6 Things to Keep in Mind When Calculating Seasonal Solar Power Generation

When calculating solar power generation, relying only on the annual average can cause you to overlook actual seasonal variations. Solar generation may seem to increase simply with higher solar irradiance, but in practice output is affected by a combination of factors such as temperature, hours of sunshine, shading, snow, soiling, equipment condition, and measurement conditions. Especially when estimating generation by month or season, it is important to separate and verify the calculation assumptions for spring, summer, autumn, and winter even for the same system capacity.

This article outlines six key points that practitioners searching for information on "solar power generation calculation" should keep in mind when calculating seasonal power output. To make them useful for generation simulations, performance comparisons, anomaly detection, maintenance inspections, and report preparation, we provide practical explanations of the on-site aspects that should be checked.

Table of Contents

• Consider solar radiation and sunshine hours separately for each season.

• In summer, we look at not only solar irradiance but also the reduction in output caused by high temperatures.

• In winter, confirm the effects of shorter daylight hours, snowfall, and low sun angle.

• Power generation tends to be more stable in spring and autumn, but don't overlook dirt and shadows.

• Ensure consistency in the comparison conditions between monthly actual results and simulations.

• In anomaly detection, distinguish between seasonal factors and equipment-related factors.

• Summary

Consider solar radiation and sunshine duration separately by season

One of the first things to be careful about when calculating seasonal solar power generation is not to treat solar irradiance and sunshine duration as the same thing. Power generation is affected by the intensity of the solar irradiance reaching the panels and the duration over which that condition persists. Even in seasons with long hours of sunshine, if clouds are frequent and direct sunlight is weak, output will be limited. Conversely, even when sunshine duration isn't very long, clear air, moderate temperatures, and favorable solar conditions can produce higher-than-expected generation.

In practice, when calculating electricity generation, it is safer to avoid the method of simply dividing the annual generation by 12 months. Monthly generation varies depending on the sun’s altitude, the length of daylight from sunrise to sunset, the climate of each region, and the orientation and tilt of the installation surface. For example, even with the same installed capacity, some regions tend to see generation increase from spring through early summer, while in other regions the rainy season and typhoons make it difficult for summer output to rise. Therefore, when calculating seasonally, the basic approach is to use monthly or seasonal solar radiation conditions that reflect regional differences.

In estimating solar power generation, a common approach is to calculate it using installed capacity, solar irradiance, and a loss factor. The important point here is that if the loss factor is kept constant over the year, seasonal characteristics can become difficult to see. In reality, output tends to drop in summer due to high temperatures, while in winter systems are more susceptible to shading effects from snow and the lower solar altitude. Spring and autumn are sometimes considered relatively favorable periods, but pollen, yellow sand (Asian dust), fallen leaves, and dust from nearby construction can increase soiling on panel surfaces. In other words, rather than calculating using only solar irradiance, it is necessary to also consider the losses that are likely to occur in each season.

Also, when comparing generation by month, check the number of calendar days. Because February has fewer days than other months, monthly generation can appear lower if you look only at the monthly total. For monthly evaluations, in addition to total generation, check daily generation, generation per unit of installed capacity, and generation curves on clear-sky days, which makes it easier to determine whether differences are due to seasonal factors or to problems on the equipment side.

A common mistake in seasonal calculations is deciding values based on broad impressions such as “summer will always be the maximum because sunlight is stronger” or “it doesn’t generate power in winter because it’s cold.” While solar radiation is stronger in summer, increases in panel temperature can reduce output. In winter, daylight hours are shorter and the sun’s altitude is lower, but low temperatures can sometimes be advantageous for output. Because seasonal generation is not determined by a single factor, it is important when calculating to separate and organize solar irradiance, hours of sunshine, temperature, shading, soiling, snow cover, and operating conditions.

In summer, consider not only solar irradiance but also output decline due to high temperatures

When calculating solar power generation in summer, avoid focusing only on the fact that solar irradiance is high. Summer has longer days and many periods of stronger sunlight, so it intuitively appears to be the season with the highest annual power generation. However, in reality an increase in panel temperature can reduce generation efficiency, so greater irradiance does not necessarily translate directly into higher power output.

Solar panels are rated for output under certain test conditions, but in the field the temperature rises on roof surfaces and mounting racks. Especially on sunny midsummer days, the panel surface temperature can become higher than the ambient air temperature. Because panels exhibit a characteristic of reduced output as temperature rises, temperature losses must be considered in summer generation calculations. If this is overlooked, simulations may predict high generation while actual results consistently fall short.

In summer calculations, note that the impact of temperature rise tends to be greater during periods of high solar irradiance. When you look at the generation curve around noon, there are cases where, despite sufficient solar radiation, the expected peak does not reach the anticipated level. In such cases, rather than simply concluding a fault or soiling, it is important to check for causes such as output reduction due to high temperature, output curtailment, the operating condition of power conversion equipment, and the ventilation conditions of the mounting surface. Installations close to the roof or in poorly ventilated locations can make it difficult for panel temperatures to drop, causing differences in power generation even within the same area.

Furthermore, in summer the weather variability caused by the rainy season, typhoons, sudden downpours, and high humidity cannot be ignored. Even if the monthly average solar radiation appears to show little difference, in reality sunny and cloudy days are often mixed, which can lead to large day-to-day variability in power generation. When calculating generation, in addition to the monthly average for the entire month, checking for extended periods of cloudy weather, sudden rainfall, shutdowns of generation due to approaching typhoons, and downtime for inspections will make it easier to explain discrepancies with actual performance.

When summer power generation is lower than expected, multiple factors may be contributing on site. In addition to output reduction due to high temperatures, there may be shading from vegetation growth, accumulation of bird droppings and dust, soiling after heavy rain, temperature rises in equipment enclosures, and data loss due to communication failures. When comparing calculated and actual values, do not simply conclude “it’s low because it’s summer”; instead, systematically isolate and rule out season-specific stressors one by one.

In practice, when calculating summer power generation, it is prudent to assume that the month with the highest solar irradiance may not coincide with the month of maximum power output. How much high-temperature loss to allow for in calculations depends on equipment specifications and installation conditions, but it is important not to rely solely on a single uniform annual coefficient. When preparing monthly generation figures, organize the summer temperature effects as a separate item and keep them so they can be reconciled with actual results later, which also makes reporting and improvement proposals easier.

In winter, check the effects of shorter daylight hours, snowfall, and low sun angles

When calculating winter solar power generation, always take into account the shorter daylight hours and the lower solar altitude. In winter the days are shorter, so the period during which generation is possible is reduced. In addition, because the sun's position is lower, shadows from surrounding buildings, trees, fences, equipment, and rooftop protrusions tend to stretch longer. Shadows that were not a problem in summer can fall on the PV surface in winter, so calculating based only on annual average shading conditions may differ from actual performance.

One thing to be especially careful about in winter calculations is that even short-duration shadows can affect power generation. Because a photovoltaic array is made up of multiple panels, even partial shading can reduce output depending on the extent of the shading and the electrical configuration. Even if calculations use only monthly insolation, on-site factors such as shadows caused by the low solar angles in the morning and evening, shadows from adjacent buildings, and how snow remains after snowfall can influence results. If you want an accurate estimate of winter generation, it is effective to check how shading occurs by time of day rather than simply inputting insolation.

In snowy regions, the impact of snow is also a major concern. When snow accumulates on solar panels, it blocks sunlight and causes a large drop in power generation. Generation may not recover until the snow is completely gone, and the reduction in output can be as short as a few hours or persist for several days. In addition, if snow remains on part of the panel surface, it acts as a partial shade and can make output unstable. In winter calculations, it is important to consider the number of snowfall days, installation angles that tend to retain snow, conditions for snow falling onto surrounding areas, and safety constraints, and not to estimate power generation based solely on a simple clear-sky assumption.

On the other hand, because temperatures are low in winter, looking only at the temperature characteristics of the panels can sometimes be favorable for output. On sunny days without snow and with little shading, there can be short periods of high output. Therefore, explaining low winter power generation simply as “because it’s cold” is not accurate. The main reasons that generation tends to decrease in winter are shorter sunlight hours, lower solar altitude, shading, snow cover, and poor weather. Because cold itself does not necessarily reduce generation, care is needed in wording when preparing explanatory materials and reports.

In winter, be careful about the evaluation period for power generation. When looking at daily generation, the difference between sunny days and snowy days tends to be large, and judging the performance of the equipment based on a short period can lead to misunderstandings. If you pick out only a week that happened to have consecutive cloudy or snowy days, the equipment may appear to be underperforming. Conversely, if you look only at a period of consecutive clear skies, the results can look better than expected even in winter. For seasonal calculations, it is desirable to combine assessments on a monthly basis, a seasonal basis, year‑over‑year for the same month, and the meteorological trends of nearby areas.

In calculating winter power generation, the conditions under which inspections and maintenance can be carried out must also be realistically anticipated. In locations with snow accumulation or icing, it may be impossible to approach roofs or mounting structures. Forcing inspections or snow removal when safety cannot be ensured should be avoided. As a result, there may be a gap between the calculated timing of power recovery and the time when a response is actually possible. In practical management of winter power generation, it is important not only to consider the power output figures but also to distinguish between areas that can be safely inspected, areas that can be assessed by remote monitoring, and areas that require on-site verification.

Spring and autumn tend to have more stable power generation, but don’t overlook dirt and shadows

Spring and autumn are sometimes considered seasons in which the conditions for calculating solar power generation are relatively favorable. Because they are less likely to get as hot as summer and their daylight hours are not as short as in winter, they tend to be periods when generation is more stable. In fact, depending on the region and installation conditions, spring and autumn can produce some of the highest generation levels of the year. However, precisely because these seasons tend to be stable, overlooking dirt, shading, or changes in the surrounding environment can make the differences between calculated and actual values difficult to explain.

In spring power generation calculations, attention is paid to soiling on panel surfaces caused by pollen, yellow sand, sand dust, and nearby soil dust. Rain may wash them away naturally, but depending on the type of soiling and how it adheres, some may remain. Especially on low-tilt installation surfaces, rainwater tends not to run off easily, and dirt can accumulate at the lower edges and corners of the panels. In generation calculations this is often treated as part of the loss factor, but during seasons when soiling tends to increase, losses can be larger than usual.

In autumn, pay attention to fallen leaves, post-typhoon soiling, shadows from branches and leaves, and changes in sunlight conditions. Trees that grew during the summer may cast shadows in autumn, and fallen leaves can accumulate in drainage channels or around panels. After typhoons or strong winds, dust, salt-containing debris, and fine litter may adhere. When power generation is lower than the calculated value, it is important not to suspect equipment faults alone but to check for seasonal soiling and changes in the surrounding environment.

In calculations for spring and autumn, because temperature conditions are relatively favorable, output loss due to temperature is generally not expected to be as large as in summer. However, there are temperature differences between morning and evening and changes in weather. Since power generation can vary greatly between periods of consecutive sunny days and periods of consecutive rain or cloudiness, the whole season should not be evaluated based only on short-term performance. In practice, it is common to forecast monthly generation based on actuals from the beginning to the middle of the month, but because results can change depending on the weather in the latter half, it is important to provide a range for the predicted values.

Spring and autumn are also periods that are easy to use as a baseline when setting conditions for calculating power generation. In regions with little extreme heat, snowfall, or prolonged low solar radiation, the power-generation curve on clear days tends to appear relatively clean, making these seasons well suited for checking equipment condition. By checking whether the power-generation curve forms a smooth, mountain-shaped peak, whether there are drops only during specific time periods, or whether there are differences compared with days of similar weather, you can more easily spot shading, dirt, or partial equipment malfunctions.

However, even when treating spring and autumn as "standard seasons," you must not ignore regional differences. In coastal areas, mountainous areas, urban areas, around farmland, and around factories, the effects of airborne debris, humidity, fog, dust, salt, and trees differ. If you use power generation calculations in practical work, you need to reflect the environment of the location where the equipment is installed rather than applying a uniform nationwide approach. In notes on calculation conditions, recording local weather trends, the surrounding environment, panel orientation, tilt, and the history of cleaning and inspections will make it easier to explain any discrepancies with actual performance later.

Align comparison conditions between monthly actuals and simulations

When calculating seasonal solar power generation, it is essential to align the comparison conditions between simulated values and actual results. Even when calculated and actual values appear to be discrepant, there are cases where the only differences are the comparison period, units, scope, or measurement points. In practice, multiple similar figures exist, such as total plant generation, generation by power conversion equipment, the energy sent to the grid, the energy used within the facility, and the values displayed by remote monitoring. If it is not made clear which figure is to be treated as the generation, season-by-season evaluations will become unstable.

The first thing to check is the period being compared. When comparing monthly power generation, simulations are often produced for calendar months, while actual data may be cut by meter-reading periods or arbitrary aggregation periods. For example, if a value treated as April actually covers the period from late March to late April, large discrepancies can occur at seasonal transitions. When evaluating monthly generation, the basic rule is to align the start and end dates.

Next, it is necessary to standardize the units. Generated electricity is usually treated as electric energy, but confusing it with instantaneous output leads to incorrect judgments. Even if the output at a given moment is high, if cloudy periods are long the daily generation will be low. Conversely, even if peak output is not very high, if it generates steadily for long periods the daily generation will increase. In seasonal calculations, treat instantaneous values, daily totals, monthly totals, and generation per unit of installed capacity separately, and make the comparison targets clear.

Also, whether you evaluate per unit of installed capacity or by total output is important. When comparing multiple installations, differences in installed capacity will lead to differences in total power generation. Looking only at totals makes larger installations appear better, but when viewed as generation per unit of installed capacity, differences in installation conditions and operating status become clearer. When comparing multiple sites by season, you need to convert them to the same metric before making a judgment.

When comparing with simulations, also verify the assumptions used in the calculations. If installation tilt, orientation, system capacity, loss rates, shading conditions, operation start date, shutdown times, how output control is handled, etc. differ from reality, there will be a mismatch in seasonal power output. In particular, if panels were later added, equipment replaced, wiring changed, nearby buildings newly constructed, or trees grew, using the simulation from the time of installation as-is will no longer match the current situation. Calculation conditions are not something you set up once and forget; it is important to review them in line with changes to the equipment and surrounding environment.

Care should also be taken with the performance data. If there were periods when remote-monitoring communications were interrupted, clock drift in measuring instruments, missing data, transcription errors from manual entry, or changes to aggregation settings, the power generation may appear lower than it actually was or be recorded on a different day. Before evaluating seasonal power generation, verify that the data itself has been correctly acquired. In particular, it is reassuring to check for any unnatural step changes in aggregated values at month-ends and beginnings, after power restoration following an outage, after replacement of communication equipment, and after setting changes.

Keeping comparison conditions consistent is important not only when investigating the causes of low power generation but also when verifying the effects of improvements. Even if it appears that power generation increased after cleaning, inspection, or equipment replacement, it may simply be that the weather improved. Conversely, if generation does not seem to increase after improvements, it might be because cloudy conditions persisted. To correctly calculate and evaluate seasonal power generation, it is essential to compare under the same conditions.

Separate seasonal factors and equipment factors in anomaly detection

One purpose of calculating seasonal solar power generation is to determine whether a decline in output is due to natural seasonal variation or an equipment fault. In practice, when you receive a report of low output, you should not immediately conclude there is a failure; first, you need to check seasonal factors. If conditions such as low solar irradiance, prolonged cloudy weather, snow accumulation, high temperatures, lengthening shadows, or dirt accumulation were present, the decrease in output can often be at least partly explained.

However, it is dangerous to explain too much by seasonal factors alone. It is true that power generation tends to decline in winter and during the rainy season, but if output is significantly lower compared with systems under the same conditions, if only specific circuits are not generating, or if the generation curve looks unnatural even on sunny days, you need to suspect a problem on the equipment side. Seasonal calculated values should be used not to rule out anomalies, but as a benchmark to determine how much variation is natural.

In anomaly detection, we first examine the relationship between weather and power generation. We separate clear, cloudy, and rainy days and compare the shapes of the generation curves. We check whether a clear day shows a bell-shaped curve, whether there are sudden drops mid-day, whether output is low only in the morning, only in the afternoon, or all day. Because the sun’s movement differs by season, summer and winter will not produce the same curve, but if there are unnatural gaps or steps, we consider possibilities such as shading, stoppage, equipment faults, or communication failures.

Next, perform a comparison within the same installation. When there are multiple systems or sections, if only some show lower power generation under the same weather conditions, it is difficult to explain this by seasonal factors alone. If output is low overall, weather, output control, or grid-side conditions may be involved; if only specific parts are low, check for soiling, shading, poor connections, equipment outages, differences in settings, and so on. Seasonal calculations provide an overall guideline, but for anomaly detection, comparisons by section are important.

Comparing with the same month of the previous year is also useful. If it is the same month, conditions such as solar altitude and sunshine hours are relatively similar. However, because the weather can differ between last year and this year, a value lower than the previous year alone does not necessarily indicate an anomaly. When examining year-on-year monthly comparisons, check the number of sunny days, number of rainy days, snowfall, typhoons, maintenance shutdowns, and whether any equipment changes occurred. By comparing while adjusting for seasonal factors, it becomes easier to detect signs of equipment degradation or malfunctions.

What you should avoid in anomaly detection is explaining the difference between calculated and actual values with only a single reason. When power generation is low, insufficient solar irradiance, high temperature, soiling, shading, downtime, measurement issues, output control, and equipment faults can overlap. Operations personnel should first clean and organize the data, check for seasonal factors, and then dig into equipment-related causes for any remaining unexplained differences; creating this flow improves the accuracy of judgments.

Calculating seasonal power generation is not merely a forecasting task; it is also about creating standards for operational management. If you understand what level of generation is typical in spring, how much it may drop in summer due to high temperatures, and how much it may decrease in winter because of shading and snowfall, you can respond calmly to inquiries and reports from the field. Conversely, if you look only at generation figures without seasonal standards, you may mistake normal variations for anomalies or attribute real anomalies to the season and overlook them.

Summary

When calculating seasonal solar power generation, it is important not to rely only on the annual average but to check seasonal solar irradiance, sunshine hours, temperature, shading, soiling, snow, and the condition of the equipment separately. Because solar power generation is strongly affected by natural conditions, even the same equipment will produce different amounts of energy depending on the month and season. In summer, although solar irradiance is high, output can drop due to high temperatures; in winter, generation is affected by short sunshine hours, low solar elevation, snow cover, and shading. Spring and autumn tend to be relatively stable, but it is important not to overlook soiling and shading caused by pollen, yellow sand (Asian dust), fallen leaves, and changes in the surrounding environment.

Also, when comparing calculated and actual power generation values, it is necessary to align the period, units, measurement points, system capacity, and simulation conditions. If the comparison conditions are misaligned, generation may appear lower than it actually is, or conversely an actual anomaly may be overlooked. In practice, it is especially essential to verify that remote monitoring data, on-site displays, monthly summaries, and figures in reports are consistent, and to clarify the assumptions behind the data.

When you suspect that power generation is low, it is effective to first identify what can be explained by seasonal factors and then treat any remaining discrepancy as an equipment-related issue. By combining the generation curve for clear days, a comparison with the same month of the previous year, system-by-system comparisons within the installation, and checks for downtime or missing data, it becomes easier to determine whether the cause is simply weather-related or an abnormality that requires inspection.

To apply seasonal power generation calculations in practice, it is more important to carefully align the assumptions than to focus on the calculation formula itself. By looking not only at solar irradiance figures but also at installation conditions, the surrounding environment, seasonal losses, and the status of actual data collection together, you can improve the accuracy of generation forecasts, anomaly detection, and improvement proposals. To efficiently carry out calculations of solar power generation and verification of field data, it is also important to establish management standards that take seasonal variations into account and to regularly review calculation conditions and actual data.