7 Calculation Axes for Assessing Solar Power Plant Prices and Annual Revenue

Table of Contents

• Assess the price of a solar power plant by examining the details of its annual revenue

• Calculation axis 1: Use actual annual generation as the baseline

• Calculation axis 2: Reflect electricity sales conditions and the remaining contract period in revenue

• Calculation axis 3: Subtract maintenance and fixed costs

• Calculation axis 4: Include equipment degradation and future repairs in revenue forecasts

• Calculation axis 5: Anticipate downtime risk and output limitations

• Calculation axis 6: Reflect land conditions and on-site management burdens in revenue

• Calculation axis 7: Consider the purchase price and the payback outlook separately

• Mistakes to avoid when comparing annual revenues

• Summary: Assess annual revenue in conjunction with on-site evidence

Judge the price of a solar power plant by examining the breakdown of its annual revenue

When considering the purchase or acquisition of a solar power plant, many practitioners compare the price and annual revenue side by side. They assess, against the proposed price, how much revenue from electricity sales can be expected and how much profit is likely to remain annually, using that to judge whether the deal is good. This is a natural practical workflow, but if one judges without checking the composition of the annual revenue, even projects that look good on the surface may result in unexpected burdens after purchase.

Annual revenue from a solar power plant is not determined solely by simple power sales revenue. It is decided by a combination of generation output, power sale terms, remaining contract period, operation and maintenance costs, fixed costs, equipment degradation, future repairs, downtime risk, land conditions, and ease of on-site management.

Even projects that appear to have large power sales revenue can yield little actual remaining income if the burdens of mowing, inspections, insurance, repairs, and emergency responses are high. Conversely, a project whose power sales revenue does not stand out can be easier to handle in the long term if its generation performance is stable, it is easy to manage, and repair risks are low.

Anyone searching for "solar power plant price" is likely looking for more than just the market price. They need to know whether the quoted price corresponds to reasonable annual revenue, what assumptions were used to calculate that revenue, and whether that revenue will continue in the future. Especially for used solar power plants, you cannot judge the reliability of annual revenue without reconciling past generation performance with the current on-site condition.

When checking annual revenue, the important thing is not to take projected figures at face value. Verify which period's generation performance is being used, whether only abnormally good years are being used as the basis, whether plant outages and output restrictions are reflected, whether operations and maintenance costs and fixed costs have been omitted, and whether future repair costs are being anticipated. Annual revenue is both a calculated result and an indicator that reflects the actual condition of the power plant.

This article organizes seven calculation axes for evaluating the price and annual revenue of solar power plants. Rather than specific monetary amounts, it focuses on the concepts that practitioners should verify before purchase, during comparative evaluation, in internal approval processes, and in price negotiations. Use this as a standard for making judgments that fit local conditions, and don’t be misled by the superficial appearance of the numbers.

Calculation Axis 1: Use Actual Annual Electricity Generation as the Reference

The primary metric for estimating annual revenue is the actual annual power generation. Revenue from a solar power plant’s electricity sales is determined based on the amount of electricity actually generated. Therefore, rather than judging annual revenue solely by installed capacity or simulations, it is important to use past generation performance as the basis. Especially for operational plants, since performance data can be verified, actual results should be weighted more heavily than desk-based assumptions.

However, it is dangerous to judge based solely on the annual total of power generation. Even if it appears stable over the year, looking at monthly data may reveal that generation drops in specific seasons. Vegetation can grow in summer and cast shadows on the panels, surrounding trees or terrain can cast longer shadows in winter, output can be affected by falling leaves or snow, and equipment outages or maintenance stoppages may occur—monthly trends can reveal site-specific issues.

Also check power generation performance over multiple years. Even if a single year’s performance is good, that year may simply have had particularly favorable weather. By confirming whether performance is stable over multiple years, is declining year by year, or has suddenly dropped from a certain point, it becomes easier to read signs of equipment degradation or changes in the local environment. If performance is declining year by year, factors such as dirty panels, malfunctioning power conversion equipment, deterioration of cables and connections, growth of surrounding trees, or insufficient vegetation management may be involved.

When using power generation performance to estimate annual revenue, it is important not to base the estimate only on good years. Even when using the historical average, you need to check the causes of any outliers. If there was a year in which generation fell sharply due to equipment failure, confirm whether that failure has been repaired or whether a risk of recurrence remains. Conversely, if there are years with exceptionally high generation, determine whether the reason was weather or superior maintenance/management.

For low-priced projects, verify that annual revenue calculations are not based on overly optimistic power generation estimates. For high-priced projects, assess whether stable power generation performance can be presented as the basis for the price. Annual power generation is the foundation of revenue calculations, and if it remains uncertain, you cannot determine whether the price is reasonable.

Calculation Axis 2: Reflecting electricity sales conditions and the remaining period in revenue

When calculating annual revenue, the terms for selling electricity and the remaining period are key factors. Even if power generation is the same, annual revenue from electricity sales will change depending on the sales conditions. Also, for operational solar power plants, a certain period has already passed since the start of operations, so you cannot make long-term revenue assessments without checking the remaining period.

When reviewing the power sale conditions, clarify the contract terms, the start date of operations, certification-related information, the procedures required for name changes or succession, interconnection conditions, and whether there are any output restrictions. Even if the conditions appear favorable on the documents, if there are inconsistencies in the location, equipment capacity, contract name, or scope of management, additional checks or procedures may be required after acquisition. It is important to confirm that the power sale conditions used to calculate annual revenue are conditions that can actually be sustained.

The remaining term affects not only annual revenue itself but also investment and repair decisions. If the remaining term is long, many opportunities for future power sales remain, but you must also anticipate equipment deterioration and future repairs accordingly. If the remaining term is short, you need to carefully assess whether you can obtain stable revenue over a short period and how much impact a major repair would have if it occurs.

When calculating annual revenue, consider not only the single-year electricity sales income but also how revenue is likely to evolve over the remaining term. Even if current power generation is stable, future generation may change due to panel soiling, the timing of power conversion equipment replacements, vegetation growth, inadequate drainage, and changes in management practices. The terms of electricity sales and the remaining term are important factors that connect current revenue to future revenue.

Check the output limits and shutdown history as well. Even if the generation equipment is capable of operating sufficiently, opportunities to sell power may be limited. Verify to what extent restrictions have occurred in the past, whether similar impacts are expected in the future, and whether these are reflected in the projected annual revenue. Revenue calculations that do not account for restrictions or shutdowns may appear better than the actual situation.

When assessing price, don't focus solely on favorable power sales conditions; instead confirm whether those conditions can be reliably maintained throughout the remaining period. Even if the power sales conditions are good, if generation output is declining, annual revenue will not increase. Even if historical generation performance has been strong, if the remaining period is short and the repair burden is large, careful judgment is required. By reflecting the power sales conditions and the remaining period in revenue calculations, you can understand the meaning of the price more concretely.

Calculation Axis 3: View after subtracting maintenance and fixed costs

Maintenance and fixed costs are easy to overlook when looking at annual revenue. Even if electricity sales revenue looks large, you won't know the actual remaining profit unless you subtract fixed costs such as inspections, monitoring, mowing, cleaning, insurance, land-related expenses, and administrative procedures. When evaluating the price of a solar power plant, it's important to look at the net income after deducting the necessary costs, not at electricity sales revenue.

Maintenance and operating costs include regular inspections, checks of electrical equipment, power generation monitoring, mowing, weeding, cleaning, management of drainage channels, repair of fences and gates, management of surrounding trees, and on-site response in case of abnormalities. These vary greatly depending on the location of the power plant and the condition of the equipment. A power plant on flat, easily accessible land and one in forested or sloped terrain will have different management burdens even with the same equipment capacity.

Fixed costs include land-related charges, insurance, communications, monitoring, tax and accounting processing, contract management, and various administrative procedures. These are costs that tend to be incurred regardless of the amount of power generated. If fixed costs are not sufficiently factored into the calculation of annual revenue, the apparent revenue can look better than the actual situation.

When looking at past operation and maintenance costs, do not simply regard low costs as a positive indicator. Low costs may mean that necessary maintenance was not performed adequately. If inspections, mowing, or drainage management were insufficient, this can lead to reduced power generation and equipment deterioration. When reviewing past costs, you should check the work performed, its frequency, the reports, and the on-site conditions together.

For annual revenue to be useful in practice, it should be viewed as revenue from electricity sales minus operation and maintenance costs and fixed costs. Even low-priced projects will see their actual returns decline if fixed costs are high. Conversely, even high-priced projects can be easier to consider over the long term if the management structure is well established, the details of fixed costs are clear, and generation performance is stable.

Maintenance and fixed costs are expenses that support the stable operation of a power plant. Rather than viewing them simply as expenditures to be cut, it is important to regard them as costs for maintaining power output, detecting abnormalities early, and reducing future repair needs.

Calculation Axis 4: Incorporate equipment deterioration and future repairs into revenue forecasts

When calculating annual revenue, you need to include not only current earnings but also equipment degradation and future repairs in the revenue outlook. Even if current power generation performance is good, equipment degrades over time. If replacements of power conversion equipment, cable repairs, repairs to racking and foundations, updates to monitoring systems, or repairs to fences and drainage facilities become necessary, they will affect annual revenue.

The equipment to be inspected is not limited to solar panels. We look at the entire power plant, including power conversion equipment, mounting structures, foundations, cables, connection equipment, monitoring devices, fences, gates, drainage facilities, and so on. Dirt or cracks on panels, shutdown history of power conversion equipment, damage to cable sheathing, corrosion of mounting structures, scouring around foundations, communication failures of monitoring devices, and damage to fences are all candidates for future repairs.

Inspection reports and repair histories are important documents for anticipating future repairs. They confirm what kinds of defects occurred in the past and how they were addressed. If there are outstanding items, they should be treated as risks of unaddressed repairs. Even when repairs are reported as completed, if it is unclear what scope was covered and how the work was carried out, it becomes difficult to assess future risk.

Future repairs are difficult to assess from a single year’s annual revenue alone. Even if one year appears profitable, substantial repairs in subsequent years can change the effective returns. It is important to consider which pieces of equipment are likely to require repairs during the remaining period, whether power generation will be halted during repairs, and whether surrounding equipment will also need to be addressed at the same time.

For low-priced properties, the low price may reflect anticipated future repair costs. For high-priced properties, the price may be based on the facilities being in good condition, a clear repair history, and the ability to more easily forecast future repairs. When assessing annual revenue, it is essential to include not only current revenue but also the expected burden of future repairs in revenue projections.

Calculation axis 5: Anticipate shutdown risk and output limitations

When calculating annual revenue, downtime risk and output restrictions are also important factors. For solar power plants, equipment outages or restrictions on the opportunity to sell electricity can affect generation output and sales revenue. Even if the projected annual revenue looks favorable, if downtime and restrictions are not adequately reflected, actual revenue may be lower than expected.

First, what I want to check is the history of past stoppages. I will check which equipment stopped, when, to what extent it was stopped, what the cause was, and how long it took to recover. The fact that there was a stoppage is not necessarily a problem in itself. What matters is whether the cause was identified, whether it was properly restored, and whether measures to prevent recurrence are in place. Unexplained stoppages or repeated occurrences of the same malfunction leave concerns about future revenue prospects.

We also verify the power generation monitoring and emergency response framework. Even if anomalies can be detected, if it is unclear who will verify them and who will arrange on-site responses, downtime may be prolonged. At power plants that are remote or have poor access roads, on-site inspections and restoration work can take time. The longer the downtime, the greater the impact on annual revenue.

Also review output limits by checking past performance and future outlook. Even if the power generation equipment is capable of producing sufficient electricity, opportunities to actually sell power may be restricted. It is important to confirm whether the impact of such restrictions is reflected in the annual revenue calculations and that the figures are not based only on the best periods.

Equipment shutdowns and output restrictions can show up in monthly power generation results. If generation drops significantly in a particular month, check whether shutdowns or restrictions, as well as weather, had an impact. It is important to identify risks on a monthly basis that are not visible from the annual total alone.

For low-priced projects, a history of downtime or the effects of restrictions may be behind the low cost. Even for high-priced projects, if the risk of downtime is low and monitoring and recovery systems are in place, there is value in revenue stability. When looking at annual revenue, you need to account not only for normal power generation but also for the potential impact of shutdowns and restrictions.

Calculation Axis 6: Incorporating land conditions and on-site management burdens into revenue

Annual revenue is not determined solely by the power generation equipment or the conditions for selling electricity. Land conditions and the burden of on-site management also have a significant impact. Solar power plants are installed outdoors, and power generation and maintenance costs vary depending on the condition of the land, the surrounding environment, drainage, road access, boundaries, and vegetation. When calculating annual revenue, the burden of on-site management must always be reflected.

We will confirm the type of land use. We will check whether it is owned land or leased land, what the contract period and renewal conditions are, and how far the management scope extends. If the land is leased, depending on the land-use conditions there may be restrictions on mowing, drainage management, maintenance of access roads, and work near the boundaries. If the land contract is unclear, it may affect future business continuity.

Access roads and entry routes also affect revenue. For power plants that are difficult to access on site, the burden of inspections, mowing, equipment replacement, and disaster recovery increases. If response to abnormalities is delayed, downtime will be prolonged and annual revenue will be affected. Conditions such as narrow access routes, muddy conditions in rainy weather, or unclear rights of way should be reflected in fixed costs and emergency response expenses.

Drainage and terrain are also important. At power plants where drainage channels are prone to clogging, sediment is likely to flow in, water tends to pool, or slopes are unstable, inspections and repairs will be necessary after heavy rain. If poor drainage affects foundations, cables, or access and maintenance walkways, it can lead to future repairs. These are on-site burdens that are easy to overlook when calculating annual revenue.

Shading and vegetation management also affect power generation and costs. If surrounding trees grow and cast shadows, power generation may decrease. Vegetation can also cast shadows on the lower parts of panels. Plants with a high frequency of mowing and tree management have increased maintenance costs. Even if generation performance appears stable, if that stability is supported by ongoing vegetation management, those management costs need to be reflected in the revenue.

In low-priced listings, the low price may reflect site conditions or the burden of on-site management. In high-priced listings, easy-to-manage site conditions may be included as part of the value. When assessing annual revenue, it is important to calculate it including the work required to maintain the site.

Calculation Axis 7: Consider the purchase price and recovery outlook separately

The final axis of calculation is to consider the purchase price and the payback outlook separately. Even if a solar power plant appears cheap, if annual revenues are unstable, the payback outlook will be weak. Conversely, even if the price appears high, if generation performance is stable and fixed costs and repair risks are clear, it may be easier to establish a long-term outlook.

The purchase price indicates the upfront cost required at acquisition. By contrast, the payback outlook depends on how steadily the annual revenue will continue. Even if electricity sales revenue looks large, once you subtract maintenance and management expenses, fixed costs, repairs, downtime risks, and land management burdens, the actual returns will change. Don't judge solely by the purchase price; it's important to examine the composition of the annual revenue.

When assessing the payback outlook, consider not only the first year but the entire remaining period. Even if current power generation is stable, future equipment degradation, replacement of power conversion equipment, vegetation growth, poor drainage, output restrictions, and changes in management arrangements can affect revenues. You need to judge whether the price is reasonable after factoring in the repairs and fixed costs expected during the remaining period.

Uncertainty is also taken into account when calculating annual revenue. If there are unclear points in power generation records, insufficient documentation, unverified on-site risks, or ambiguous repair histories, uncertainty will remain in the revenue forecast. Ignoring this uncertainty can lead to overvaluing projects that appear to be low-priced.

Separating the price from the return outlook makes it easier to explain a purchase decision. It allows you to clarify why this price can be considered reasonable, which parts of the annual revenue are at risk, and which fixed costs and repair expenses you are anticipating. What matters to practitioners is not the level of the price but the ability to explain the predictability and stability of annual revenues.

Judgment Mistakes to Avoid When Comparing Annual Revenues

When comparing annual revenue, what you should avoid is judging based only on revenue from electricity sales. Even if revenue from electricity sales looks large, unless you deduct operation and maintenance costs, fixed costs, repair costs, downtime risks, and the burden of land management, you won't know the actual revenue that remains. If you assess the price based solely on surface-level revenue, you're likely to overlook the burdens after purchase.

Next, you should also avoid relying solely on generation performance from a good year. Solar power generation is affected by weather and local conditions. If you project annual revenue based on a single year’s performance, you may end up with an overly optimistic estimate of future revenue. You need to review monthly data over multiple years and identify the causes of any outliers or downward trends.

Underestimating fixed costs is also a problem. Even if past management fees have been low, it may simply be that necessary maintenance was not performed. If proper management is carried out after purchase, fixed costs can end up higher than expected. It is important to check the scope of management, not just the amount of the costs.

Overlooking future repairs is a major misjudgment. Even if current power generation performance is good, if replacement of power conversion equipment, cable repairs, repairs to mounting structures or foundations, or updates to monitoring devices become necessary, revenue will change. Annual revenue should be projected not only for the present but also taking into account the burdens over the remaining period.

You should avoid assessing site risks based only on documents. Even if power generation records and drawings are in order, there may be issues on site such as shading, vegetation, drainage, road access, boundaries, or equipment degradation. To view annual revenue realistically, it is essential to cross-check the documents against the actual site.

Summary: Annual revenue should be assessed together with local evidence

When looking at the price and annual revenue of a solar power plant, it is important to check seven calculation axes: annual power generation, power sales terms and remaining duration, operation and maintenance costs and fixed expenses, equipment degradation and future repairs, downtime risk and output restrictions, land conditions and on-site management burden, and purchase price and payback outlook. The figures for power sales revenue alone do not reveal the actual value of a solar power plant.

There may be reasons why a power plant is priced low. Reduced power output, heavy fixed costs, equipment deterioration, future repairs, uncertain land conditions, and lack of documentation may be behind it. There are also reasons for a high-priced power plant. If generation performance is stable, fixed costs are clear, equipment and land risks are low, and the documentation matches the site, it can be considered an easier proposition to evaluate over the long term.

What is important for operational staff is to organize annual revenue as a basis that can be used for internal reporting. Power generation data, electricity sales terms, operation and maintenance costs, inspection reports, repair history, land contracts, drawings, and on-site inspection results need to be linked so that the basis for the revenue can be explained. This is particularly important for used solar power plants, where reconciling past operational history with current on-site conditions is indispensable.

In on-site surveys, it is useful to record inspection points likely to affect annual revenue together with precise location information. If equipment near boundaries, drainage channels, trees causing shade, the extent of vegetation overgrowth, power conversion equipment, cable damage, fence damage, and candidate repair locations can be recorded with location data, it becomes easier to organize the basis for revenue declines and future costs.

If you want to assess a solar power plant’s annual revenue with on-site evidence, using LRTK (an iPhone-mounted GNSS high-precision positioning device) is also effective. If you can record equipment locations within the plant, drainage channels, causes of shading, the extent of vegetation, candidate repair locations, and points of caution near boundaries together with high-precision position information, you can reconcile discrepancies between drawings and the site and make it easier to share factors that affect annual revenue among stakeholders. When judging the price and annual revenue of a solar power plant, it is important not only to rely on desk calculations but also to build up evidence that can be verified on site.