Resource Yield Assessment
Executive Summary
Key Takeaways
- ✓ A resource yield assessment estimates the expected energy resource available to a generation asset, expressed at defined exceedance probability confidence levels such as P50 and P90.
- ✓ P50 is the yield level expected to be exceeded in 50 percent of years (the median case); P90 is the more conservative level expected to be exceeded in 90 percent of years.
- ✓ P50 should feed base case forecasting, and P90 (or an equivalently conservative case) should feed conservative lender debt sizing — using the wrong confidence level for the wrong purpose is a common structural error.
- ✓ A resource yield assessment should be prepared by an independent technical party, not the developer's own in-house estimate, since lenders and investors rely on its independence for their own risk assessment.
- ✓ Degradation is applied on top of, not within, the resource yield assessment's output figures, and the two should be modelled as separate, explicit schedules.
Definition¶
Resource yield assessment is a technical study estimating the expected energy resource available to a generation asset — solar irradiance, wind speed, or hydrology, depending on the technology — expressed at defined confidence (exceedance probability) levels, typically prepared by an independent engineering or technical consultancy.
P50 vs. P90 and Other Exceedance Levels¶
P50 is the yield level expected to be exceeded in 50 percent of years — the median, or base case, estimate. P90 is a materially more conservative level, expected to be exceeded in 90 percent of years, meaning only a 10 percent chance actual yield falls below it in any given year. Other exceedance levels (P75, P95, P99) are used depending on the specific purpose and lender or investor requirement.
Matching Confidence Level to Purpose¶
P50 should feed base case revenue forecasting, since it represents the statistically expected outcome. P90, or an equivalently conservative exceedance level, is commonly used for lender debt sizing, since it represents a level of resource availability the project is highly likely to meet or exceed, giving the lender confidence in debt service coverage even in a below-median year. Using the optimistic P50 case for a conservative purpose such as debt sizing, or applying a single blended yield figure to both purposes, is one of the most common structural errors in this asset class — see Financial Modelling Best Practices for Renewable Energy and Financial Model Audit for Renewables.
Independence of the Assessment¶
A resource yield assessment should be prepared by an independent engineering or technical consultancy, not derived solely from the developer's own in-house estimate. Lenders and investment committees rely on this independence to form their own view of the project's revenue risk, and an assessment without a credible independent basis carries materially less weight in a financing or investment decision.
Relationship to Degradation and Capacity Factor¶
Resource yield assessment estimates the energy resource available at the outset of the asset's life. Degradation rate is a separate, subsequent schedule reducing output over the asset's operating life to reflect equipment performance decline, and should be applied on top of, not folded within, the resource yield assessment's base output figures. The resulting output, expressed relative to the asset's nameplate capacity, is the project's capacity factor.
Audit Considerations¶
- Confirm the resource yield assessment is sourced from an independent technical party, with the source clearly referenced in the model.
- Confirm P50 and P90 (or other stated exceedance levels) are each applied to their correct, distinct purpose, not blended into a single yield figure.
- Confirm degradation is applied as a separate schedule on top of the resource yield assessment's base output, not embedded within it.
Common Errors¶
| Error | Description | Risk |
|---|---|---|
| Blended yield figure | A single yield assumption used for both forecasting and debt sizing | Conflates base case and conservative purposes |
| Wrong confidence level for purpose | P50 used for debt sizing, or P90 used for base case forecasting | Misstates debt capacity or base-case return |
| No independent source | Yield assumption based solely on the developer's own estimate | Understates the evidentiary basis for the assumption |
| Degradation embedded in yield | Degradation folded into the yield assessment rather than a separate schedule | Obscures which factor drives an output shortfall |
Continue Reading¶
Prerequisites¶
- Power Project Financial Model Structure — the parent guide
- Energy Financial Modelling
Related Glossary¶
Related Industries¶
How OXXON tests thisRun a free structural check with FMAE
Frequently Asked Questions
What is a resource yield assessment?
A technical study estimating the expected energy resource available to a generation asset — solar irradiance, wind speed, or hydrology, depending on the technology — expressed at defined confidence (exceedance probability) levels, typically prepared by an independent engineering or technical consultancy.
What do P50 and P90 mean?
P50 is the yield level expected to be exceeded in 50 percent of years — the median or base case estimate. P90 is a more conservative level expected to be exceeded in 90 percent of years, meaning there is only a 10 percent chance actual yield falls below it in a given year.
Why does it matter which confidence level is used for which purpose?
Because P50 and P90 serve different purposes — P50 is appropriate for base case revenue forecasting, while P90 (or an equivalently conservative exceedance level) is commonly used for conservative lender debt sizing. Using the optimistic P50 case for debt sizing overstates the coverage ratio a lender is actually protected by.
Why should the resource yield assessment come from an independent party?
Because lenders and investors rely on the assessment's independence to form their own view of the project's revenue risk — a developer's own in-house yield estimate, without independent verification, does not carry the same evidentiary weight in a financing or investment decision.
How does resource yield assessment relate to degradation?
Resource yield assessment estimates the energy resource available at the outset; degradation is a separate, subsequent reduction applied to output over the asset's operating life reflecting equipment performance decline — the two should be modelled as distinct, explicit schedules rather than combined into a single blended output figure.
Related Articles
Power Project Financial Model Structure
A power generation financial model is architected around a technical output schedule — generation volume for a variable-output asset or available capacity for a dispatchable one — that drives every downstream calculation: the electricity revenue stack, the operating cost build, and, where the asset is project-financed, debt sculpting and covenant testing. This guide sets out that architecture as a sequence of explicit, separately built modules, distinct from a standard corporate model's revenue-growth-first structure.
Capacity Factor
Capacity factor expresses a generation asset's actual energy output over a period as a percentage of the output it would have produced running at full nameplate capacity continuously over that same period. It is the core utilization metric for comparing generation assets and technologies, distinct from availability, which measures uptime rather than realized output.
Degradation Rate
Degradation rate is the annual decline in equipment output over a generation asset's operating life, reflecting expected panel, turbine, or other equipment performance decline. It should be applied as an explicit, consistent annual schedule reconciled to the technical basis used elsewhere in the model, since even a small inconsistency compounds materially over a multi-decade asset life.
Financial Modelling Best Practices for Renewable Energy
Renewable energy financial models combine project finance debt mechanics with technical resource-yield, degradation, and curtailment assumptions specific to the energy source. This page sets out how such a model should be constructed: building the yield and degradation schedule at the correct confidence level for its purpose, modelling the PPA-to-merchant-tail transition explicitly, and sculpting debt against the resulting cash flow. It addresses the construction question as a discipline applied while the model is built, distinct from the audit-risk perspective covered on Financial Model Audit for Renewables.
Financial Model Audit for Renewables
Renewable energy financial models combine standard project finance debt sculpting with technical assumptions specific to the energy source, resource yield (solar irradiance or wind speed), equipment degradation over the asset life, and curtailment risk, that directly determine the cash flow feeding the debt structure. Power purchase agreement pricing and tenor, and the merchant tail risk once a PPA expires, add a further layer of revenue structure specific to this sector. This page sets out the modelling risks specific to renewables, the audit findings that recur across solar, wind, and storage financings, and what lenders typically expect before financial close.