When checking a solar power generation simulation, one aspect many practitioners easily overlook is the generation decline in winter. Even if the annual generation looks sufficient, a month-by-month breakdown can reveal that the power balance becomes tight only in winter. This is especially important for self-consumption systems, corporate facilities, factories, warehouses, shops, agricultural facilities, and public facilities, where winter generation declines can greatly affect electricity bill savings, expected feed-in revenue, battery operation, and contract capacity reviews. This article explains, from a practical viewpoint, how to read winter generation declines from solar power generation simulations and how to link those readings to design decisions.

# 目次

• 冬の発電低下を読む前に押さえる基本

• 冬に発電量が下がる主な理由

• 月別シミュレーションで見るべき読み取りポイント

• 日射量と日照時間を冬季補正として確認する

• 影の影響を季節ごとに分けて読む

• 積雪・汚れ・低温による発電低下をどう扱うか

• 年間発電量だけで判断しないための実務視点

• 自家消費・売電・蓄電の冬季リスクを読む

• 現地条件を反映してシミュレーション精度を高める

• 冬の発電低下を前提にした設計判断とまとめ

# 冬の発電低下を読む前に押さえる基本

When reading winter generation declines in solar power generation simulations, it is important not to simply decide that “generation will always drop in winter.” Winter tends to be a season when generation drops because daylight hours shorten and the solar altitude is lower. On the other hand, lower temperatures can improve photovoltaic conversion efficiency. Therefore, winter generation is determined by a combination of solar irradiance, solar altitude, shading, snow, panel tilt angle, regional differences, system layout, and seasonal variations in power consumption.

In practice, if decisions are based only on the annual total generation, winter power shortages or unexpected purchased electricity may be overlooked. For example, even if annual generation is sufficient, facilities that use more heating and lighting in winter may see consumption increase at the same time generation falls, significantly changing self-consumption rates and electricity savings. In other words, winter generation decline is not merely a seasonal fluctuation but an important item that can determine post-installation power balances.

The purpose of using solar power generation simulations in practice is not to stare at generation numbers. They are used to decide how large the installed capacity should be, where on roofs or land to place panels, whether to avoid or accept shading, and how to combine battery storage and power contracts. Carefully reading winter figures helps avoid excessive expectations and makes it easier to plan operations that reflect reality.

When reading for winter specifically, it is necessary to check month-by-month generation, hourly generation trends, shading occurrence times, tilt angle, azimuth, snow assumptions, and the condition of nearby buildings and trees separately. Even if simulation results show a winter decline, if you do not understand the cause of that decline, you cannot devise improvements. Conversely, if you understand the causes, you can take concrete measures such as revising tilt angles, changing layouts, avoiding excessive capacity increases, or adjusting operational plans.

# 冬に発電量が下がる主な理由

The most basic reason solar generation declines in winter is that the time the sun is up becomes shorter. In summer, generation can occur from early morning to evening, but in winter sunrise is later and sunset is earlier. Thus, with the same system capacity, the total generation hours decrease. Because photovoltaic generation depends on solar irradiance, reduced generation hours translate directly into reduced monthly generation.

The next major factor is the lower solar altitude. In winter the sun’s path is lower and solar irradiance comes in at a shallow angle. If the panel tilt and azimuth match winter solar conditions, some of that irradiance can be captured; if roof slope or installation azimuth are not suitable, panels may not receive sufficient irradiance. Also, low solar altitude causes long shadows from surrounding buildings, trees, fences, equipment, and rooftop upstands. Obstacles that were not an issue in summer can cast significant shadows on generating surfaces in winter.

Shading is extremely important when reading winter generation declines. Even partial shading of modules can cause large drops in generation. The impact varies depending on whether shading occurs only in the morning, only in the afternoon, or only on specific rows. If a simulation shows a large monthly drop in winter, you need to check whether the decline is not only due to lack of irradiance but also due to the extended shadows caused by low solar altitude.

In snow regions, generation stoppage or reduction due to snow is also significant. When snow accumulates on the panel surface, irradiance cannot reach the panel and generation drops sharply. If snow falls off quickly, the impact is limited, but in conditions like shallow roof slope, surfaces that hamper sliding, low ambient temperatures, or continuous snowfall, generation can fall for several days at a time. If a simulation does not adequately consider snow losses, actual winter generation may fall below expectations.

On the other hand, lower temperatures in winter can favor module performance. Photovoltaic modules tend to suffer output loss at high temperatures and perform better at low temperatures. Thus, on clear winter days with sufficient irradiance and little shading or snow, instantaneous generation output can appear high. However, when viewed over a month, the shorter daylight hours, weather, shading, and snow impacts usually dominate, so overall winter generation tends to be lower. Therefore, winter generation decline should be read as the result of multiple overlapping factors, not a single cause.

# 月別シミュレーションで見るべき読み取りポイント

When confirming winter generation declines in a solar power generation simulation, the first thing to check is month-by-month generation. Annual totals do not reveal how much is generated in each month. To read winter declines, you need at least a monthly view of peaks and troughs to identify which month is lowest. Generally the decline appears in winter months, but depending on region and system conditions, the rainy season or typhoon season may be lower. Therefore, it is important not to look at winter with preconceptions but to position winter within the annual context.

When looking at month-by-month generation, merely finding the minimum month is insufficient. Evaluate how much lower winter generation is compared to the annual average, how it differs from summer and spring, and whether the winter drop is abrupt or gradual. If generation drops sharply only in winter, it may indicate strong effects from shading, snow, unfavorable azimuth, or nearby conditions, not just irradiance. Conversely, a relatively gentle winter decline may indicate that tilt and azimuth are suitable for winter irradiance, shading is minimal, and snow risk is low.

Also read month-by-month generation together with power consumption. For decision-making, it is important not only whether generation is high or low but whether it coincides with facility demand in time and season. Facilities that increase heating, lighting, ventilation, hot water, snow melting, or equipment operation in winter are more likely to see increased purchased electricity due to winter generation declines. Even if the simulated annual self-consumption rate looks high, if self-consumption is insufficient in winter the actual reduction effect may be much weaker than expected.

In monthly simulations, rather than judging “low in winter = bad,” consider “which uses will it be insufficient for?” For example, in a plan focused on selling electricity, winter declines can be evaluated as part of annual feed-in. In contrast, in self-consumption-centered plans, increased purchases in winter become an operational issue. If emergency power supply is included as an objective, carefully read how much power can be met by the minimum winter generation.

Month-by-month numbers can also be used for design comparisons. Comparing simulations under different conditions—changing panel azimuth, changing tilt, reducing rows to avoid shading, arranging for snow considerations—reveals how much winter decline improves. A design that stabilizes winter generation may be operationally preferable even if annual generation drops slightly. In practice, it is important to consider not only annual maximization but also lifting the winter baseline.

# 日射量と日照時間を冬季補正として確認する

To read winter generation declines correctly, confirm the assumptions for solar irradiance and sunshine hours used in the simulation. Solar generation is not determined solely by system capacity, module efficiency, and loss factors; it is heavily influenced by how much irradiance reaches the installation site. If irradiance data deviate from regional reality, simulation results can diverge from actual generation. Because regional differences are large in winter, checking irradiance assumptions is indispensable.

Even within the same country, regions with many clear winter days, regions prone to cloudy winter weather, areas with heavy snow, coastal areas with variable weather, and mountain areas with short sunshine hours exhibit very different generation trends. Judging winter generation solely by latitude can misrepresent reality. If mountains surround a site, the low winter solar altitude can further shorten generation time due to mountain shadows after sunrise or before sunset. Looking only at standard irradiance conditions for flat ground risks missing such topographical effects.

When reviewing simulation results, be aware of how winter irradiance is set month-by-month, whether long-term average data are used, or whether values reflect specific years’ weather. Solar generation is subject to natural variability, so actual generation varies year to year. If poor winter weather persists, a year may underperform the simulation; conversely, more sunny winters may exceed assumptions. Thus, winter generation decline should be read as a forecast with a range, not a single-year fixed number.

Checking sunshine hours includes verifying the length of time generation is possible. Winter days are short and peak generation times are compressed compared with summer. In corporate facilities where demand is high in the morning and evening, winter generation hours may not align with demand peaks. For example, facilities that see increased power use immediately after start of business or in the evening may not have their demand peaks sufficiently covered by winter solar alone. In such cases, it is safer to check not only monthly but, if possible, hourly generation tendencies in the simulation.

As a winter adjustment approach, do not blindly trust irradiance averages—also consider lower-case scenarios. Simulations should not be documents that make benefits look brighter; they should visualize risks for business and equipment decisions. By confirming whether electricity balances work even under weak winter irradiance scenarios and avoiding excessive expectations in investment decisions, you can reduce post-installation gaps.

# 影の影響を季節ごとに分けて読む

When reading winter generation declines, shading is the item to confirm most carefully. Locations that receive almost no shading in summer can be heavily shaded in winter because the solar altitude is lower and shadows lengthen. Therefore, a year-round quick shade check is insufficient. Check how shadows form around the winter solstice, on winter mornings, and on winter afternoons when the sun is low.

On rooftops, common shading causes include adjacent buildings, chimneys, outdoor HVAC units, water tanks, rooftop railings, penthouses, signs, antennas, lightning protection equipment, and upstand walls. For ground-mounted systems, surrounding trees, utility poles, fences, terracing, neighboring buildings, and mountain shadows affect performance. If winter simulation shows substantial generation declines, confirm that these obstacles are not blocking the low winter sun.

Shading affects not only generation declines but also electrical circuit design. Even the same shaded area can cause different losses depending on which panel strings or rows are shaded, at what times, and to what extent. Morning-only shading and midday shading have different impacts on generation. Because winter mid-morning to midday is a short but important window, shading in that window can significantly reduce monthly generation.

In practice, don’t judge shading by feel like “a little shading is fine.” Compare a simulation that includes winter shading conditions with an idealized simulation without shading. If the difference is large, winter declines are not just due to irradiance but indicate scope to improve layout. Rather than maximizing panel count, changing the layout to avoid some shading can stabilize effective winter generation.

Also, accuracy of on-site surveys is critical for shading checks. Even if drawings suggest no shading, in reality neighboring building heights, tree growth, ground elevation differences, or placement errors of rooftop equipment can create shading. In existing facilities, older drawings often don’t match current conditions. To read winter declines correctly, confirm that shading conditions in the simulation reflect real on-site conditions using field measurements and photo records as well as drawings.

# 積雪・汚れ・低温による発電低下をどう扱うか

Snow handling is a major branching point for winter generation declines. In regions with almost no snow, main winter decline factors are daylight hours, irradiance, solar altitude, and shading. But in snow regions, panels can be covered by snow for extended periods, causing large generation drops. If simulations do not consider snow loss, winter generation may be shown higher than reality.

Snow impacts depend not only on snowfall amounts but also on panel tilt, surface condition, roof slope, temperature, irradiance, wind, presence of snow guards, and clearance under panels. Adequate tilt may help snow slide off, but if falling snow could land on people, vehicles, or equipment, safety measures are necessary. Conversely, designs that make snow difficult to remove may prolong low-generation periods. Decisions should consider safety and maintenance, not only generation.

Soiling also affects winter generation. Snow, meltwater deposits, dust, fallen leaves, bird droppings, and frozen grime can remain on panel surfaces in winter. Some regions see natural washing by rain, but in dry periods or on shallow slopes soiling can persist and reduce generation. If a simulation assumes standard loss rates, it may not reflect local soiling conditions or ease of cleaning.

Low temperatures should be understood as an element that can boost generation performance rather than cause decline. Since modules lose output at high temperatures, cold clear days can be advantageous for efficiency. However, the benefit of low temperature does not completely offset insufficient irradiance, short daylight hours, shading, or snow. When reading winter generation, consider not just “colder = better efficiency” but also whether there is enough time and irradiance to generate.

In simulations, check to what extent conditions such as snow, soiling, and low temperature are input. Even if not explicitly listed, they might be included in loss factors or regional coefficients; if not considered at all, result reliability changes. Practically reading winter declines requires not only looking at magnitudes but confirming the assumptions behind those numbers.

# 年間発電量だけで判断しないための実務視点

Annual generation is the most immediately visible metric in solar power generation simulations. While annual generation is the base for investment decisions, it is not enough to read winter generation declines. Systems with the same annual generation can differ in whether they generate steadily in spring and autumn, spike in summer, or have a small winter dip, and these differences affect practical usability.

What matters in practice is not total generation but whether it matches demand. For example, a system that generates much in summer and little in winter can be highly effective for a facility with large cooling demand in summer. Conversely, for facilities with large heating demand in winter, significant winter drops may limit purchase-reduction effects. Even if annual generation appears sufficient, if winter demand is not met by generation, operational measures must be considered.

Also, designing solely to maximize annual generation may not be optimal. Adding more panels may increase annual generation, but forcing installation into areas that are shaded in winter can increase the number of underperforming panels. If installed capacity rises but effective winter generation does not, this can be disadvantageous in cost-effectiveness and maintenance. By reading winter declines, you can shift perspective from “install as many as possible” to “choose locations that generate efficiently.”

To avoid judging by annual generation alone, check the minimum monthly generation, total winter generation, winter self-consumption rate, winter reduction in purchased electricity, and winter surplus energy. These reveal how much power can be met in winter and how much will be lacking. In corporate projects, winter usage patterns vary greatly by industry, so read simulations together with actual facility operation.

Winter declines also affect stakeholder communication. Emphasizing only annual generation in pre-installation proposals can lead to distrust when stakeholders find winter generation lower than expected. Sharing month-by-month variability, reasons for winter declines, and the expected range beforehand reduces recognition gaps after commissioning. Simulations are both persuasive materials and tools to set realistic expectations.

# 自家消費・売電・蓄電の冬季リスクを読む

Winter generation declines are especially important for self-consumption-type solar systems. These systems aim to reduce purchased electricity by using generated power on-site. But because winter generation tends to drop while heating and lighting demands increase, the balance between generation and demand can break down. Simulations must check to what extent winter generation meets demand.

When reviewing self-consumption rates, confirm winter values as well as annual averages. Even if annual self-consumption is high, if purchases rise in winter and surpluses are large in summer, perceived electricity savings will vary by season. Facilities with little daytime operation or many holidays may still have mismatches despite winter generation. Checking the time-of-day overlap between generation and demand gives a more realistic evaluation.

In plans that include feed-in, winter declines appear as reduced feed-in volumes. For feed-in-focused plans, annual total feed-in is important, but revenue forecasts must be based on the premise of lower winter generation. Underestimating winter drops in monthly feed-in projections can skew financial and cash flow planning. Even if feed-in is not the main objective, checking how much surplus appears in winter helps set operational expectations.

When combining battery storage, winter declines become even more important. Batteries store excess generation for later use, but if winter surplus is scarce, days when batteries cannot fully charge increase. When evaluating battery effects in simulations, check not only annual charge/discharge but whether winter charging shortfalls occur. If batteries are also intended as emergency power, carefully assess how much power can be secured during extended winter bad weather.

A key point in reading winter risks is not to try to solve shortages solely by adding capacity. Even increased capacity has limited benefit if installed in heavily shaded or snow-prone locations. Combine options such as operational demand adjustments, reviewing battery capacity, prioritizing placement in high-generating spots, and confirming power contracts. While some winter declines are unavoidable, reading them correctly allows you to adopt measures during the planning stage.

# 現地条件を反映してシミュレーション精度を高める

To read winter declines correctly, it is important how well site conditions are reflected in the simulation. Simulation results depend heavily on input conditions. If the installation azimuth, tilt, surrounding obstacles, topography, roof geometry, building heights, snow conditions, or irradiance conditions are inaccurate, results will be inaccurate. In winter, when the solar altitude is low, small differences in heights or positions affect shading, so the precision of site information directly affects results.

For rooftop installations, confirm the exact orientation and slope of roof planes, steps, equipment locations, parapet heights, and relationships to neighboring buildings. Even with drawings, verify consistency with current conditions. Renovations, extensions, or equipment updates can introduce obstacles not on older drawings. To correctly read winter shading, combine on-site measurements and photos with drawings.

For ground-mounted systems, also check land contours, surrounding trees, adjacent boundaries, planned earthworks, drainage, and how snow accumulates. Because winter sun comes in at a low angle, slight ground elevation differences or surrounding structures can function as shadows. Row spacing is important in ground installations: if rows are too close, front-row shadow can fall on back rows in winter, reducing generation. Maximizing installation area and avoiding winter shading may not align, so compare with simulations.

When reflecting site conditions, precision in positioning and dimensions matters. If roof edges, obstacle positions, equipment heights, site boundaries, or panel placement reference points are ambiguous, shading reproduction accuracy falls. Especially when evaluating winter declines, position errors from tens of centimeters (tens of inches) to several meters (several ft) can affect shading assessments. Accurately capture site positional information and reflect it in simulations and layout drawings to make generation estimates closer to reality.

To improve simulation accuracy, don’t stop at a single calculation—compare conditions. Compare standard conditions, conservative shading scenarios, conditions with expected snow losses, different tilt angles, and alternative layouts to identify which factors influence winter declines. Seeing differences in results clarifies where to take measures. Rather than adopting a single number, making decisions with a range is a safer practical approach.

# 冬の発電低下を前提にした設計判断とまとめ

Winter generation decline is an unavoidable seasonal variation in solar power, but unavoidable does not mean there are no countermeasures. By carefully reading solar power generation simulations, you can determine how much generation will drop in winter and whether the causes are irradiance, shading, snow, or layout. Once causes are clear, you can separate what can be improved by design or operation from what must be accepted as a premise.

In design decisions, first check the minimum winter generation and whether that value is acceptable for the facility’s purpose. If self-consumption is a priority, confirm how much winter demand can be covered. If feed-in is the priority, include winter feed-in declines in annual finances. If combining batteries, verify whether sufficient winter charging is possible. If expecting value as emergency power, carefully evaluate how usable the system will be during poor winter weather.

Next, perform design comparisons to suppress winter declines: panel azimuth and tilt, layout, row spacing, excluding locations prone to shading, installation methods for snowy regions, and maintenance access should be examined. The important point is not to try to maximize annual generation at all costs. Rather than forcing panels into poor winter-performing spots, prioritizing locations that generate well and are easy to maintain may lead to more stable long-term operation.

When explaining to stakeholders, do not hide winter declines—present simulation results clearly to build trust. Many complaints that “it doesn’t generate as much in winter as expected” stem from insufficient sharing of monthly variations beforehand. If you organize the reasons for winter declines, the expected magnitude of decline, what countermeasures will be taken, and what will be accepted as natural conditions, you can reduce post-commissioning gaps.

Solar power generation simulation is not merely a prediction exercise; it is a decision-making tool linking installation planning, power use, finances, maintenance, and site surveys. Being able to read winter generation declines correctly helps avoid overestimation and makes realistic equipment planning easier. Especially in winter, shading and snow effects are large, and accurate site positioning and obstacle recognition determine result precision.

Establishing a workflow to accurately record roof shapes, equipment positions, site boundaries, and surrounding obstacles in site surveys and to reflect those in generation simulations is the fastest way to reduce winter misreadings. A useful tool for this is LRTK (iPhone-mounted GNSS high-precision positioning device). If high-precision positional information obtained on site makes it easier to grasp the relationships among roofs, sites, and obstacles, you can consider winter shading and layout conditions more realistically. Incorporating LRTK-based positioning and records to link simulations to site-based decisions is an effective option for practitioners.