5 Methods to Calculate and Compare Solar Power Generation Between Summer and Winter

When calculating solar power generation, it's easy to judge a system's merits solely by its annual kWh. Of course the annual figure is convenient for comparing system size, but in practice that's not enough. In particular, if you don't grasp how much generation changes between summer and winter, prospects for self-consumption, the appearance of surplus, and the operational image of the system will become quite vague.

Summer has longer hours of sunlight and stronger solar radiation, so it subjectively appears to be the season that generates the most power. On the other hand, high temperatures can cause output to drop, so production may not increase as much as expected. In winter, shorter sunlight hours and a lower solar altitude tend to reduce generation, but low temperatures can be advantageous for equipment efficiency. In other words, the difference between summer and winter is not simply “more in summer, less in winter”; you need to consider solar radiation, temperature, shading, snowfall, and even the timing of demand.

For practitioners who search for "solar power generation calculation," what matters is how to translate these seasonal differences into numbers. In this article, we organize five approaches for comparing summer and winter and clearly explain which method should be used at each stage. We convert differences that are not apparent from annual input values into forms that can be used in practice.

Table of Contents

• Why comparing summer and winter is necessary

• Method 1: Compare summer and winter by monthly power generation

• Method 2: Compare summer and winter by daily power generation

• Method 3: Compare solar radiation and sunshine hours separately

• Method 4: Compare with corrections for temperature, snow cover, and shading

• Method 5: Compare including self-consumption and time-of-use

• Common calculation mistakes in summer–winter comparisons

• How practitioners can improve the accuracy of comparisons

• Summary

Why a Summer–Winter Comparison Is Necessary

The reason for comparing solar power generation between summer and winter is that it reveals how a system is actually used in ways that annual figures alone cannot. For example, a figure of a system that produces 10,000 kWh per year does not tell you how much it generates in summer or how much it falls off in winter. In practice, however, that difference is very important. In facilities with high air-conditioning demand, the amount generated in summer is directly tied to the self-consumption rate, and in facilities where power demand is high in winter, the winter decline directly affects system evaluation.

Also, the amount of electricity generated is not determined solely by the equipment capacity in kW. kW denotes the size of the installation, while the actual electrical energy produced is expressed in kWh. Comparing summer and winter means looking at how solar irradiance, sun angle, temperature, shading, and, in some cases, snow change for the same equipment capacity, and how those changes are reflected in kWh. In other words, comparing seasonal differences is also the process of converting equipment capacity into the on-site realities.

Also, looking at the summer–winter differences makes it easier to judge the appropriateness of the system configuration. For example, even if a south-facing installation appears to have a higher annual total, a facility with large afternoon demand might find that including west-facing surfaces more practical. Conversely, if many surfaces that are heavily shaded in winter are being used, that installation’s contribution in winter may be weak relative to its annual value. In other words, seasonal comparisons help evaluate a system’s value not only by total output but also by its composition.

In practice, when comparing summer and winter, treating the annual average as if it can simply be split evenly in half is likely to lead to errors. In summer, solar irradiation hours are longer but there are high-temperature losses; in winter, lower temperatures increase efficiency but the solar incidence angle and duration become more severe, so multiple conditions change at the same time. For that reason, it is better to put things into calculable form and compare them rather than rely on a simple impression.

Comparing summer and winter directly affects the overall evaluation of the equipment. Because it pertains not only to power generation but also to self-consumption, surplus, operation, and the ease of explanation, it is an extremely important perspective for operational staff.

Method 1: Compare summer and winter using monthly electricity generation

The clearest method is to compare summer and winter by monthly power generation. Rather than looking at the annual kWh as a whole, extracting a representative summer month and a representative winter month and comparing them makes the seasonal differences of the system much easier to visualize. For example, decide on representative months according to the region and usage—such as August and January, or July and December—and compare the power generation for those months.

The basic calculation is: Monthly generation (kWh) = system capacity (kW) × average equivalent generation hours for the month (h) × number of days in the month × correction factor. For example, with a 10 kW system, if the representative summer month has average equivalent generation hours of 4.0 h, number of days in the month 30 days, and a correction factor of 0.82, the monthly generation is 10 × 4.0 × 30 × 0.82 = 984 kWh. On the other hand, if the representative winter month has average equivalent generation hours of 2.5 h, number of days in the month 31 days, and a correction factor of 0.80, then 10 × 2.5 × 31 × 0.80 = 620 kWh. This alone shows that there can be a significant difference between summer and winter.

The advantage of this method is that it most directly and clearly shows seasonal variations. Differences that were buried in the annual average become quite apparent simply by breaking the data down to the monthly level. Moreover, because it is easy to overlay monthly usage for facilities or households, it also facilitates evaluation of self-consumption and surplus. Practical points—such as both generation and demand being high in summer, while generation falls but demand is high in winter—can be tied directly to the numbers.

Also, having a comparison of monthly power generation makes explanations during proposals easier. Rather than just saying 10,000 kWh per year, showing that summer months are around 1,000 kWh and winter months around 600 kWh conveys a much better sense of the system. This monthly comparison is especially effective for facilities where electricity usage varies by season.

However, it is important not to treat the representative months for summer and winter too rigidly. Because the rainy season, snowfall, cloud cover, and temperature conditions differ by region, the way summer–winter differences appear is not uniform. Therefore, monthly comparisons are very effective, but being mindful of regional characteristics and site conditions as a premise will further improve accuracy.

Method 2: Compare summer and winter by daily power generation

The second method is to compare summer and winter by power generation per day. While monthly generation figures are easy to understand, they can be somewhat misleading because the number of days differs. Converting to daily generation makes differences in equipment performance more intuitively visible. Another strength of this method is that it is easier to relate to everyday loads and operations.

As a way of thinking, daily generation (kWh) = installed capacity (kW) × average equivalent generation hours (h) × correction factor. For example, with a 5 kW system, if the summer average equivalent generation hours is 4.0 h and the correction factor is 0.82, the daily generation is 5 × 4.0 × 0.82 = 16.4 kWh. If the winter average equivalent generation hours is 2.5 h and the correction factor is 0.80, 5 × 2.5 × 0.80 = 10 kWh. Rather than looking only at the annual total, it becomes easier to understand as day-to-day differences.

This method is effective because it makes comparison with demand easy. For example, for a facility that uses about 15 kWh during the daytime, a summer daily generation of 16.4 kWh will contribute considerably to self-consumption, whereas 10 kWh in winter may not be enough. In other words, even with the same system size, how usable it is by season becomes quite apparent on a daily basis.

It also becomes easier to get a sense of the time-of-day distribution. Daily generation directly ties to how much surplus or shortfall will occur during the day, so it is meaningful both for projects based on self-consumption and for those based on selling electricity. For example, even if daily generation in summer is high, its value changes depending on whether most of it is concentrated around noon or distributed from morning through afternoon, so starting with a comparison of daily totals makes it easier to move on to time-of-day–specific evaluations.

However, when comparing daily electricity generation, it is important not to confuse the maximum values on sunny days with an average day. For practical use, it is more stable to view things under the assumption of a typical summer day and a typical winter day. Thinking on a sunny-day basis makes the system’s capacity easy to understand, but it is less useful for actual year‑round operation. It is clearest to use it only as the average of representative seasonal days.

Method 3: Compare solar radiation and sunshine duration separately

The third method is to compare solar irradiance and sunshine duration separately. When considering the differences between summer and winter, many people first focus on the length of sunshine. Indeed, days are longer in summer and shorter in winter. However, differences in power generation are not determined solely by the length of sunshine. In fact, the intensity of solar irradiance, solar altitude, temperature, and the occurrence of cloudy skies all act together. Therefore, judging seasonal differences based only on sunshine duration can easily lead to errors.

For example, in summer the days are longer and solar irradiance is stronger, but because temperatures are higher the system efficiency tends to decrease. In winter the sunshine duration is shorter and the sun’s elevation is lower, but lower temperatures can be advantageous for efficiency. In other words, you cannot simply say that summer is better because the days are longer and winter is worse because the days are shorter. To compare power generation correctly, you need to consider sunshine duration and solar irradiance as separate factors, and then apply corrections for temperature and shading.

In practice, irradiance data are sometimes treated directly as generated energy, but this is dangerous. Irradiance is merely the amount of solar energy received, and it is only converted into kWh through installed capacity and loss conditions. Therefore, when comparing summer and winter, it is clearer not only to see which has the more favorable irradiance conditions, but also to consider how much those irradiance conditions translate into equivalent generation hours.

Also, with this way of thinking, it becomes easier to understand structures such as: in summer, even though solar radiation is strong, high temperatures cause a slight drop; in winter, although sunlight hours are short and shadows tend to lengthen, low temperatures help a little. In other words, the difference in power generation between summer and winter is not due to a single cause but is the result of multiple factors combined. Once you understand this, explaining the estimates becomes considerably easier.

This method is especially useful when you want to explain differences in annual or monthly values with reasons. It lets you avoid ending with explanations like "more in summer because summer is longer" or "less in winter because winter is shorter" for the value of equipment, and instead explain why those differences occur. For practitioners, it is an important way to understand what lies behind the numbers.

Method 4: Compare by applying corrections for temperature, snow cover, and shadows

The fourth method is to compare summer and winter by applying corrections for temperature, snow cover, and shading. If you want the seasonal difference to be closer to real-world practice, you should include these corrections. This is because in summer output losses due to high temperatures tend to be more pronounced, while in winter the effects of snow cover and shading from low solar elevation tend to be stronger. Differences in insolation or sunshine duration alone cannot adequately represent these site-specific variations.

For example, in summer solar radiation is stronger and daylight hours are longer, so theoretically it appears to be the easiest season for power generation. However, because panel temperatures rise and efficiency falls, output may not increase as much as one might intuitively expect. Conversely, in winter, low temperatures tend to improve efficiency, but daylight hours are shorter, the sun’s altitude is lower, shadows tend to be longer, and in snowy regions the receiving surface can be covered. In other words, to accurately assess the seasonal difference between summer and winter, it is better to treat summer temperature losses separately from winter shadow and snow losses.

In this method, we first determine baseline generation values for summer and winter, and then apply seasonal corrections. The idea is to include a modest high-temperature correction for summer and to apply stronger shading and snow corrections for winter. For example, because solar irradiance is stronger in summer it is slightly reduced by the high-temperature correction, whereas in winter the irradiance itself is weaker and is further reduced by shading and snow. This approach makes the monthly and daily differences much closer to actual site conditions.

Also, this method makes it easy to represent regional differences. In areas with heavy snowfall, you need to give stronger weight to winter adjustments, while in relatively warm regions the weighting of high‑temperature losses can become larger. In other words, even when comparing summer and winter, where you place the emphasis changes depending on regional conditions. Understanding this makes it far more practical in the field than a uniform nationwide approach.

If you use summer–winter comparisons for equipment evaluation, this method is quite important. That's because, if you don't capture the differences that actually affect on-site performance, readings of equipment usage and interpretations of economic benefits can easily be skewed. It is effective as a method for seeing not only the annual total but also how much value exists in each season.

Method 5: Compare including self-consumption and time-of-use

The fifth method is to compare summer and winter while including self-consumption and time of day. This is a way to connect comparisons of generation output to the usability of the equipment. That generation differs between summer and winter is important in itself, but what matters more in practice is how that difference overlaps with demand. In other words, summer and winter comparisons need to look not only at differences in total output but also at differences in how the output is used.

For example, in summer, at facilities with high air-conditioning demand, self-consumption tends to increase during periods of high generation. As a result, summer generation more directly leads to electricity bill savings. On the other hand, in winter generation is lower, and at facilities where heating loads tend to fall in the early morning or evening, the overlap between generation periods and demand may be weak. In this case, even if the annual total is the same, the value of the equipment can differ significantly between summer and winter.

Also, when installations include east-facing or west-facing equipment, this difference becomes even clearer. East-facing systems produce more power in the morning, while west-facing systems produce more in the afternoon. If afternoon demand is strong in summer, west-facing may be advantageous, and if morning demand is strong in winter, east-facing can be meaningful. In other words, comparisons between summer and winter are more realistic when you look at system orientation together with demand timing.

The advantage of this method is that it can explain seasonal differences including self-consumption rates and how surpluses occur. In summer, generation is high and self-consumption tends to increase. In winter, generation is low and demand is high, but the temporal overlap is weak. Understanding this structure makes it easier to organize assessments of the system’s financial balance and payback. Rather than ending with simply “more in summer” and “less in winter,” it conveys the meaning of those differences.

Of course, this method requires more man-hours. However, it is very valuable for projects premised on self-consumption or for projects that aim to optimize equipment configuration. In practice, differences in equipment value that are not visible from comparisons of total amounts become much easier to see with this method.

Common calculation mistakes in summer–winter comparisons

One common calculation mistake in summer–winter comparisons is to simply assume that summer produces the most power because daylight hours are longer, and winter produces less because daylight hours are shorter. This line of thinking is not entirely wrong, but if you ignore summer’s high-temperature losses and winter’s efficiency gains due to low temperatures, you can somewhat exaggerate the difference. In other words, comparing only by daylight hours can lead to misjudging the actual difference in power generation.

Another common mistake is to arbitrarily allocate summer–winter differences from the annual average. For example, simply dividing the annual power generation by 12 and assigning slightly more to summer and slightly less to winter tends to overlook the month-by-month realities and the effects of snowfall and cloudy weather. In winter in particular, not only shorter daylight hours but also shadows, snow cover, and the low solar elevation combine, so a simple allocation becomes excessively coarse.

Also, a common mistake is to compare summer and winter only by power generation and not consider self-consumption and demand. Even if generation is higher in summer, if demand increases similarly the self-consumption rate can rise. In winter, even if generation is lower, it can still be valuable if it can be used during the necessary time periods. In other words, it’s important not to judge the quality of an installation solely by how much it generates.

Furthermore, failing to account for orientation and time of day in seasonal comparisons is problematic. East-facing systems tend to generate more in the morning, while west-facing systems tend to generate more in the afternoon. This difference changes how generation aligns with demand in summer versus winter. Even if the total annual kWh looks similar, the value can differ when you consider how it's used in summer and winter. In other words, seasonal comparisons that ignore time of day are insufficient.

To improve the accuracy of summer–winter comparisons, it is necessary to layer not only the total amount but also solar radiation, temperature, shading, time of day, and demand in that order. If this layering is omitted, the calculations may be simple, but the resulting figures tend to be impractical for on-site use.

How Practitioners Should Proceed to Improve Comparative Accuracy

If operational staff want to improve the accuracy of summer–winter comparisons, it is clearer to first establish the annual value as an entry point, then proceed step by step to monthly, daily, and overlaps with demand. Trying to do everything in detail from the start increases workload and tends to be wasteful when the assumptions are not yet solid. Conversely, ending with only the annual value makes the meaning of seasonal differences unclear. Gradually increasing accuracy in stages is the most practical approach in operational work.

First, get an outline of the annual generation from the system capacity and the regional coefficient. Next, select representative months for summer and winter and compare monthly or daily generation. After that, apply adjustments not only for sunshine hours but also for solar irradiance, temperature, shading, and snow. Furthermore, for projects where you want to examine self-consumption or time-of-use value, overlay the demand profile to assess the system's value. Following this order makes it much easier to see where the numbers changed.

Also, the accuracy of on-site conditions is important. If the roof surface orientation, the positions of surrounding obstructions, elevation differences, winter shadows, and how snow remains are unclear, winter corrections and azimuth (orientation) corrections will be less precise. Especially when comparing summer and winter, estimates of winter conditions tend to directly affect the system evaluation, so accurately grasping the site conditions becomes even more important.

Also, if possible, using actual results and data from nearby similar projects will make the analysis even stronger. On paper you may have anticipated a certain difference between summer and winter, but if the actual results show that winter performance fell more than expected or that high-temperature losses in summer were larger, you can reflect those differences in the next estimate. In other words, the summer-winter comparison should not end with a single calculation; thinking of it as something to be refined using real-world results makes it more practical for actual work.

Summary

As methods to calculate and compare solar power generation between summer and winter, five approaches are practical in the field: comparing monthly generation, comparing generation per day, separating and comparing solar irradiance and sunshine duration, comparing with corrections for temperature, snow cover, and shading, and comparing including self-consumption and time of day. Each serves a different purpose, and combining them according to the project stage makes the meaning of the difference between summer and winter considerably clearer.

The difference in power generation between summer and winter is not determined solely by differences in sunlight hours. In summer, high-temperature losses come into play; in winter, a low sun angle, shading, and, in some cases, snow have an effect. Moreover, the system’s orientation and the time-of-day value affect how easy it is to self-consume. For that reason, summer–winter comparisons are not merely a supplement to annual totals but an important perspective for evaluating the usability of the system.

Also, if you truly want to improve comparative accuracy, it is essential to accurately understand the on-site conditions. If the roof surface orientation, the positions of surrounding obstacles, elevation differences, and the way shadows fall remain unclear, no matter how carefully you correct for summer/winter differences, the final figures will still tend to fluctuate. In particular, under winter conditions, estimates will change depending on whether the site's positional relationships have been captured accurately.

In that respect, LRTK, an iPhone-mounted GNSS high-precision positioning device, is very effective as a means of accurately grasping on-site positional relationships. Because it makes it easier to precisely record candidate equipment locations and the positions of surrounding obstacles in the field, it facilitates linking to summer–winter comparative estimates that take shading and layout conditions into account. If you want to compare solar power generation between summer and winter using numbers that are truly usable, properly capturing on-site conditions with a method like LRTK is a major advantage.