Four considerations for power generation calculations that differ between surplus electricity sales and selling all generated electricity

When calculating solar power generation, even with the same system capacity the figures to look at and the way calculation results are used differ between surplus power sales and selling all generated electricity. The basic approach to determining the amount of energy generated is the same, but for surplus power sales the amount sold after deducting self-consumption becomes important, while for selling all generated electricity it is important how stably the generated energy can be sold. Confusing the purpose of the calculations can easily lead to wrong judgments in profitability forecasts, considerations of system capacity, and evaluations of post-installation generation performance. This article organizes and explains four conceptual differences between surplus power sales and selling all generated electricity that practitioners should grasp when calculating solar power generation.

Table of Contents

• The purpose of the calculation differs depending on whether you sell surplus electricity or sell all generated electricity.

• Approach 1: Calculate generated electricity and sold electricity separately

• Approach 2 Change the way self-consumption is estimated

• Approach 3: Consider how to reflect losses and output control on the supply‑demand side

• Approach 4: Change the criteria to consider when comparing results

• A summary to avoid mistakes when calculating generation for surplus and full-feed-in electricity sales

• Summary

Calculation purposes differ between surplus electricity sales and selling all generated electricity

In calculating solar power generation, it is important first to distinguish whether you want to know the amount of electricity generated, the amount of electricity that can be sold, or the reduction in electricity consumption achieved through self-consumption. The difference between surplus electricity sales and selling all generated electricity is not simply a difference in how you sell, but also a difference in which figure the calculations are tied to.

When all generated electricity is sold, most of the generated energy is treated as saleable, so the calculated generation tends to be close to the expected amount of electricity to be sold. Of course, equipment downtime, measurement errors, output control, conversion losses, wiring losses, and so on mean that generated energy and the actual metered sold electricity may not match exactly. Even so, the focus of the calculation is on how realistically to estimate annual or monthly generation from photovoltaic (PV) capacity, solar irradiance conditions, installation tilt, azimuth, temperature effects, equipment losses, and other factors.

On the other hand, for surplus electricity sales, in addition to the amount of power generated itself, it is important to know “how much of the generated power is used on-site and how much is left over to be sold.” For example, even with the same monthly generation, a building that uses a lot of power during the daytime will have higher self-consumption and a lower amount available for sale. Conversely, a building with low daytime consumption will have a larger surplus and is more likely to see increased electricity sales. In other words, when calculating surplus electricity sales, simply determining generation is insufficient; you must also consider load timing, operating days, holidays, seasonal variations, and equipment operation patterns.

One thing to note here is not to simply assume that “the more power generation, the better.” With surplus electricity sales, it is important whether generation occurs during hours when it can be consumed on-site. Even if generation is large, if demand is low during those hours, the share sold will be high. In plans that prioritize self-consumption, making the system capacity too large can increase surplus more than expected and cause the plan to deviate from its intended goals.

Under a full-feed-in scheme, maximizing generation and the operational stability of equipment are the central focuses of the calculations. Under a surplus-feed-in scheme, the calculations focus not only on maximizing generation but also on the degree of alignment with demand. Understanding this difference makes it easier to determine which figures to prioritize when reviewing solar power generation calculation results.

Also, in practice the terms “annual generation,” “monthly generation,” “amount of electricity sold,” “self-consumption,” and “reduction in purchased electricity” are sometimes used interchangeably. However, these are actually different figures. Annual generation is the amount of electricity the system produces, while the amount of electricity sold is the amount that flowed to the grid. Self-consumption is the portion of generated power used on-site, and the reduction in purchased electricity indicates how much the amount of electricity bought from outside has decreased. This distinction is particularly important in the case of surplus sales.

Therefore, the first thing to decide is not the formula itself but the purpose of the calculation. The required level of detail also varies depending on whether it is a pre-installation study, a verification of estimate contents, or a post-startup performance evaluation. If it is before installation, a rough estimate may be enough to grasp the general direction, but if it will be used for investment decisions or internal briefings, you may need breakdowns by month, by time of day, and by weekdays versus holidays. After operations begin, you need to compare calculated values with measured values while checking weather conditions and equipment status.

Correctly understanding the difference between surplus electricity sales and full-volume electricity sales not only improves the accuracy of power generation calculations but also provides the foundation for avoiding misinterpretation of the calculation results.

Approach 1: Calculate generated and sold electricity separately

The first thing that changes between surplus feed-in and full feed-in is to what extent "generated energy" and "sold energy" can be treated as the same thing. In solar power generation output calculations, you calculate the amount of electrical energy a generation system produces over a given period. Generally, you estimate annual or monthly generation by taking into account system capacity, local solar irradiation conditions, orientation, tilt, temperature effects, and various losses. This generation figure is the basic number for evaluating the performance of the solar power system itself and its installation conditions.

However, the amount of electricity sold is not the same as the amount generated. With full-feed-in sales, since the generated power is basically sold, the relationship between generation and sales becomes relatively straightforward. Even so, conversions by the power conditioner, losses in cables, equipment downtime, differences in metering locations, and output control can cause discrepancies between the calculated generation and the actual amount sold. You should avoid assuming that, simply because it is full-feed-in sales, the generated amount will be sold as-is.

With surplus electricity sales, you need to think about things even more distinctly. The volume sold under surplus electricity sales is not well represented by simply subtracting a fixed percentage from total generation. The amount sold is the electricity that could not be consumed on-site at the moment it was generated. Therefore, even if annual generation is the same, the amount sold can vary greatly depending on daytime electricity usage patterns.

For example, in facilities that use a lot of air conditioning, lighting, machinery, and charging equipment during the daytime, generated power is easier to use on-site. In this case, surplus electricity sold back to the grid is reduced and self-consumption increases. Conversely, facilities with low daytime operation or many holidays, where daytime electricity demand is small, tend to have surpluses during periods of high generation. Even if the amount of electricity sold appears large, that may not align with objectives that prioritize reducing purchased electricity through self-consumption.

To clarify these differences, it is effective to divide the calculations into stages. First, calculate the power generation from the equipment conditions. Next, reconcile that generation with the site's electricity demand by time of day. Then, classify the portion of generation used on site as self-consumption and the portion that could not be used as surplus electricity sold. If you calculate only totals on a monthly or yearly basis, you may end up treating power generated during the day as if it were used at night, which can deviate from reality.

Especially in the case of surplus electricity sales, you should avoid comparing generation and consumption solely by the same monthly totals. Even if monthly consumption is greater than monthly generation, a surplus can occur if there is no demand during the times when power is generated. Conversely, even if monthly generation is less than consumption, selling to the grid can occur if demand during generation hours is low, while purchases may remain for nighttime or early morning. Because solar power generation is governed by sunlight, temporal alignment is critically important.

In calculations for full-feed-in sales, matching to time-of-day demand is not as important as it is for surplus feed-in. Of course, it is necessary to check seasonal variations in generation and solar irradiance conditions, but allocation calculations based on the presence or absence of self-consumption are not central. Therefore, full-feed-in assessments focus on items such as the generating capacity of the equipment, outage rates, the potential for output control, and losses up to the metering point.

On the other hand, for surplus electricity sales, demand-side information is indispensable in addition to the generation capacity of the equipment. By anticipating building or facility operating hours, peaks in electricity usage, holiday loads, seasonal air-conditioning loads, and demand drops during lunch breaks or downtime, the accuracy of calculations for self-consumption and electricity sales improves. In practice, checking past electricity usage records and daily usage patterns, and using time-of-day data when possible, makes it easier to make decisions.

Separating generated power and sold power also helps with verification after installation. You cannot determine by looking only at sold power whether generation is lower than expected, or whether generation is as expected but the self-consumption ratio differs from assumptions. If you manage generated power, sold power, and self-consumption separately, it becomes easier to isolate the effects of weather, equipment faults, and changes in demand patterns.

The basics of calculating solar power generation are to accurately estimate the amount generated. However, when considering the difference between selling surplus electricity and selling all generated electricity, it is essential, after calculating generation, to clarify where that electricity flows.

Approach 2: Change the Way You Estimate Self-Consumption

The most important factor in surplus electricity sales is estimating self-consumption. Under full-feed-in schemes, calculations assume that generated power is sold, so demand-side electricity usage patterns tend not to be central to generation calculations. In contrast, in surplus electricity sales, self-consumption directly determines the amount sold. Therefore, after calculating generation, how to estimate the portion that will be self-consumed becomes a major practical issue.

When estimating self-consumption, something to avoid is assuming “because annual electricity consumption is large, most of the generated power can be used.” Even if annual consumption is high, if usage is concentrated at night or in the early morning, it will not align with solar power generation. Conversely, even if annual consumption is not particularly large, facilities with a stable daytime load can have a high self-consumption rate.

Solar power generation fluctuates with weather and seasons, but it basically occurs during daytime. When calculating surplus electricity sales, you need to overlay this daytime generation curve with the site's electricity consumption curve. During periods when generation falls below demand, the generated electricity is more likely to be used for self-consumption. During periods when generation exceeds demand, the excess is allocated to surplus electricity sales. Simply adopting this way of thinking yields a calculation that more closely reflects reality than a simple comparison of annual totals.

The way to estimate self-consumption also varies by type of facility. Offices, factories, warehouses, retail stores, agricultural facilities, and other sites where people and machines operate during the daytime are more likely to have their generation period and power-use period overlap. However, even facilities of the same type can produce different results depending on holidays, operating shifts, air-conditioning usage, and the operating rate of production equipment. Therefore, rather than deciding based only on facility type, it is preferable to check actual electricity usage data.

The same applies to homes and small facilities. The amount of surplus electricity sold varies depending on whether you spend a lot of time at home during the day or hardly use electricity during the day. How much of the electricity generated during daytime you can use depends on lifestyle patterns and the operating hours of equipment. Combining the operation of storage systems and power-consuming devices also affects estimates of self-consumption. However, adding equipment does not necessarily lead to a greater effect, so it is important to check the relationship between generation times and usage times.

When calculating surplus electricity sales, attention must also be paid to monthly self-consumption. Solar power generation varies by season, and electricity demand also changes seasonally. In summer, air-conditioning loads can increase daytime electricity use, making self-consumption more likely. In winter, demand changes depending on how heating and water-heating systems are used. In spring and autumn, generation tends to be relatively higher while air-conditioning loads are lower, which can increase surplus. Estimating based only on the annual average can overlook differences in monthly electricity sales and reductions in purchased power.

Also, the difference between weekdays and holidays is important. For facilities that operate on weekdays, even if the self-consumption rate is high on weekdays, demand can fall on holidays and surpluses may increase. For facilities with year-end/new-year holidays, long-term shutdowns, or scheduled stoppages, generation during those periods may be less likely to be self-consumed. When using solar power generation calculations in practice, it is easier to explain the results if you clarify the number of operating days per year and how non-operating or holiday days are treated.

When all generated electricity is sold, calculating self-consumption is not usually a primary issue. The main considerations are the output of the generation equipment, generation conditions, losses, stoppages, and output control. Therefore, even when performing the same solar power generation calculations, organizing demand data becomes important for surplus sales, while organizing the conditions on the generation equipment side becomes important for full sales.

When estimating self-consumption, you also need to consider the granularity of the calculations. At the rough-estimate stage, you can grasp broad trends from monthly generation and monthly electricity consumption. However, when refining the appropriateness of system capacity and revenue/profit projections, checks by time of day often become necessary. Furthermore, in facilities where demand during peak hours is low, generation peaks are more likely to be sold back to the grid. Because the self-consumption rate may decrease as system capacity increases, comparing different capacities is also useful.

Under surplus feed-in, the more self-consumption increases, the less electricity is sold. However, that is not necessarily a bad thing. The reduction in electricity purchased from external sources due to self-consumption is also part of the installation’s benefits. Whether you want to increase only the amount sold or prioritize reducing electricity consumption through self-consumption will change the optimal system capacity and operating method. If you compare only generation output without clarifying these differences in objectives, your judgment can easily be skewed.

In other words, when calculating generation for surplus electricity sales, estimating self-consumption is as important as calculating the generated output. While selling all output focuses on the stability of the produced energy, with surplus sales the key to improving calculation accuracy is focusing on whether the generation can be used at the time it is produced.

Approach 3: Consider how to reflect losses and output control in the supply-demand balance

When calculating solar power generation, you need to consider not only simply estimating output from the installed capacity but also the losses that occur in actual operation. Whether for surplus power sales or full-output power sales, the concept of losses itself is important. However, which figures those losses affect and how they are reflected in the financial balance and perceived benefits depends on the sales scheme.

Losses to be considered in solar power generation include output reductions due to temperature rise, conversion losses in the power conditioner, wiring losses, the effects of dirt and shading, output degradation with aging, equipment outages, and shutdowns during inspections. These are factors that reduce the amount of electricity generated itself. If generation falls, under a full-feed-in scheme the amount sold tends to decrease. Under a surplus-feed-in scheme, reduced generation can affect both self-consumption and the amount sold.

When all generated electricity is sold, losses appear relatively straightforwardly as a reduction in the amount of electricity sold. The amount of electricity remaining after subtracting losses from the calculated generation becomes the basis for revenue from electricity sales. Therefore, it is important to estimate the expected annual generation, the operating rate, equipment downtime, and the potential for output curtailment. Assuming an excessively high generation will widen the gap with actual performance and affect financial projections.

In surplus electricity sales, the way the same loss manifests is a bit more complex. If a loss occurs during a period when generation exceeds demand, it may mainly reduce the amount of surplus sold. Conversely, if a loss occurs during a period when generation is below demand, the amount of electricity available for self-consumption decreases and the amount purchased from external sources may increase. In other words, whether the loss appears as a reduction in sold electricity or as a decrease in the effect of reducing purchased electricity depends on the supply–demand relationship during the time period.

Therefore, when it comes to surplus electricity sales, assessing losses only by applying a single annual correction makes it difficult to understand the breakdown of effects. Correcting the total amount of generated energy itself is necessary, but it is important to verify how the corrected amount of electricity is split between self-consumption and sales. In particular, for plans that prioritize self-consumption, it is crucial to know how much generation can be secured during daytime hours when demand is high.

Perceptions of output curtailment also vary depending on the electricity sales method. Output curtailment refers to the suppression of a power generation facility’s output due to grid-side conditions or other factors. When output curtailment occurs, the amount of electricity that would have been generated may not actually be generated or transmitted. Under full feed-in sales, the curtailed portion is easily regarded as a reduction in the amount sold. Under surplus sales, it is necessary to clarify whether it affects self-consumption or the portion sold.

In practice, when accounting for the effects of output curtailment or shutdown, you should be clear about which amount of electricity is being deducted. Whether it is deducted from available generation, from the amount available for sale, or considered to affect only the surplus after self-consumption will change the results. Because the treatment varies depending on equipment configuration, contract terms, and the control mechanism, it is important not to draw conclusions based solely on a simple fixed percentage.

Also, if you underestimate losses in calculations, the amount sold will be overestimated under a full-feed-in scheme, and both self-consumption and the amount sold will be overestimated under a surplus-feed-in scheme. Conversely, if you overestimate losses, you will underestimate the benefits of installing the equipment. When calculating generation, separating and checking optimistic, standard, and conservative scenarios makes it easier to provide a range for decision-making.

In handling losses, monthly differences should not be overlooked. Temperature effects change with the seasons. At high temperatures, solar cell output tends to decrease, and even during periods of high solar irradiance, generation may not increase as much as expected. The impacts of soiling and shading also vary with the season and the surrounding environment. Fallen leaves, snow accumulation, shading from nearby structures, and differences in irradiance conditions due to topography can be difficult to detect in annual averages.

In the case of selling all generated electricity, such losses are reflected in the financial results as a reduction in generated output. In the case of surplus electricity sales, you check whether the reduction in generation is affecting self-consumption or electricity sales. For example, at facilities with high daytime demand, a drop in generation may appear as a shortfall in self-consumption. At facilities with low daytime demand, it may appear as a reduction in the amount of surplus electricity sold.

To understand these differences, it is effective not to leave the results of power generation calculations as a single annual value but to break them down by month and by time of day. When selling all generated power, forecasts tend to focus on expected annual sales volumes, but viewing them month by month makes it easier to detect equipment abnormalities and seasonal differences. For selling surplus power, not only monthly breakdowns but also time-of-day matching of supply and demand is important.

In solar power generation calculations, including losses is not the goal in itself. What matters is understanding which financial items the calculation results affect after accounting for losses. Under full-feed-in (selling all generated power), distinguish impacts on the amount sold; under surplus-feed-in (selling only surplus), distinguish allocation impacts between self-consumption and the amount sold. Doing so makes the calculations more practical for real-world use.

Approach 4: Change the criteria for comparing track records

During operation after installation, there are situations where calculated values are compared with actual results. At that time, the criteria for comparison change between surplus electricity sales and full-volume electricity sales. When evaluating operations using the results of solar power generation calculations, simply looking at whether the amount of electricity sold is higher or lower than expected can lead to misjudging the cause.

Under a full-feed-in arrangement, the focus of performance comparisons is generation output or the amount of electricity sold. You check how the actual electricity sold compared to the expected monthly generation, and distinguish factors such as solar irradiance conditions, equipment outages, output curtailment, soiling, shading, and equipment faults. Because demand-side electricity usage has little direct impact on the amount sold under full-feed-in, you can concentrate checks on the generation equipment and external conditions.

In the case of surplus power sales, it is difficult to assess the condition of the generation equipment by looking only at the amount of power sold. If the amount sold is lower than expected, it may be because generation was low or because self-consumption increased more than expected. Conversely, if the amount sold increases, it may not be due to increased generation but simply because on-site daytime demand decreased and the surplus grew. Therefore, for surplus power sales, it is necessary to check generation, sales, self-consumption, and purchased power together.

For example, suppose the amount of electricity sold as surplus power has decreased compared with the same month of the previous year. In this case, before concluding there is an abnormality in the power generation equipment, you need to check electricity consumption for the same period. If daytime operation has increased and self-consumption has risen, the amount sold may decrease even though there is no problem with the generation equipment. Conversely, even if the amount sold remains unchanged, if self-consumption falls and generation also decreases, attention should be paid to the equipment condition and operational conditions.

Under a full-feed-in arrangement, a decrease in the amount of electricity sold relatively directly leads to checks on the generation side. However, it is still important to assess while excluding the effects of weather. In months with low solar irradiance, a drop in generation is natural. Comparing only with the same month of the previous year may not sufficiently reflect differences in weather. For performance comparisons, it is desirable to view calculated generation values, past performance, solar irradiance conditions, and equipment outage information together.

With surplus electricity sales, it is necessary to also account for demand fluctuations. Factors such as increased factory production, changes in store opening hours, changes in air-conditioning operation, a different number of holidays, or the addition of equipment can change on-site self-consumption. Even if there is no fault in the generation equipment, the amount of electricity sold can vary greatly. Therefore, when evaluating the performance of surplus electricity sales, it is important not to directly attribute increases or decreases in sold electricity to the quality of power generation.

In performance comparisons, pay attention to differences in measurement points. The generation output recorded on the power-generating equipment side, the energy measured at the receiving point or by the meter used for electricity sales, and the building's total electricity consumption each have different meanings. For surplus electricity sales, it is necessary to clarify which measurement value is being used to calculate self-consumption. When the measurement points differ, the way conversion losses and wiring losses are treated also changes.

Also, setting the period for performance comparisons is important. On a daily basis, weather has a large impact and causes variability. Monthly comparisons make trends easier to see, but are affected by differences in the number of holidays and operating conditions. On an annual basis, the overall trend is easier to view, but it becomes harder to pinpoint when anomalies occurred. For full-volume power sales, comparisons of sold power tend to focus on monthly or yearly amounts, while for surplus power sales it is often necessary to verify a combination of time-of-day, daily, and monthly comparisons.

What's particularly useful for surplus electricity sales is looking at generation output and the self-consumption rate together. The self-consumption rate indicates the proportion of generated electricity that was consumed on-site. If this ratio is higher than expected, the amount sold tends to decrease, but it can also be seen as progress in on-site utilization. If it is lower than expected, there may be a large surplus, which provides an opportunity to check whether equipment capacity and operating methods match demand.

Under a full-feed-in scheme, we focus more on equipment utilization, generation efficiency, downtime, and output curtailment impacts than on the self-consumption rate. If actual results are lower than calculated values, check in order: differences in solar irradiance conditions, shading, soiling, equipment shutdowns, and communication or measurement malfunctions. Do not judge based only on the amount of electricity sold; by examining the operating status of each installation and month-by-month changes, it becomes easier to narrow down the cause.

In practice, during the pre-installation calculation stage, it is easier to manage if you decide in advance which figures you will compare after installation. For surplus power sales, design the system to separately manage generated power, self-consumption, surplus power sold, and purchased power, which makes later verification easier. For full power sales, it is effective to organize generated power, sold power, downtime, and whether output control is applied.

The calculation of solar power generation is not something that ends with the pre-installation forecast. It is also used to compare with post-installation actual performance to check the condition of the equipment and changes in operation. If you understand that the criteria for comparison differ between surplus sales and full-feed-in sales, you can avoid being swayed solely by increases or decreases in the amount of electricity sold and make more accurate judgments.

How to Avoid Mistakes When Calculating Power Generation for Surplus Feed-in and Full Feed-in

When calculating power generation while taking into account the differences between surplus feed-in and full feed-in, it is important to clearly identify the items that need to be organized before performing the calculation. The first thing to confirm is whether the calculation target is the amount of power generated, the amount sold, or the amount consumed for self-use. If this distinction remains unclear, it will be difficult to explain the calculation results and may cause misalignment among stakeholders.

At the stage of calculating power generation, we confirm the system capacity, installation location, azimuth, tilt, solar irradiation conditions, temperature effects, loss rates, presence or absence of shading, assumptions about equipment downtime, and so on. This is common to both surplus feed-in and full feed-in. Unless you estimate how much electrical energy the generation equipment can produce, subsequent calculations of the amount sold and self-consumption cannot be carried out.

Next, we outline the branches for each electricity sales scheme. In the sell-all scheme, we reflect required losses and the effects of downtime in the generated output and confirm the expected amount of power to be sold. In the surplus-sale scheme, we match generation with on-site demand and split it into self-consumption and surplus for sale. It is important to note that, in surplus-sale calculations, information from the demand side influences the calculation accuracy.

Demand information required for calculating surplus electricity sales includes monthly energy consumption, energy consumption by time of day, differences between weekdays and holidays, seasonal equipment operation, long-term shutdowns, and daytime load fluctuations. Even if you cannot grasp all of these in detail, it is important to clearly state the assumptions used for the calculations. If the assumptions are explicit, you can later compare them with actual results and determine which conditions deviated.

In calculations for selling all generated electricity, assumptions about the equipment side are more important than demand information. Confirm the total system capacity, the power conditioner (inverter) capacity, whether there is overloading (oversizing), wiring distance, installation angle, effects of shading, the possibility of output control, and downtime due to maintenance. In selling all generated electricity, because increases or decreases in generation are easily reflected in the amount sold, it is important not to be overly optimistic about generation-side conditions.

When performing calculations, it's important not to rely on a single result. Solar power generation is affected by weather, so actual performance may not necessarily match calculated values. In addition to the expected annual generation, checking monthly variations and adopting conservative assumptions will broaden the range of explanations. For surplus electricity sales, also verify the impacts of increased or decreased demand and of changes in the self-consumption rate to make it easier to use in practice.

Also, when deciding equipment capacity, the points of emphasis differ between surplus power sales and full power sales. For full power sales, taking into account the installable capacity, generation efficiency, grid conditions, and the impact of output control, you assess whether generation can be secured stably. In surplus power sales, if equipment capacity is made too large the surplus increases and may not align with plans that prioritize self-consumption. It is necessary to consider this while balancing the maximization of generation and the self-consumption rate.

What practitioners often overlook are the names of the calculation results. If “generated electricity,” “sold electricity,” “consumed electricity,” and “reduction” are mixed together in the same document, readers can easily be misled. For surplus power sales in particular, notation that does not confuse generated electricity with sold electricity is required. Even with total power sales, the generation measured on the generating equipment side and the amount sold at the metering point can differ, so it is necessary to clarify which location the electricity quantity refers to.

Furthermore, if you are looking ahead to post-installation management, it is important to record the assumptions used in the calculations. Keeping a record of solar irradiance conditions, loss rates, assumed equipment outages, self-consumption rates, the period of demand data, how holidays are treated, and so on will make it easier to verify when actual results differ. If only the calculation results remain and the assumptions are unknown, it becomes difficult to trace the cause of discrepancies.

In surplus power sales, even if generation is as expected, the amount sold varies with changes in demand. In full-feed-in sales, equipment condition and solar irradiance have a greater effect than demand fluctuations. Bearing this difference in mind, when explaining calculation results, organizing "which numbers change and which outcomes they affect" makes it easier to explain to stakeholders.

The purpose of calculating solar power generation is not simply to determine the amount of electricity generated. It is to produce figures that can be used for equipment planning, forecasts of electricity sales, the effects of self-consumption, operations management, and anomaly detection. By organizing the calculations with an awareness of the difference between surplus sales and selling all output, you can make the results more useful in practice.

Summary

Even though the basic calculations for solar power generation are the same for surplus feed-in and full feed-in, the way you interpret the calculation results changes significantly. With full feed-in, the focus is on how stably the generated electricity can be sold. With surplus feed-in, the focus is on how much of the generated electricity is self-consumed and how much is sold as surplus.

Treating power generation and electricity sold separately is the first important point. Under a full-feed-in arrangement, power generation and electricity sold tend to be closely related, but differences due to losses, outages, and output control must be taken into account. With surplus feed-in, the electricity sold is the amount remaining after subtracting self-consumption from generation, so an accurate assessment cannot be made without examining demand-side electricity usage patterns.

When estimating self-consumption, annual electricity consumption alone is not sufficient. You need to check whether the times when power is generated and when it is used coincide, how demand changes between weekdays and holidays, and how air conditioning and equipment operation vary by season. For selling surplus electricity, the degree of alignment with this demand significantly affects calculation accuracy.

The way losses and output control are handled also appears differently depending on the electricity sales scheme. With full feed-in, a drop in generation tends to show up as a drop in the amount sold. With surplus sales, it is important to distinguish whether losses affect self-consumption or the surplus electricity sold. Rather than simply subtracting a fixed percentage, you need to confirm which revenue/expenditure items the calculation results are impacting.

Even in comparisons of post-installation performance, the criteria to consider differ between surplus feed-in and full feed-in. With full feed-in, you check equipment condition and solar irradiation conditions focusing on generated output and the amount of electricity sold. With surplus feed-in, you need to look not only at the electricity sold but also self-consumption, purchased electricity, and demand fluctuations together. If you simply conclude that low electricity sales indicate generation problems or that high electricity sales mean everything is going well, you may misinterpret the actual situation.

To make solar power generation calculations usable in practice, it is important to separate and organize generation, electricity sold, self-consumption, and reductions in purchased electricity, and to document the assumptions used in the calculations. Under surplus-sale arrangements, cross-checking with demand data is particularly important; under full-sale arrangements, verifying equipment-side conditions and operational stability is especially important.

Power generation calculations are involved not only in pre-installation estimates but also in considering equipment capacity, internal explanations, post-operation performance evaluation, and early detection of abnormalities. Calculating with the differences between surplus power sales and full-feed-in power sales in mind clarifies the meaning of the numbers and improves the accuracy of decisions. If you want to practically organize power generation calculations for solar power generation facilities and site-by-site generation forecasts, it is important to establish a system that can separately manage power generated, electricity sold, self-consumption, and electricity purchased.