight is present, power cannot be generated. Also, even when only part of a panel remains snow-covered, depending on the configuration of the power generation circuit and the way bypass diodes operate, output reductions can occur that are difficult to explain by area ratio alone.

In other words, when calculating solar power generation in snowy regions, it is necessary to separate the meteorological solar irradiance conditions from whether the equipment surface is in a state capable of generating power.

Also, even among snowy regions, conditions are not uniform. Some areas receive heavy snowfall, some have low temperatures where snow melts slowly, some have wet snow that easily adheres to panels, and some are windy and prone to snowdrifts, so the reasons for reduced power generation differ from site to site. Snow quality and the way solar radiation is distributed also vary between mountainous areas, plains, and coastal areas. When calculating power generation, it is important not to lump places together by regional name alone, but to check the monthly trends at the specific site.

In pre-installation estimates, rather than emphasizing only the annual generation figure, it is necessary to clarify monthly variations. In snowy regions, even if generation is relatively stable from spring through autumn, there can be a significant drop during several winter months. Differences that are averaged out and hard to notice when looking only at the annual value become easier to see when broken down by month, revealing the impact of snow. In practice, before confirming annual payback or the effects of self-consumption, it is important to be able to explain under which assumptions the decline in winter generation was incorporated.

Furthermore, in snowy regions, equipment design and operational policies are directly linked to power generation calculations. Whether the installation is roof-mounted or ground-mounted, how much tilt angle can be achieved, whether there are safety issues where the snow will fall, and whether snow will be cleared manually or left to shed naturally — all of these factors can change expected power generation even under the same weather conditions. If you try to handle this only with calculation coefficients, you risk producing figures that diverge from the actual on-site operational realities.

Therefore, when calculating power generation in snowy regions, it is easier to organize the process if you first decide how much accuracy is required. For preliminary estimates before installation, applying a conservative winter adjustment is effective. For verification of existing installations, you need to combine meter-reading data, remote monitoring data, daily weather records, and post-snowfall operating status to make corrections that reflect actual performance. Clarifying the purpose of the calculation also makes it easier to distinguish between items that require detailed checking and those that can be handled approximately.

Note 1: Estimate the monthly periods of obstruction due to snowfall

The biggest concern in snowy regions is how to account for periods when the panel surface is covered by snow. Basic generation calculations use the approach of multiplying solar irradiance by system capacity and loss factors to estimate output, but when panels are covered with snow they may not generate power even if sunlight is present, or output may be greatly reduced. Ignoring this effect will lead to overestimating winter generation compared with reality.

The shading period cannot be judged simply by the number of days with snowfall. Not only the days it snowed, but also the number of days that snow remains on the panels after snowfall are important. In regions with low temperatures, snow can remain even when snowfall has stopped and it is sunny, delaying the resumption of power generation. Conversely, in regions where temperatures rise more easily or where the installation has a steep tilt, snow may fall off relatively quickly. In other words, when calculating power generation you need to consider not the "number of days it snowed" but the "time the panel surface was effectively exposed."

In practice, it is convenient to set shading rates on a monthly basis. For example, the approach might be to reduce the number of days available for generation only in specific winter months, or to apply a snowfall correction to monthly generation. However, do not fix the correction rate uniformly; set it based on the region’s snowfall tendencies, installation angle, past performance, and snow-removal policy. In the initial stage, it is realistic to assume conservative values and update the corrections once measured data have been collected.

What we want to avoid here is simply deducting a fixed percentage from the annual power generation. Subtracting a lump sum from the annual figure is easy, but it can fail to properly represent declines in winter generation. Because the effects of snowfall are concentrated in winter, reflecting them within monthly calculations makes it easier to assess actual cash flow, on-site consumption, surplus electricity, and the relationship with storage systems. For facilities whose electricity demand increases in winter, estimating how much generation to expect in winter is a particularly important factor in decision making.

Also, the condition in which snow remains on part of a panel should not be overlooked. It is often regarded as less severe than the panel being completely covered in snow, but even partial shading can cause a noticeable drop in power output. The way snow remains varies by site: it may stay on the lower edge, on part of a row, clump near the frame, or accumulate in drifts at roof steps and around edges. Even in calculations that do not handle detailed electrical circuitry, it is prudent to assume that "when partial snow remains, power generation may not recover in proportion to the clear area."

Historical photos and inspection records are also useful for confirming the duration of shading. Looking only at power generation data can make it difficult to determine whether a decrease is caused by snow, cloudy weather, or equipment outages. If you keep photos of the panels’ condition after snowfall, it becomes easier to link generation declines to residual snow and analyze them. In new projects where photos are not available, be careful not to apply overly optimistic corrections, and use nearby similar installations, roof shapes, and local snowfall conditions as references.

Expectations about snow shading affect not only projected power generation but also explanations of equipment operation. If you do not share in advance that generation will drop in winter, after installation people are likely to form the impression that it is "producing less than expected." In calculation sheets and internal documents, organize the winter decline in generation as a regional characteristic rather than an anomaly, and clearly state under which conditions the figures were calculated; doing so will make later explanations smoother.

Precaution 2: Check the panel angle and the ease of snow shedding separately

In calculating photovoltaic power generation in snowy regions, the installation angle of the panels is important. The tilt angle affects not only the efficiency of receiving solar radiation but also how easily snow sheds. Typical generation calculations consider angles that are favorable for the area's solar conditions, but in snowy regions it is also necessary to take into account reducing residual snow in winter. If the angle is determined solely by generation efficiency, snow is more likely to remain, and as a result actual winter power generation can decrease.

However, increasing the tilt does not necessarily provide an advantage. A steep slope can make snow more likely to slide off, but there are trade-offs with roof shape, racking structure, wind effects, the safety of areas where snow may fall, and installation conditions. When installing on a roof, the existing building pitch often dictates the tilt, so you may not be able to freely choose the ideal angle. For ground-mounted systems, increasing the angle also requires consideration of inter-row shading, wind loads, and maintenance space. Even if calculations suggest that energy production will improve, it is important to confirm that this does not conflict with the actual installation conditions.

How easily snow falls off is affected not only by the tilt angle but also by the condition of the panel surface, the shape of the frame, the temperature, and the type of snow. Wet snow tends to stick to panels and, at low temperatures, can freeze and remain. If snow catches on the lower edge of a panel or on steps, the upper part may become exposed while snow remains below, prolonging reductions in power generation. In power generation calculations, you should not simply conclude "snow will fall because there is a slope"; you need to assume conditions that make residual snow likely.

Also, the installation angle affects seasonal solar irradiance capture. In winter, the sun's altitude is lower, so the amount of solar radiation received changes with the angle. In snowy regions, rather than omitting angle considerations because winter generation is low, it is important to calculate separately the potential winter generation when there is no snow and the reduction in output caused by snow. After the snow has fallen off and the panel surface is exposed, an angle suited to the low solar altitude may contribute to generation.

On the other hand, increasing the tilt angle can also affect summer power generation. Whether you want to maximize annual generation, secure as much winter generation as possible, or which season has higher demand for self-consumption will change the way you approach the appropriate tilt. If a factory or facility has high electricity demand in winter, calculations need to emphasize winter supply capacity as well as the annual total. Conversely, some approaches assume significant snow impacts in winter and evaluate generation primarily from spring through autumn.

In practice, it is useful not only to treat the installation angle as an input value but also to leave a note on how that angle affects snow retention. For example, even for the same power generation estimate, if there are conditions such as “assume natural shedding during snowfall,” “expect reduced output for several days after snowfall due to remaining snow,” and “prioritize safety where falling snow will land and do not change the angle,” the meaning of the calculation results becomes clearer. If you share only the generation figures, the safety and operational decisions embedded in the angle are not visible, so it is important to write the assumptions carefully in explanatory materials.

Note 3: Consider both reflected light and shadows after snowfall

In snowy regions, snow not only has a negative impact on power generation but, under certain conditions, reflected light can also contribute to generation. This is because the ground and surrounding snow surfaces can reflect sunlight and deliver additional solar irradiance to the panel surface. Ground-mounted installations and higher racking in particular are more likely to receive reflections from snow surfaces. However, if this effect is overestimated, power generation calculations can become overly optimistic, so it should be handled carefully and treated separately from losses due to shading.

Snow-surface reflection assumes the panel surface is exposed. If the panel itself is covered by snow, surrounding reflected light will not contribute sufficiently to power generation. Therefore, when accounting for reflected light in calculations for snowy regions, first confirm that residual snow on the panel surface has been cleared. Power generation may increase on sunny days after the snow has cleared, but if the panels remain covered immediately after snowfall, shading losses will outweigh the effect of reflected light.

Also, the effect of reflected light varies depending on installation height and the surrounding environment. Installations close to the ground, placements where building shadows are likely to intrude, or locations with nearby walls or trees may not receive sufficient reflection from the snow surface. For roof installations, reflections from snow on the roof or from surrounding buildings can also have an impact, but it is not always appropriate to assume the same level as for ground installations. In calculations, it is safer to treat the reflection-induced increase as a supplementary factor rather than as a standard value.

On the other hand, the impact of shadows caused by snow is an easily overlooked point. When accumulated snow remains at the lower edge of panels or around the front rows, shadows tend to extend farther in winter when the sun’s altitude is low. For ground-mounted installations, if a mound of snow forms in front of a row, shadows can fall on the lower parts of the panels even on sunny days. On roof-mounted installations, roof ridges, snow guards, nearby equipment, or piled-up fallen snow can also create shadows. Even on days when solar irradiance data indicate power can be generated, on-site shadows can reduce output.

When considering the impact of shading, it is important to be mindful of the sun’s altitude in winter. Low obstacles that are not a problem in summer can cast longer shadows in winter and reach the panels. In areas where snow changes the height of the ground surface, snowdrifts can form in locations that normally would not cast shadows, which can affect power generation. During the calculation stage, checking not only surrounding obstacles but also where cleared snow will be piled and where fallen snow will accumulate makes it easier to realistically estimate winter power generation.

Reflected light and shading are both influenced by site conditions, so they are aspects that are difficult to judge by desk calculations alone. For existing installations, checking winter generation data on sunny days makes it easier to grasp the trends. Comparing sunny days with no snow, sunny days with snow-covered ground, and sunny days with residual snow on the panels reveals upward deviations in reflection and downward deviations from shading. For new installations, it is important to set conditions in a way that avoids overestimation while taking into account local meteorological tendencies and the site layout.

In calculating power generation in snowy regions, rather than simplifying to "power generation will always decrease because there is snow," it's easier to explain if you separate the effects into: "snow covering panels causes loss," "reflection from surrounding snow surfaces can act beneficially depending on conditions," and "shading from snowdrifts or residual snow can become additional losses." When this separation is made, it is also easier to judge which factors to review when checking the validity of the calculation results.

Precaution 4: Check power generation characteristics and equipment limitations at low temperatures

In snowy regions, not only snow but also low temperatures affect power generation calculations. Because solar panels tend to lose output at high temperatures, low temperatures can, under certain conditions, be advantageous for output. On clear winter days, if the air temperature is low and the panel surfaces are exposed, relatively good generation can be obtained. However, emphasizing only the benefits of low temperatures can lead to overlooking snow shading and shorter sunshine hours. Winter power generation is determined by the combined effects of temperature-related output characteristics, solar irradiance, solar altitude, and snow shading, so it needs to be considered comprehensively.

When calculating power generation, the temperature coefficient is sometimes taken into account. This is a way of estimating how much the output will change when the panel temperature deviates from the reference conditions. In summer, a drop in output due to high temperatures is anticipated, and in winter an increase in output at low temperatures may be considered. However, in practical rough estimates, the downtime when generation is impossible because of snow can have a greater impact than any upside from temperature. In snowy regions, even if lower temperatures may improve generation efficiency, the period of lingering snow must not be ignored.

Also, it is important to check voltage conditions at low temperatures. When ambient temperature is low, the open-circuit voltage of solar panels tends to increase. In equipment design, it is necessary to confirm, taking expected low-temperature conditions into account, that the input allowable range of devices such as power conditioners will not be exceeded. This is more a verification of equipment conditions for safe and proper operation than a calculation of power generation itself, but it is not unrelated to personnel handling estimates of generation output. If equipment is limited or shut down, the expected power generation cannot be obtained.

In snowy regions, the environment around power conditioners, junction boxes, wiring, and racking also affects winter operations. Low temperatures, freezing, snow accumulation, meltwater, and condensation can influence inspections and operating conditions. Even if these factors are not reflected in detail in power generation calculations, if the environment increases the risk of shutdowns in winter, they should be documented as maintenance conditions. In particular, for equipment with a history of winter shutdowns, it is important to treat equipment downtime separately rather than simply applying a weather correction.

To separate the effects of low temperatures and snow, it is effective to look at power generation trends on sunny days. If generation increases on a sunny winter day when the panel surface is exposed, favorable conditions due to low temperatures may be at work. Conversely, if generation is low even on sunny days, you should suspect other factors such as residual snow, shading, equipment limitations, soiling, or shutdowns. Because it is difficult to judge from monthly generation alone, if possible check daily or hourly data to make it easier to break down the causes.

In calculation documents, it is important not to casually assume a large upside from low temperatures. Actual winter power generation is constrained by shorter sunshine hours, a tendency toward unstable weather, and shading from snow. Although low temperatures can be advantageous for panel performance, that alone does not necessarily offset the decline in winter generation. When explaining, organizing the message along the lines of "low temperatures provide some favorable aspects in panel characteristics, but prioritize evaluation based on snow shading and solar irradiance conditions" helps avoid excessive expectations.

Note 5: Include snow removal, inspection, and safety management in the calculation conditions

In calculating power generation in snowy regions, it is important to include in the calculation conditions not only the performance of the equipment itself but also the operational policies for snow removal and inspections. Whether you wait for accumulated snow to fall off naturally or perform snow removal as needed changes the time until generation can resume. Assuming snow removal may allow for higher estimated winter generation, but that assumption does not hold unless there is actually a system in place to carry out the work safely.

Snow removal work on rooftop equipment carries inherent hazards. You must take into account slips and falls, falling snow, damage to roofing materials or panels, risk of electric shock, and effects on nearby passersby. It is inappropriate to assume unsafe snow removal solely to increase power generation. Even if calculations indicate increased output, without proper safety management and work procedures it may not be feasible in actual operation. Therefore, when basing calculations on the assumption that snow will be removed, you must verify the work scope, the criteria for deciding to perform work, measures to ensure worker safety, and methods for protecting the equipment.

Even for ground-mounted installations, snow removal is not necessarily simple. When snow accumulates in front of the panels it creates shading, so you may need to clear snow not only from the panel surface but also from the space in front. If you do not plan in advance for access routes, access around equipment, the movement paths of snow-clearing machines and manual crews, and where to pile the snow, winter inspections and recovery may be delayed. In power-output calculations, the assumption is not simply that snow on the panel surface will be removed, but whether the entire site can be kept in a condition capable of generating power.

Snow-removal policy can also be reflected in adjustments to monthly power generation. If operation relies only on natural snow shedding, you need to assume longer periods of stoppage or reduced output after snowfall. If operations include periodic snow removal within a safe range, you may be able to expect shorter snow-cover periods. However, because snow removal cannot always be carried out immediately, you must also take into account weekends, nighttime, bad weather, and the availability of workers. In practice, assuming an operationally realistic snow-removal approach that can actually be sustained, rather than an idealized one, will reduce the gap between calculated results and actual performance.

Inspection planning also affects power generation. Due to snowfall and freezing, inspection frequency may decrease in winter. Even if equipment malfunctions, if on-site verification is delayed, downtime may be prolonged. Even with a system that can detect generation drops remotely, if the issue cannot be repaired on site, recovery of generation will be delayed. In power generation calculations, even if detailed equipment failure rates are not included, at sites with winter inspection constraints it is advisable to document this as an operational risk in explanatory materials.

There is also a risk of damaging the panels during snow removal. If the choice of tools or work methods is inappropriate, it can lead to surface scratches, frame deformation, or added stress on wiring components. As a result, this may cause reduced power generation or other malfunctions. Work carried out to increase power output must be decided carefully so that it does not impair long-term equipment performance. If snow removal is included as a calculation condition, confirm that the procedure is also reasonable from the standpoint of protecting the equipment.

In snowy regions, it is important not to separate power generation calculations from safety management. Looking only at power output, more frequent snow removal may seem advantageous. However, when work risks, staffing, equipment protection, and falling-snow countermeasures are taken into account, it can be more realistic to base operations on natural snow shedding and only address snow where necessary. In calculation reports, rather than using only simple labels such as "with snow removal" and "without snow removal", leaving a written description of the conditions under which action will be taken will be helpful when reviewing them later.

Note 6 Continuously review winter corrections using measured data

In calculating solar power generation in snowy regions, it is important not to rely solely on pre-installation forecasts but to review winter adjustments using actual measured data. The impact of snow varies greatly by region and by site, making it difficult to estimate accurately with initial calculations alone. By accumulating actual power generation, snowfall days, remaining-snow conditions, whether snow removal was performed, and outage history, you can improve forecasting accuracy for subsequent years.

The basic principle of review is to cross-check monthly generation with weather and snowfall conditions. If winter generation is low, distinguish whether the cause is snow coverage, insufficient solar irradiance, equipment shutdown, or shading. Because monthly totals can mix causes, check daily data if possible. If generation drops sharply immediately after snowfall and then recovers slowly even in sunny weather, lingering snow is suspected. If the panels are exposed on sunny days but output is low, also check for shading or equipment-related causes.

Even if you only have meter-reading data, a review is possible. Compare the monthly power generation with the same month in the previous year or with months under similar conditions to check whether it drops significantly only during winter. Also, keeping notes on months with heavy snowfall, months when low temperatures caused snow to remain, or months when snow removal was delayed can be useful for later analysis. Even without detailed hourly data, combining site records with generation data can improve the adjustment parameters.

For existing installations, it is important not to fix the winter adjustment as a single constant. If the first winter had little snow, basing the adjustment only on that year's results may lead to overestimating power generation in subsequent heavy-snow years. Conversely, if the first year experienced heavy snowfall, using that overly strict adjustment unchanged can cause an underestimation of generation in normal years. If possible, review performance over multiple years and categorize conditions into near-average, severe, and favorable to broaden the range of explanations.

When reviewing power generation calculations, it is important to record not only the difference between actual and forecast but also the reasons that caused the difference. Simply lowering the correction factor because output was lower than forecast will not lead to improvement. If snow shading is the main cause, there is room to review panel angle, snow removal, where accumulated snow falls, and the locations of snowdrifts. If equipment outages are the main cause, it is necessary to review inspection protocols and alarm-checking procedures. If insufficient solar irradiance is the main cause, it may need to be treated as regional interannual variability. Separating the causes leads not only to improved calculation accuracy but also to operational improvements.

When handling measured data, pay attention to how units and time periods are aligned. Confirm whether the generation figures are AC-side or DC-side values, whether the meter-reading period matches calendar months, whether downtime is included, and whether there were system expansions or equipment replacements; otherwise comparisons can be skewed. In snowy regions, a difference of a few days in winter can affect monthly generation, so differences in meter-reading dates may also be non-negligible. Operational personnel should prepare not only the calculation formulas but also the conditions of the source data.

Also, it is effective to record the results of winter correction reviews in internal documents and management ledgers. When personnel change, it can become unclear why a particular correction rate was used. If you briefly record the basis for the correction—covered period, snowfall conditions, generation actuals, site photos, snow-clearing measures, and outage history—it will be easier to review the following year and apply the approach to other sites. Calculating power generation in snowy regions is not something you get exactly right on the first try; treating it as a process of improving accuracy using actual results will bring the figures closer to those that fit your operations.

In snowy regions, the precision of organizing conditions determines the results of power generation calculations

In snowy regions, when calculating photovoltaic generation you need to consider not only the usual system capacity and insolation but also factors such as obscuration by snow, duration of residual snow cover, panel tilt, reflected light, shading, low‑temperature characteristics, snow‑removal operations, inspection regimes, and reassessment of measured data. In particular, during winter, even if there is solar irradiance, there will be periods when the panel surface cannot generate power if it is covered by snow. Conversely, when the snow falls off and the panel surface is exposed, under clear skies and low temperatures generation can be relatively higher. It is important not to handle these upward and downward factors crudely with a single coefficient, but to organize them by month and by site.

In practical power-generation calculations, it is more important to establish explainable assumptions than to pursue excessively detailed theoretical values. Clarifying why winter generation was estimated conservatively, whether snow removal is assumed or natural snow shedding is assumed, to what extent snow-surface reflection is taken into account, and whether equipment stoppages and inspection constraints are included will increase confidence in the calculation results. Rather than presenting numbers alone, recording the conditions and their rationale together makes the material more useful for deployment decisions and operational improvements.

Also, in snowy regions it is essential to link pre-installation estimates with post-installation performance verification. The initial calculations are only forecasts that include assumptions. After operation begins, by accumulating meter-reading data, daily power generation, snowfall conditions, photos of remaining snow on the panels, and records of snow removal measures, you can improve the accuracy of winter adjustments. Because generation does not occur under the same conditions every year, it is realistic to take a broad view that also accounts for differences between low-snow years and heavy-snow years.

To reduce failures in calculating solar power generation in snowy regions, you need an approach that identifies under which conditions generation is possible and under which conditions generation is likely to stop, rather than treating winter generation declines as merely a negative factor. Expect shading periods on a month-by-month basis, check angle and snow-shedding performance separately, consider reflected light and shadows simultaneously, confirm equipment conditions at low temperatures, assume safe snow-removal operations, and update corrections with measured data. By addressing these six points, you can minimize the gap between desk calculations and on-site reality.

In the future, when introducing solar power installations or reassessing power generation in snowy regions, it will be important to record calculation conditions and establish a mechanism to continuously improve them by cross-checking with on-site data. The greater the winter uncertainty in a region, the more useful it is to visualize power generation and to have a system that can compare planned values with actual results. Rather than ending with generation calculations alone, managing measured values, meteorological conditions, snow cover status, and snow-clearing and inspection records together makes it easier to carry out power generation evaluations in snowy regions that reflect on-site realities.