6 Steps to Calculate Solar Power Generation for 100 kW｜Basic Knowledge Before High Voltage

A solar power system in the 100 kW range is somewhat too large to be considered simply an extension of residential or small-scale installations, and it is a scale that requires clarifying assumptions as a commercial installation. In the government's classification, commercial solar power is treated in categories of low-voltage under 50 kW and high-voltage 50 kW and above but less than 2,000 kW, and 100 kW is, in practice, a capacity band where attention to the high-voltage side is already required. In addition, photovoltaic power generation facilities with output of 50 kW or more, or those electrically connected to high-voltage equipment, are treated as private electrical installations under the Electricity Business Act. In other words, when considering 100 kW, you need to evaluate expected generation not only by the size of the installation but on the premise that high-voltage interconnection, safety management, and accountability (including the obligation to provide explanations) are included. ([Ministry of Economy, Trade and Industry][1])

Therefore, when calculating the power generation of a 100 kW system, compared with small-scale systems such as 5 kW or 10 kW, it is better to carefully organize the system configuration, regional differences, orientation, shading, losses, and overlap with demand, so that subsequent explanations and comparisons are more stable. In this article, we divide the basic procedures for estimating the annual power generation of a 100 kW solar power system into 6 steps and organize them, including the premise of taking high-voltage projects into account. ([Ministry of Economy, Trade and Industry][1])

Table of Contents

• What to know before calculating a 100 kW system

• Step 1 Confirm the components of the 100 kW system capacity

• Step 2 Establish the reference generation for each region

• Step 3 Determine orientation and installation angle

• Step 4 Adjust for shading and surrounding conditions

• Step 5 Calculate the annual generation reflecting system losses

• Step 6 Review monthly figures and the demand side to convert them into business figures

• Common calculation mistakes in 100 kW projects

• How practitioners should proceed to improve accuracy

• Summary

Things to know before calculating a 100 kW system

First, what you need to understand is that the figure 100 kW refers to the system capacity, not the amount of electricity generated. kW denotes the output scale of the equipment, while the quantity generated over a year is expressed in kWh. In other words, only by applying regional conditions, installation conditions, and loss factors to the initial figure of 100 kW does the annual generation become clear. If you skip this step and mechanically treat 100 kW as about 100,000 kWh per year, differences in site conditions can easily be overlooked. ([Ministry of Economy, Trade and Industry][1])

Also, at the 100 kW scale it becomes difficult to treat it with the same mindset as small-scale installations from a regulatory perspective. In government materials, commercial solar is categorized as low-voltage for systems under 50 kW and high-voltage for systems of 50 kW or more and under 2,000 kW, and 100 kW falls into the high-voltage bracket. Therefore, when calculating generation, it is more practical to view it on a commercial-use basis rather than simply as an extension of rooftop-mounted residential systems. ([経済産業省][2])

The important point here is not to perform complicated calculations from the outset. Calculate the initial annual power generation, and then add conditions typical of high-voltage projects. If you organize them in the order of installed capacity, regional differences, orientation, shading, losses, and the demand side, the structure of the calculation becomes sufficiently easy to understand even for 100kW. ([Ministry of Economy, Trade and Industry][1])

Step 1: Determine the contents of equipment with a capacity of 100 kW

The first step is to determine the composition of a system capacity of 100 kW. This does not suffice to simply say "it's a 100 kW project"; it means organizing how many panels, on which faces, and how they are arranged to make up the 100 kW. For example, if the panels are 0.4 kW each, you would need 250 panels; if 0.42 kW, about 238 panels, to reach around 100 kW. The system capacity itself is calculated from the number of panels and the output per panel in this way. ([経済産業省][1])

However, in practice it is important not to adopt the theoretical maximum number of modules as-is. For rooftop installations there are edge setbacks, inspection walkways, equipment, upstands, and structural conditions, and for ground-mounted installations clearance, maintenance space, row layout, and shading conditions are added. Even if it appears that you could place more than 100 kW, the capacity that can actually be adopted often drops into the 95 kW or 98 kW range. At the 100 kW level, this difference can translate directly into a difference of several thousand kWh per year, so the initial capacity setting must be done carefully. ([Ministry of Economy, Trade and Industry][1])

Furthermore, at 100 kW it often cannot be accommodated on a single surface and is frequently distributed across multiple surfaces or multiple rows. If you have a breakdown by surface—for example, south-facing 60 kW, west-facing 20 kW, and east-facing 20 kW—keeping that makes later orientation and shading corrections much easier. For commercial installations, the basics are to organize capacity as a configured composition rather than only as a total amount. (Ministry of Economy, Trade and Industry [3])

Step 2: Set the baseline power generation for each region

The next step is to establish a region-specific reference generation value. This is an initial guideline for how much a 1 kW installation in that region could generate in one year, and is used to grasp the outline of annual generation. The basic idea is: Annual generation (kWh) = system capacity (kW) × region-specific reference generation (kWh/kW·year). ([Ministry of Economy, Trade and Industry][1])

For example, under standard conditions, if you assume 1,050 kWh/kW·year, the annual generation for a 100 kW system is 105,000 kWh. If you use 1,100 kWh it becomes 110,000 kWh. Conversely, under somewhat harsher regional conditions, if you assume around 1,000 kWh it will be around 100,000 kWh. For the 100 kW class, a difference of just 50 kWh per kW results in an annual difference of 5,000 kWh, so it is particularly important not to ignore regional differences. ([経済産業省][3])

The point here is not to make these figures uniform across the country. Even in national documents, classifications and deployment status of commercial solar power are organized by capacity bands, and they show that even within the same high-voltage band there are large differences in project conditions. Regional coefficients are merely an entry point, but by placing them appropriately here, the foundation for subsequent adjustments is stabilized. ([Ministry of Economy, Trade and Industry][3])

Step 3 Organize the orientation and installation angle

The third step is to determine the orientation and tilt angle. In the 100-kW class, installations often span multiple surfaces or multiple rows, and differences in orientation and tilt have a significant effect on annual power generation. Even for the same 100 kW, a system that is mainly south-facing and one distributed east–west will have different annual kWh output. ([経済産業省][1])

At this stage, rather than treating the total capacity as a single condition, it is ideal, if possible, to organize it by orientation. For example, if you divide it into 40 kW on the south-facing side, 30 kW on the east-facing side, and 30 kW on the west-facing side, you can apply a relatively higher correction to the south-facing side and slightly more modest corrections to the east and west sides. In practice, simply having this per-orientation organization greatly improves the explanatory power for the power generation. (Ministry of Economy, Trade and Industry [3])

Also, for the 100 kW class, row layout and racking tilt angle are important. For ground-mounted installations, inter-row shading effects matter; even for rooftop installations, differences in roof slope affect winter shading and irradiance conditions. As can be seen from the fact that national regulatory arrangements treat high-voltage systems of 50 kW and above as a single category, 100 kW projects require a perspective based on layout conditions rather than a simple extension of small-scale installations. ([Ministry of Economy, Trade and Industry][1])

Step 4: Correct Shadows and Surrounding Conditions

The fourth step is to correct for shading and surrounding conditions. With 100 kW installations, because the installation covers a larger area, the effects of shading and nearby structures often appear only on parts of the system. On roofs, typical factors are adjacent buildings, upstands, and equipment; for ground-mounted installations, typical factors are fences, trees, neighboring structures, and elevation differences. If shading is uniformly handled as simply “some” or “none,” the forecast for annual power generation becomes coarse. ([経済産業省][1])

In practice, it is easier to understand if you organize things using a shading correction factor. If there is almost no shading, the value is close to 1.0; if there is a slight effect, 0.97 or 0.95; and if it is more severe, lower than that — this method looks at how much conditions deviate from the ideal. In the 100 kW-class, even small differences in the correction can amount to several thousand kWh per year, so assessing shading is more important than for small-scale projects. ([Ministry of Economy, Trade and Industry][1])

Shadows also change with the seasons and time of day. It's not uncommon for shadows to be long only in winter, to affect only the morning, or to repeatedly impact only certain rows. For that reason, assessing shadow conditions not just on paper but, if possible, in conjunction with on-site conditions improves accuracy. In 100 kW projects where high-voltage considerations are important, these site-specific differences can directly translate into differences in profitability, so it's safer not to take them lightly. ([経済産業省][1])

Step 5: Calculate annual power generation reflecting system losses

The fifth step is to account for system losses and calculate the final annual electricity generation. So far, we have accounted for installed capacity, regional variations, orientation, and shading, but those are still theoretical values. In practice, generation is further reduced by losses in conversion equipment, wiring losses, efficiency decreases due to high temperatures, soiling, differences between modules, and other factors. ([Ministry of Economy, Trade and Industry][1])

For example, if the input value for a 100 kW facility is 105,000 kWh, and the azimuth correction is 0.95, the shading correction is 0.97, and the system loss coefficient is 0.85, the annual generation is 105,000×0.95×0.97×0.85, which is about 82,955 kWh. The difference from the input value may appear large, but for installations on the order of 100 kW it is important to separate theoretical values from practical projected values. ([Ministry of Economy, Trade and Industry][3])

It can also be useful at this stage to break the data down by month. Rather than stopping at an annual aggregate, checking seasonal differences—stronger in spring and autumn, weaker in winter and the rainy season—will make later estimates of self-consumption and electricity sales considerably more stable. This is especially true for commercial applications, where seasonal differences on the demand side and the generation side overlap; therefore it’s worth reviewing the monthly profile as well as the annual values. ([経済産業省][3])

What's important here is to treat the loss coefficient not simply as a conservative number, but as a figure for converting input values into values usable on-site. If you use the theoretical value as-is, explanations tend to become inconsistent later. Conversely, keeping both the theoretical value and the corrected practical value makes it considerably easier to use for internal explanations and comparisons. ([Ministry of Economy, Trade and Industry][1])

Step 6: Check monthly and demand-side data and convert them into business figures

The final step is to check monthly generation and the demand side to produce figures that are meaningful for commercial use. For a 100 kW facility, because the total generation is large, simply looking at how many kWh are produced in a year is not sufficient. Only by seeing how much of that electricity can be self-consumed and how much becomes surplus for sale will the value of the installation and its profitability become clear. ([Ministry of Economy, Trade and Industry][2])

The approach is to separate annual self-consumption and sold energy. Sold energy (kWh) = Annual generation (kWh) − Self-consumption (kWh). For example, if annual generation is about 83,000 kWh and daytime demand is 35,000 kWh, the amount sold is about 48,000 kWh. Conversely, if daytime demand is 50,000 kWh, the amount sold is about 33,000 kWh. Even with the same 100 kW, the outcome can look quite different depending on demand-side conditions. ([Ministry of Economy, Trade and Industry][2])

Also, looking at it by month makes the meaning even clearer. In summer, generation is higher, but if air-conditioning loads are large the self-consumption rate tends to be higher, and in spring and autumn surpluses may be more likely to occur. In winter, while generation falls, demand can be high due to heating and equipment operation. In other words, for the 100 kW class, examining the overlap with monthly demand gives a picture closer to the actual commercial situation. ([Ministry of Economy, Trade and Industry][3])

For projects at a scale that require consideration of high-voltage, it is fundamental to organize not only the size of the annual kWh but also how those kWh will be used. Thinking of this as the final step to convert generation calculations from "equipment theory" into "business figures" makes it easier to understand. ([経済産業省][1])

Common calculation mistakes in 100 kW projects

One common calculation mistake in 100 kW projects is treating the installed capacity of 100 kW as if it directly implies a fixed annual output of around 100,000 kWh. While this is convenient as an initial annual estimate, it often does not account for orientation, shading, and losses, and the figure tends to decrease upon later detailed verification. At the 100 kW scale, this difference can amount to several thousand to several tens of thousands of kWh per year, substantially affecting financial results and explanations. ([Ministry of Economy, Trade and Industry][1])

Another common practice is to convert the theoretical maximum number of panels or maximum area directly into capacity. In the 100 kW class, the effects of end clearances, maintenance spaces, equipment, and row layout conditions become very significant. Differences that were not noticeable in small-scale projects translate directly into large kWh differences in a 100 kW system. If the initial capacity estimate is too aggressive, all subsequent figures will be inflated, so caution is necessary. ([経済産業省][3])

Also, confusing generated electricity with electricity sold is a typical mistake. For 100 kW systems, self-consumption can be substantial, so treating annual generation itself as if it were electricity sold can easily lead to incorrect business decisions. For high-voltage systems, it is fundamental to consider the separation between self-consumption and electricity sales. ([Ministry of Economy, Trade and Industry][2])

How Practitioners Can Improve Accuracy

If practitioners want to improve the accuracy of a 100kW estimate, the first step is to keep the initial rough estimate and the corrected practical value separate. First, grasp the annual outline using regional coefficients, then reflect surface-specific conditions, shading, and losses, and finally link these to monthly figures and the demand side. Having such a stepwise process makes it easier to see where the numbers changed. ([経済産業省][1])

Also, it is important to always retain the assumptions. For example: how many panels were used to make up 100 kW, how many kW are placed on each surface, what the regional coefficient is, whether shading has been confirmed on site, and what is included in the loss coefficient. If these are organized, you will not be at a loss when recalculating later. Conversely, if only the annual kWh remains, it becomes difficult to explain why that value was obtained. ([Ministry of Economy, Trade and Industry][1])

Furthermore, the accuracy of acquiring on-site conditions is particularly important for 100kW projects. If the roof surface orientation, obstacle positions, elevation differences, and the feasibility of row arrangements are ambiguous, assessments of shading and layout conditions become less precise. In the sense that the quality of the input conditions determines the quality of the results, the impact is greater for high-voltage-side projects like 100kW. ([Ministry of Economy, Trade and Industry][1])

Summary

To calculate the solar generation for a 100 kW system, first confirm the composition of the installed capacity, use region-specific reference generation to derive an annual baseline value, and then sequentially account for orientation and tilt, shading and surrounding conditions, and system losses. On top of that, by reviewing monthly generation and the demand side and organizing the structure of self-consumption and power sales, you arrive at figures suitable for high-voltage projects. In national classifications, commercial solar is considered low-voltage if under 50 kW and high-voltage if 50 kW or more and under 2,000 kW, and 100 kW is precisely a scale that should be treated with high-voltage considerations. (Ministry of Economy, Trade and Industry [1])

Especially in the 100 kW class, how you set the installed capacity at the point of entry, per-surface conditions, shading, and estimates of losses can directly lead to differences of several thousand to tens of thousands of kWh per year. That's why it's important not to rush to conclusions based solely on installed capacity, but to carry the figures through to the corrected practical values. Also, it's only when you reinterpret generation into self-consumption and electricity sales that its meaning as a business becomes clear. ( [Ministry of Economy, Trade and Industry][3])

Furthermore, for field personnel who want to capture on-site positional relationships with high accuracy, LRTK, an iPhone-mounted GNSS high-precision positioning device, is useful. Because it makes it easier to accurately record candidate equipment locations and obstacle positions on site, it facilitates linking to power generation estimates for 100kW installations that account for shading and layout conditions. For 100kW projects, accurately capturing on-site conditions is as important as knowing the calculation methods. Utilizing LRTK can significantly strengthen that initial step.