Seven Conditions and Calculation Examples to Estimate Solar Power Generation

When calculating solar power generation, looking only at panel capacity is not sufficient. Actual generation varies with multiple conditions such as solar irradiance, installation angle, shading, temperature, equipment losses, soiling, and aging. This article organizes the seven conditions that practitioners searching for "solar power generation calculation" should check to obtain a rough estimate of generation, and provides calculation examples that are easy to use on site.

Table of Contents

• A guideline for solar power generation can be derived from the basic formula.

• Requirement 1: Accurately determine the system capacity

• Condition 2: Confirm solar radiation and regional differences

• Condition 3: Examine differences in power generation due to azimuth and tilt angle

• Condition 4: Account for shadow effects by time of day

• Condition 5 Consider output reduction due to temperature rise

• Condition 6 Include conversion losses and wiring losses

• Condition 7: Consider soiling and aging on the safe side

• Provide an annual estimate using an example calculation of solar power generation

• Check the difference between measured and calculated values

• Summary: When calculating power generation, it is important to keep the conditions consistent

Estimating Solar Power Generation Based on the Basic Formula

When estimating solar power generation, it is important to first clarify the basic equation. In practice, detailed simulations may be performed, but for initial assessments and rough checks, a method that multiplies system capacity, solar irradiance, and loss factors is easy to work with.

The basic idea is to estimate expected generation as "system capacity × conditions equivalent to insolation × a coefficient that accounts for losses." For example, you estimate how much a 1 kW solar PV system will generate for each region and installation condition, and then multiply that by the system capacity to obtain an overall estimate.

However, it should be noted that the calculation results are only an estimate and do not mean the same amount of power will be generated every day. Solar power generation is greatly affected by the weather, so output varies significantly between sunny, cloudy, and rainy days. Moreover, even on the same sunny day, results can fluctuate depending on temperature, atmospheric clarity, the season, and how shadows fall.

When considering annual power generation, it is closer to reality to look at solar radiation conditions throughout the year and monthly variations rather than simply multiplying a daily generation figure by 365. In practice, it is especially important not to confuse the planned value before equipment installation, the expected value at inspection, and the reference value for abnormality judgment. The planned generation at the planning stage is an estimate based on design conditions, and during inspections it should be compared taking into account that day’s weather and operating conditions.

In calculating solar power generation, the assumptions you make are more important than the formula itself. Even with the same system capacity, the expected annual energy output changes depending on whether you assume a larger or smaller loss coefficient. Therefore, rather than recording only the calculation results, it is useful for later review and explanation to record the capacity used in the calculation, the solar irradiance conditions, the loss rates, whether shading is present, and the assumed operating conditions.

Requirement 1: Correctly understand system capacity

The first requirement for estimating solar power generation is to correctly determine the system capacity. System capacity generally refers to the total of the nominal maximum outputs of the solar modules. For example, when multiple solar modules with a fixed output per module are installed, the sum of those outputs is treated as the capacity of the power generation system.

One point to note here is that system capacity and the power actually produced on a continuous basis are not the same. The rated maximum power of a photovoltaic (PV) module is a value measured under specific test conditions. In real outdoor environments, factors such as irradiance, temperature, shading, soiling, wiring, and conversion equipment losses mean it does not always generate power equal to the rated maximum.

When operations personnel calculate power generation, they first confirm the system capacity of the equipment in question using installation drawings, as-built documents, equipment registers, single-line wiring diagrams, and so on. If the installation is divided into multiple systems or sections, it is useful to understand not only the total capacity but also the capacity of each system. This is because, when investigating the cause of a drop in generation, a reduction that does not appear to be a major anomaly overall may be occurring only in a specific system.

Also, when calculating power generation, you need to treat the DC-side PV capacity and the AC-side output conditions separately. Even if the PV capacity on the DC side is large, the rated output or control settings of the power conversion equipment can cause the output to be capped during certain periods. In such cases, the instantaneous output may not rise above a certain level during times of strong solar irradiance and can appear flat on a graph. This may not be a major issue for rough annual generation estimates, but it is an important point to check when comparing with measured values.

When checking system capacity, make sure not to overlook the history of expansions or partial shutdowns. If some circuits were disconnected in the past or the capacity changed after replacements, calculations based on outdated documentation will not match reality. To accurately estimate power generation, the basic principle is to use the capacity of equipment currently in operation as the baseline.

Condition 2: Check solar radiation and regional variations

A major factor that determines photovoltaic generation is solar irradiance. Because solar power generates electricity by receiving sunlight, annual output will vary between regions with high and low solar irradiance even for systems with the same capacity. In areas with many clear days, heavy snowfall, or that are prone to the rainy season or cloudy conditions, the underlying assumptions differ even when the same calculation formula is used.

When assessing solar radiation, it is important to check not only whether there are many sunny days but also the trends throughout the year. In summer, days are longer and solar radiation is greater, but output reductions due to rising temperatures are more likely to occur. In winter, lower temperatures can be favorable for solar cell output, but daylight hours are shorter and the sun's altitude is lower. In snowy regions, it is also necessary to consider the impact of periods when snow remains on the panel surface.

Roughly, people sometimes use an indicator of how much electricity is generated per 1 kW per year. Under typical installation conditions, a commonly used guideline is roughly 1,000 kWh per 1 kW per year, but this varies depending on the region and installation conditions. In locations with favorable conditions it can exceed this, while in places with shading, snow accumulation, or an unfavorable installation angle it can be lower.

In practice, when performing calculations using solar radiation, it makes later explanations easier if you clearly state which location's data you referenced. If the site of the power plant or building is far from the location of the referenced solar conditions, the meteorological conditions may differ. In mountainous, coastal, and urban areas, cloud formation, temperatures, and snow conditions can also change.

If the purpose of estimating power generation is for pre-installation assessment, it is common to use average yearly solar irradiation conditions. On the other hand, for inspections or abnormality detection of existing equipment, you need to compare under conditions similar to the solar conditions during the period in question. For example, even if a month’s generation is lower than the same month in the previous year, that does not necessarily indicate an equipment fault if that month had many rainy or cloudy days. It is important to evaluate generation together with solar conditions, not by generation alone.

Condition 3: Examine power generation differences due to azimuth and tilt angle

When estimating expected solar power generation, orientation and tilt angle are also important factors. The amount of solar radiation received changes depending on which direction the photovoltaic modules face and at what angle they are installed. In general, the closer the orientation and angle are to those that receive sunlight most effectively, the more the power generation tends to increase, but optimal installation may not be possible due to building shape or site conditions.

When it comes to orientation, the closer a system faces south, the easier it is to secure annual power generation. However, being east- or west-facing does not mean it won't generate electricity. East-facing systems produce more in the morning, while west-facing systems produce more in the afternoon. Total annual generation may be lower than for south-facing systems, but depending on the timing of electricity use and the purpose of the installation, they can still be practically useful.

Regarding the tilt angle, power generation changes depending on its relationship with the sun’s elevation. When the tilt is small, the panels are more exposed to summer sunlight, but attention may be needed regarding soiling and water runoff. When the tilt is large, it can be advantageous for the low sun in winter, but you need to check the trade-offs with wind effects and installation conditions.

When performing calculations in practice, if you can finely correct azimuth and tilt angle, use correction coefficients; for rough estimates, there is a method of taking a slightly conservative view compared with standard conditions. For example, take a south-facing installation with little shading and an appropriate tilt as the baseline, and assume lower power generation for east- or west-facing sites or when tilt conditions are unfavorable.

If an installation is divided across multiple faces, calculating each face separately will be closer to reality than calculating the whole at once. In systems that mix south-, east-, and west-facing surfaces, the times of peak generation differ. Checking not only the total annual generation but also the generation trends by time of day makes it easier to compare with measured values.

Azimuth and tilt angle conditions are relevant not only to pre-installation planning but also to diagnosing declines in power generation. If the angles assumed in the design differ from the actual installed angles, or if changes in the surrounding environment alter solar irradiance conditions, calculated values and measured results may not match. During on-site verification, it is important to check not only the information on the drawings but also the actual orientation, tilt, and surrounding obstructions.

Condition 4: Consider the effects of shadows by time of day

One factor that is easy to overlook when calculating solar power generation is the effect of shadows. Even partial shading of a photovoltaic module can reduce its power output, depending on the circuit configuration and how the shadow falls. Shadows are not constant throughout the year; their position and length change with the seasons and time of day, making them difficult to capture with a simple average.

Causes of shading include buildings, trees, utility poles, railings, antennas, adjacent equipment, and mountain ridgelines. Even locations that posed no problem at the time of installation can experience increased shading later due to tree growth or the construction of nearby buildings. Also, because the sun is lower in the sky in winter, shadows cast by obstacles that are not an issue in summer can extend much farther.

When estimating expected power output, it’s important not to judge the presence of shading simply as "present" or "absent", but to check when it occurs, over what area, and for how long. The impact on annual power generation differs if shading occurs only in the morning, around midday, or only in the evening. In particular, shading during periods of strong sunlight tends to have a greater impact on power output.

In calculations, there is a method to incorporate the effect of shading as a loss coefficient. If shading is minimal, estimate it as small; if shading occurs only during certain periods, take those times and seasons into account and adopt a conservative assumption. For installations where shading has a large impact, treating it solely with a simple annual factor can diverge from reality, so it is preferable to verify on a monthly or time-of-day basis.

When investigating a decline in power generation in practice, the effects of shading can appear on the power-output graph. If output drops at the same time of day every day despite clear skies, shading, control actions, or equipment limits should be suspected. Declines caused by clouds tend to change shape from day to day, whereas shadows from fixed objects often appear in a similar shape at roughly the same time of day.

When checking shadows, in addition to taking photos on site, record the time the photo was taken, the orientation (azimuth), and the season, as this makes later assessment easier. Even if an on-site check in the morning shows no issues, a different shadow may appear in the afternoon. Recording how shadows were treated as an assumption in the power generation calculations makes it easier to explain any differences between calculated and measured values.

Condition 5: Consider the output reduction due to temperature rise

Solar power generation tends to increase with stronger solar irradiance, but when the temperature of photovoltaic modules rises, their output tends to decrease. On sunny summer days, although solar irradiance is high, module temperatures also tend to be high, so estimating generation solely from irradiance can lead to overestimation.

The output of a photovoltaic module is specified under certain temperature conditions. However, in real outdoor environments the module temperature on a roof surface or on a mounting structure can be higher than the ambient air temperature. In particular, installations with poor ventilation or those mounted close to the roof surface can tend to trap heat. Conversely, under conditions of low ambient temperature and good ventilation, the same solar irradiance may result in relatively higher output.

When estimating expected power generation, it is common to include temperature-related losses in the overall loss factor. For detailed calculations, temperature coefficients and module temperature are used, but for rough estimates it is realistic to account for temperature losses as a factor that prevents output from reaching its full potential during summer.

The impact of temperature is also important when looking at power generation graphs. On a clear midsummer day, even with favorable solar irradiance conditions, performance may not appear as high as in spring or autumn. In such cases, it is not necessarily equipment malfunction; a rise in module temperature may be involved. Conversely, on clear winter days, even though sunshine duration is short, the instantaneous output may appear relatively high.

In practice, when selecting a reference for comparing power generation, it is necessary to take seasonal differences into account. Simply comparing summer and winter by the same criteria can easily lead to incorrect conclusions, because solar irradiance, sunshine duration, temperature, and solar altitude all differ. When judging whether generation is low, it is desirable to compare under the same season, similar weather, and comparable solar irradiance conditions.

Including temperature losses makes calculated values less likely to be overly optimistic. Because estimated power generation is often later compared with measured values, it is important to set conditions that reflect actual operating environments as well as ideal conditions.

Condition 6 Include conversion losses and wiring losses

The electricity generated by solar photovoltaic modules does not directly become the amount of electricity ultimately used or sold. Electricity generated as direct current is converted to alternating current, and certain losses occur during that process. There are also losses due to wiring, connection points, collection equipment, and measurement conditions. When estimating generation, these conversion losses and wiring losses need to be included.

A common calculation mistake is to multiply the solar panel capacity only by the solar irradiance and assume almost no losses. In that case, the calculated generation tends to be higher than the actual output, so later measured values appear lower. In reality, conversion equipment efficiency, wiring length, the condition of connection points, equipment operating conditions, and other factors combine to affect the final power generation.

When setting a loss factor, it is important to be clear about which losses are being included. You can combine temperature losses, conversion losses, wiring losses, soiling, shading, and degradation into a single factor, but in that case the breakdown becomes hard to see. To check later why energy production is lower than expected, it is useful to separate and organize the loss factors as much as possible.

Conversion equipment has a rated output and operating range. Depending on the relationship between the capacity on the solar cell side and the capacity of the conversion equipment, the output may not increase beyond a certain level during periods of strong solar irradiance. This is not necessarily a malfunction and can occur due to system design or control conditions. However, if there are unexpected output limits or shutdowns, they can cause a reduction in generated power.

Wiring losses can become harder to ignore as the scale of the installation increases. When wiring distances are long, connection points are many, or conditions differ between systems, it is necessary to check not only the overall power output but also the output by section and by system. If there are poor connections or deterioration, since this also affects safety, it is important not to judge solely by a drop in power output and to arrange for a professional inspection when necessary.

In calculations of power generation, losses are included to translate theoretical values into actual operational values. Including losses is not intended to make generation performance look worse, but to provide a more realistic estimate. In explanatory materials and internal review documents, explicitly stating that the power generation figures include losses makes it easier to avoid later misunderstandings.

Condition 7: Consider soiling and aging on the safe side

The final factor to consider when estimating solar power generation is soiling and aging. When dirt accumulates on the surface of a photovoltaic (PV) module, it reduces the amount of light received and can affect power output. Types of soiling include sand and dust, yellow sand (Asian dust), pollen, bird droppings, fallen leaves, exhaust-derived deposits, and salt-containing deposits in coastal areas.

The effects of soiling vary: some types are easily washed away by rain, while others tend to remain in certain areas. In particular, localized obstructions of light, such as bird droppings or fallen leaves, can have a greater impact than they appear. Equipment installed at a low tilt often has poor drainage, causing dirt to remain near the edges. Because the pattern of soiling depends on the surrounding environment, it is important not to rely solely on general coefficients but to check the actual on-site conditions.

Regarding aging, consider that photovoltaic (PV) modules and ancillary equipment may gradually degrade in performance with years of use. Equipment does not remain in the same condition as when it was first installed; during long-term operation the condition of modules, wiring, connections, power conversion equipment, and metering devices will change. When estimating annual energy generation over the long term, it is necessary to account for changes over multiple years, not just the first year.

If soiling and aging are not taken into account in generation calculations, the gap with measured values can appear to widen year by year. Of course, a decline in generation does not necessarily mean everything is due to aging. Other factors include increased shading, equipment outages, grid troubles, measurement errors, output curtailment, and unfavorable weather. That is why it is important not to assume soiling and aging as a given, but to verify them alongside inspection records, cleaning history, and system-specific data.

In rough calculations, dirt and aging are sometimes accounted for as conservative losses. Especially when using an estimate of power generation as a reference for internal briefings or maintenance planning, calculating only for ideal conditions makes it difficult to explain the gap with actual performance. To produce a usable on-site guideline for expected power generation, it is necessary to provide margins appropriate to the local environment.

Estimate annual solar power generation using a calculation example

From here, we will consider an estimate of annual power generation using a concrete calculation example. Here, we calculate under simplified conditions where the photovoltaic (PV) system capacity is 10 kW, the annual generation per 1 kW is 1,000 kWh, and various losses are already included to some extent. In this case, the estimated annual generation is 10 kW × 1,000 kWh, resulting in approximately 10,000 kWh per year.

This calculation is very simple, but it is a convenient method for preliminary assessments. However, in practice it varies depending on the region, orientation, tilt angle, shading, temperature, losses, soiling, and so on. For example, if installation conditions are good, shading is minimal, and solar irradiation conditions are favorable, the annual generation per 1 kW may exceed this estimate. Conversely, under east–west orientation, with shading, with snowfall, or in conditions prone to soiling, it may fall below it.

Next, consider a simple calculation that separates the losses. Assume a system capacity of 10 kW, a theoretical generation equivalent to 12,000 kWh under reference annual solar irradiation conditions, and a loss coefficient of 0.8. In this case, the estimated annual generation is 12,000 kWh × 0.8, or about 9,600 kWh. The 0.8 here is considered to collectively include temperature, conversion, wiring, soiling, and minor shading.

In this way, even for the same 10 kW installation, depending on the calculation method you may see it as 10,000 kWh or 9,600 kWh. What matters is not which one is always correct, but whether the assumptions are clearly stated. Be careful not to confuse whether the value was derived from annual generation per 1 kW or by applying a loss factor to the theoretical value.

When estimating monthly power generation, simply dividing the annual generation by 12 is insufficient. Solar power generation varies by season, so it tends to be higher from spring through summer and lower in winter. However, because temperature increases in summer also affect output, the maximum is not necessarily in midsummer. If you need monthly estimates, allocating based on monthly insolation conditions and past performance will more closely reflect actual conditions.

When estimating daily power generation, take into account the weather on the target day. Even for a system that produces 10,000 kWh per year, the daily average is about 27 kWh, but in practice it will exceed that on sunny days and fall well below it on rainy days. Therefore, using only the daily average to determine anomalies can lead to incorrect judgments. For day-to-day comparisons, the graph shape on sunny days, past data from the same season, and comparisons with nearby or similarly conditioned systems are useful.

When using calculation examples in practice, it's safer not to present the result as a single definitive number but to explain it with a range. For example, you can use an annual power generation of approximately 9,500 kWh to 10,500 kWh as a guideline and explain that it will actually vary depending on weather and operating conditions to avoid excessive certainty. It is important to make clear that power generation calculations are forecasts and not guaranteed values.

Check the difference between measured and calculated values

Calculating solar power generation does not end with producing a rough estimate. When a system is actually operating, comparing calculated values with measured values allows you to identify the system’s condition and operational issues. However, it is premature to conclude that there is a fault simply because the calculated and measured values do not match. You need to isolate the reasons for the discrepancy one by one.

First, check the period being compared. If you compare annual calculated values with monthly actuals, or monthly calculated values with daily actuals, the units will not match and misunderstandings can occur. In comparisons of power generation, it is fundamental to align the target period, units, aggregation timing, and measurement points. Especially when comparing remote monitoring data with on-site meter readings, the measurement targets and aggregation timing may differ.

Next, check the impact of the weather. Calculated values are often based on average conditions, so if the actual period is cloudier or rainier than normal, the power generation will be lower. Conversely, if there are more sunny days, generation may exceed the calculated values. Looking at solar irradiance and weather records alongside generation makes it easier to determine whether the cause is an equipment malfunction or natural variability.

Also check for shading, soiling, downtime, output control, and equipment errors. If, on a sunny day, the generation graph does not form a smooth, bell-shaped curve and drops only during specific time periods, shading or partial derating may be suspected. If there is a flat plateau around midday, output limiting or the rated conditions of the power conversion equipment may be involved. If there are periods when generation is zero or extremely low, you should also check the shutdown history and communication anomalies.

When dealing with discrepancies between calculated and measured values, it is important not to attribute the cause to a single factor. Power generation declines can result from a combination of weather, equipment, measurement, and operational conditions. For example, if cloudy weather during the rainy season, dirty panels, and missing communication data occur simultaneously, the reasons why generation appears low become complex. In practice, it is efficient to first check the major factors and then, as necessary, break the analysis down by system, by time of day, and by day.

Keeping records of power generation calculations is also important. If you record the capacity used in the calculations, solar irradiance conditions, loss factors, how shading was treated, and the measured period used for comparison, the calculations can be reproduced later. Even if the person in charge changes, having the assumptions documented reduces variation in decision-making. Estimates of power generation are not merely numbers; they can be used as a basis for equipment management and decisions about improvements.

Summary: Consistent conditions are essential for calculating power generation

To estimate expected solar power generation, you need to consider not only system capacity but also solar irradiance, regional variations, orientation, tilt angle, shading, temperature, conversion losses, wiring losses, soiling, and degradation over time. Even if the calculation formula is simple, leaving the assumptions vague makes it easy for discrepancies to arise when comparing with measured values or when explaining things internally.

A practical approach for use in the field is to first set a guideline for annual generation per 1 kW, then multiply by the system capacity to obtain a rough estimate. After that, check whether site conditions are more favorable or less favorable than the standard and adjust loss factors as needed. For conditions such as shading, susceptibility to soiling, the presence of snow, or constraints on orientation or tilt, it is important to assume on the safe side.

Also, the calculated values are not guaranteed values but a guideline for assessment. Even if the power generation is lower than the calculated value, this may be caused by adverse weather or differences in aggregation conditions. Conversely, just because values are close to the calculated value does not mean all systems are operating normally. By checking total power generation, system-specific data, time-of-day graphs, and on-site conditions together, you can make a judgment that more closely reflects the actual situation.

To apply calculations of solar power generation on-site, it is important to record the assumptions, compare them with measured values, and be able to isolate causes when discrepancies occur. By visualizing generation output, organizing inspection records, and continuously comparing with solar irradiance conditions, it becomes easier to detect declines in generation early and to inform maintenance and improvement decisions.