4 methods to calculate solar power generation including equipment degradation

In calculations of solar power generation, the value of a system is sometimes judged solely by the first-year annual kWh. Of course, first-year annual generation is convenient for comparing system sizes and for early-stage planning. However, if you want figures that are truly usable in practice, you must incorporate system degradation into your assessment. This is because a solar power system's performance at the moment of installation does not remain constant, and its output characteristics change gradually from year to year.

In particular, what practitioners searching for "太陽光発電量 計算" want to know is not merely the theoretical first-year kWh, but how much generation can be expected including several years out and how to translate those figures into financial performance and payback period. What becomes important here is clarifying which unit to use to assess degradation, whether to treat it as an annual rate, whether to separate the initial drop from the subsequent trend, whether to consider the period up to equipment replacement, and whether to include impacts on self-consumption.

Also, when we talk about equipment degradation, it does not necessarily mean only a drop in the output of the solar panels themselves. Conversion equipment, wiring, dirt, heat, the surrounding environment, and operating conditions are just some of the multiple factors that can reduce actual power generation. Therefore, how you model degradation significantly changes the projected energy output. If you leave this ambiguous and simply explain it as “it will decrease slightly in the long term,” both the equipment assessment and the explanation are likely to be weak.

This article organizes and explains four methods for incorporating equipment degradation into power generation calculations. These are: viewing degradation as an annual rate, separating initial degradation from subsequent degradation, incorporating replacements of major equipment, and extending the analysis to cover self-consumption and recovery. The material is summarized so that the flow of calculations connects naturally and can be used directly in practice even without diagrams.

Table of Contents

• Basics to Understand Before Accounting for Equipment Degradation

• Method 1 Calculate by Incorporating a Constant Annual Degradation Rate

• Method 2 Calculate Initial Degradation and Subsequent Age-Related Degradation Separately

• Method 3 Calculate by Separating Replacement Timing of Major Equipment

• Method 4 Calculate Including Degradation for Self-Consumption Volume and Payback Period

• Common Misconceptions in Calculations Including Degradation

• Order in Which Practitioners Create Degradation-Inclusive Estimates

• Summary

Fundamentals to Grasp Before Accounting for Equipment Degradation

When considering solar power generation including equipment degradation, the first thing to clarify is the difference between kW and kWh. kW is the output capacity of the equipment; for example, figures like a 5 kW system or a 10 kW system indicate the size of the installation. In contrast, kWh is the amount of electricity actually generated. Degradation mainly affects this kWh. In other words, rather than the installed capacity itself decreasing numerically, it's easier to understand if you think that even with the same installed capacity, the amount of electricity that can be generated gradually changes year by year.

For example, even if a system can generate 10,000 kWh in its first year, it doesn’t necessarily maintain the same 10,000 kWh a few years later. Depending on the condition of the equipment and surrounding circumstances, the amount of power generated may gradually decline. How to model this “gradually” is the starting point for calculations that include equipment degradation. In other words, considering degradation is not about changing the system’s kW rating, but about deciding how the annual kWh output will change.

Also, the reason for taking degradation into account lies in long-term equipment evaluation. If you calculate the payback period using only the first year's annual power generation, it tends to diverge somewhat from the reality of long-term operation. This is especially true for projects that assume self-consumption or include electricity sales: small year-to-year changes in generation alter the amount of savings and how revenue is realized. In other words, degradation is not merely a technical matter—it is connected to cash flow and investment decisions.

Furthermore, misinterpreting deterioration can cause the numbers to appear either overly strong or, conversely, too weak. If you simplistically assume a constant annual decline rate, you may misjudge the initial pattern. Conversely, if you ignore deterioration entirely, you may overstate the long-term value of the equipment. That is why it is important to clarify which period you are looking at and which equipment elements you are considering, and to use an approach that incorporates deterioration.

In other words, considering solar power generation with equipment degradation means putting numbers on the time axis beyond the first year’s generation. Understanding this makes the following four methods much easier to organize.

Method 1 Calculate by incorporating a constant annual degradation rate

The first method is a calculation that incorporates an annual degradation rate at a constant percentage. This is the most basic approach and the easiest to use in practice. Using the first year’s annual power generation as the baseline, assume that generation decreases by a fixed percentage each year and list the generation for each year. It is suitable when you want to roughly grasp long-term trends without introducing complex assumptions.

A practical approach is to first calculate the annual generation in the first year, and then apply the annual degradation rate to that value for the second year onward. For example, if the first year's annual generation is 10,000 kWh and you assume an annual degradation of about 0.5%, the second year will be slightly lower than 10,000 kWh, and the third year slightly lower still. In words, the generation in each year is "first-year annual generation × remaining rate due to aging". This remaining rate is the figure that reflects the year-by-year degradation.

The good thing about this method is that it is very easy to understand. For comparing system sizes, making long-term estimates, or roughly assessing payback periods, this method is often perfectly adequate to use first. It makes it easy to visualize what the first year’s annual kWh will be and how much it is likely to decline after 10 or 15 years. In other words, it is an excellent first step for adding a time dimension to long-term power generation estimates.

This method is also suitable for comparing system capacities. For example, when comparing a 5 kW system and a 10 kW system, you can line up and consider not only the difference in the first year but also the difference after 10 years. Assuming the same degradation rate, larger system sizes will produce more total generation over the long term, but this method makes it easier to see to what extent that difference is maintained. In other words, it is a convenient way to compare system value not only in the first year but also over the medium to long term.

However, there are caveats to this method. By treating all degradation as "declining at the same rate each year," it simplifies actual equipment behavior. As will be discussed later, in practice it may be more appropriate to assume that the initial behavior and the subsequent steady-state behavior differ. Even so, creating an initial estimate that includes degradation is still quite meaningful and very easy to use as a starting point. The basic approach for calculations that include degradation is to first try incorporating a constant rate.

Method 2 Calculate initial deterioration and subsequent age-related deterioration separately

The second method is to calculate by separating initial degradation from subsequent aging-related degradation. This is a step beyond the constant annual rate method. When considering the long-term behavior of solar power systems, rather than treating the way performance drops in the first year and the way it declines each subsequent year as the same, it is often easier in practice to explain if you separate the initial change from the later steady change.

The idea is to reflect initial performance changes in the first year or the first few years of power generation, and then have it evolve at a smaller annual rate afterward. For example, even if the annual generation in the first year is 10,000kWh, rather than keeping that level unchanged in subsequent years, you would start from a baseline that has been slightly adjusted initially and then apply the annual degradation rate thereafter. Doing so makes it easier to explain the equipment’s aging over time in a natural way, compared with simply assuming it declines at the same rate every year.

The advantage of this method is that it makes long-term explanations more persuasive. In practice, first-year results can differ slightly from theoretical values. Rather than immediately attributing those differences entirely to ageing-related deterioration, separating the initial settling from the subsequent trend increases the credibility of the numbers. In other words, if a constant annual-rate approach is a simple entry point, this method can be regarded as a model that is somewhat more grounded in on-site practice.

It is also easy to use for projects with different purposes, such as factories, warehouses, and residences. This is because, in the early stages of operation, not only the condition of the equipment itself but also how it gets dirty, the temperature conditions, and the management status may not yet be firmly understood, and actual performance may stabilize later. In other words, it means you have room to adjust the initial assumptions while watching the differences from the actual results.

However, what you need to watch out for with this method is not making it too complicated. There is meaning in separating the initial phase and what follows, but if you introduce an overly detailed model from the start, the many assumptions can actually make it harder to handle. In practice, it is clearer to first capture the outline with a constant-rate model and then, if necessary, separate the initial and stable phases. In other words, this method is effective when you want to make the entry point a little more on-the-ground.

Method 3: Calculate by Staggering the Replacement Timing of Major Equipment

The third method is to calculate by separating the replacement timing of major equipment. When considering power generation including equipment degradation, it can be somewhat insufficient in practice to stop after looking only at changes in panel output. This is because an entire solar power system is composed of multiple pieces of equipment, some of which are better considered for long-term upgrades or replacement. In other words, if you want to view time-based changes in power generation more practically, you should also be mindful of the separate timing for major equipment.

The approach is to create a baseline for annual power generation and then treat the timing when equipment replacement is expected as a separate segmentation. This makes it easier to organize the overall equipment timeline in a more practical, operational way than viewing output as simply declining at the same rate every year. For example, if you allow for the possibility that system conditions may change at a certain time, long-term generation estimates can be given a more realistic range.

The advantage of this approach is that it does not treat the entire installation as a single "degrading box." In practice, the long-term operation of an installation involves considerations such as inspections and equipment replacements. In other words, it's not only the panels' power-generation characteristics that change over the years; the way the installation as a whole is maintained affects how power output is perceived. When discussing the long-term asset value of an installation, being aware of this distinction makes it easier to explain.

Also, this method is particularly effective for projects where payback and long-term cash flow are being evaluated. Rather than simplifying the first year through the 20th or 30th year into a single continuous period, inserting intermediate breakpoints makes long-term projections more realistic. This is because estimates that include equipment degradation lead not merely to a discussion of simple annual declines but to an equipment assessment that incorporates maintenance and upkeep.

However, it is important not to make things too complicated here either. If you try to include the timing of equipment replacements and their impacts in strict detail, the assumptions will multiply and it will become unwieldy. In practice, it is sufficiently meaningful to simply incorporate the idea that, over the long term, there are also breakpoints for major equipment. In other words, it is helpful to regard this method as an approach to make degradation-inclusive power generation estimates a little closer to the operational realities of the facility.

Method 4 Calculate including degradation for self-consumption and payback period

The fourth method is to calculate, including degradation, the self-consumption and the payback period. The real purpose of considering generation that accounts for equipment degradation is not merely to know the long-term generation amounts themselves. It is to understand how changes in that generation will affect self-consumption and surplus energy, electricity bill savings, the amount of electricity sold, and the payback period. In other words, degradation-inclusive calculations are not only for reading the trajectory of kWh, but also for interpreting the time-based change in equipment value.

For example, consider a project with an annual generation of 10,000 kWh in the first year, of which 4,000 kWh can be self-consumed. If this system's generation declines slightly each year, the amount of kWh that can be self-consumed will also change gradually in the future. The surplus will, of course, change as well. In other words, if you calculate electricity cost savings using only the first year's self-consumption amount as fixed, you may overstate the system's value in the long term.

The payback period is the same. If you simply calculate the payback period based only on the first year’s economic effect, it tends to deviate somewhat from the reality of long-term operation. Of course this can be used as an initial estimate, but if you factor in equipment degradation, it’s more natural from a long-term perspective to pay at least some attention to year-to-year changes in power generation. This perspective is especially important for projects that emphasize self-consumption, because the amount of savings tends to change in line with any reduction in generation.

Also, an advantage of this method is that it can be used to compare system sizes. While the difference between 5 kW and 10 kW is easy to see from first-year annual generation, when viewed over the long term it is hard to see how that difference will persist and how the self-consumption rate will change unless degradation is included. In other words, if you want to compare long-term equipment value, it is more practical to include degradation and carry the analysis through to self-consumption and payback.

Using this method, calculations that include equipment degradation shift from a technical discussion of “declining slightly each year” to an economic discussion of “how useful it will be over the long term.” In practice, this often becomes the most important point. Connect the trend in power generation to the trend in equipment value. With this perspective, degradation-inclusive estimates become quite meaningful.

Common misconceptions in calculations that include degradation

One common misconception in calculating power generation that includes equipment degradation is believing that the more degradation you assume, the more realistic and better the result will be. Of course, including degradation in long-term equipment evaluations is important. However, if you assume too high a degradation rate, or stack too many factors—constant annual rate, initial changes, equipment replacement, and losses—you will instead undervalue the equipment. In other words, including degradation does not mean that assuming a stronger effect is better.

Another common issue is confusing equipment degradation with general losses. For example, if you put high-temperature losses, conversion losses, and the effects of soiling entirely into an annual degradation rate, the way you view first-year annual power generation becomes ambiguous. Conversely, if you do not separate first-year losses from long-term degradation, it becomes difficult to identify where the power generation is declining. In other words, although degradation and losses may appear closely related, in practice it is clearer to treat them as distinct items with different time horizons.

Also, it's a common misconception to base evaluations solely on first-year generation and simply extend the payback period or savings unchanged into the long term. While that can be used as a rough estimate for initial screening, it is somewhat insufficient to explain the long-term asset value. Conversely, if you want to examine long-term degradation but completely ignore changes in self-consumption rate or surplus energy, the asset value can easily be seen as either too high or too low. In other words, when performing calculations that include degradation, it's better to consider not only the kWh of generation but also how that energy is used.

Furthermore, there are cases where the degradation rate is treated as a single fixed value without any supporting rationale. Assuming an annual rate in itself is not wrong, but unless you are clear whether that value is being assumed to remain the same from the first year onward or to represent an average that includes initial changes, it becomes difficult to explain. In other words, rather than the degradation rate number itself, it is important to clarify what that number actually means.

Order in which practitioners prepare estimates including degradation

If you are preparing a power generation estimate that includes equipment degradation in practice, the first priority is to produce as reasonable an estimate as possible of the first year's annual generation, taking into account system capacity, local conditions, orientation, shading, and losses. If you include degradation while this remains unclear, the long-term projection will be of little use. In other words, the foundation of any calculation that includes degradation is, after all, a reasonable first-year generation figure.

After that, the next things to look at, in order, are whether a constant annual rate is sufficient, whether it’s better to separate the initial change from what follows, and whether you want to examine it down to the breakdown by major equipment. For an initial rough estimate, a constant rate is often enough, while if a longer-term explanation is required it can be better to divide it a bit. It’s most practical to deepen this step by step according to the project’s objectives.

Furthermore, keeping self-consumption and surplus separate makes the long-term value of the system much easier to understand. This is because you can view not only generation but also the overlap with daytime demand, the meaning of the self-consumption rate, and the impact on savings and payback in a single, continuous view. In other words, estimates that account for degradation should be regarded not merely as tables of declining generation but as tables showing the progression of the system’s value.

Finally, to improve the accuracy of these long-term estimates, the precision of acquiring site conditions is, after all, crucial. If equipment capacity, effective area, shading conditions, orientation, or tilt are ambiguous, first-year kWh will vary, and that variation will carry through into long-term generation. For that reason, estimates that include degradation require carefully confirming the initial site conditions.

Summary

To calculate solar power generation output including equipment degradation, there are four basic approaches: incorporating a constant annual degradation rate, separating initial degradation from subsequent long-term degradation, segmenting the timeline to account for the replacement timing of major components, and linking self-consumption volumes with the payback period while including degradation. Each serves a different purpose, and it’s easy to choose between them—use a simple method for an initial rough estimate, and a more site-specific approach for long-term equipment evaluation.

What’s important is not to understand accounting for degradation merely as “it decreases a little every year.” You need to take a reasonable first-year generation as the foundation and, from there, consider how to view subsequent changes in kWh, how to change the meaning of self-consumption and surplus, and how to link those to payback and long-term value. In other words, calculations that include equipment degradation are not just for knowing long-term generation but for assessing the true value of the equipment.

Also, if you truly want to improve this level of accuracy, it is essential to accurately understand the on-site conditions. If elements such as roof edges, obstacles, elevation differences, the way shadows fall, and the locations of equipment remain ambiguous, the first-year kWh will fluctuate, and that fluctuating value will then be repeatedly multiplied by the annual degradation rate. In particular, usable area and shading conditions can greatly influence the starting point of long-term projections.

In that regard, LRTK, an iPhone-mounted GNSS high-precision positioning device, is highly effective as a means of accurately capturing on-site spatial relationships. Because it makes it easier to accurately record the positions of roof edges and obstacles on site, it becomes easier to improve the accuracy of first-year power generation estimates that take into account orientation, shading, and layout conditions. If you want solar power generation figures that are genuinely usable, including equipment degradation, properly capturing site conditions with measures such as LRTK is a significant practical advantage.