Energy Yield Assessment (EYA): Bankable P50/P90 Reports

Quick answer

An Energy Yield Assessment (EYA) is an engineering study that forecasts a solar plant's long-term energy production, usually with PVsyst or similar software, and expresses uncertainty as P50 and P90 values. P90 is the annual energy expected to be exceeded with 90% probability. Lenders, investors and buyers use EYAs for project financing.

Quick Facts

Field	Detail
Term	EYA, Energy Yield Assessment
Category	Engineering Math / Bankability
Engineering Discipline	Project Finance, Energy Modeling
Standard Reference	IEC 61724-1, NREL TP-7A40-60628
Use Case	Bankable project finance
Difficulty Level	Advanced

What’s in an EYA

Executive summary: Plant capacity, P50/P90 yields, PR, LCOE.
Site description: Coordinates, terrain, climate.
Weather data analysis: Multi-year GHI, POA, temperature, soiling. Source citations.
System description: Module, inverter, racking, ILR, GCR.
Simulation methodology: PVsyst version,.PAN/.OND files, transposition model, shading scene.
Loss diagram: Per-loss-category breakdown.
Production projection: Hourly, monthly, annual. 25-year degradation curve.
Statistical analysis: P50, P75, P90, P95, P99 with uncertainty breakdown.
Sensitivity analysis: Impact of soiling, degradation, availability, weather extremes.
Compliance check: IEC 61724-1, manufacturer specs.
Reviewer commentary: Limitations, assumptions, recommendations.
Appendix: PVsyst.PRJ file, raw outputs, weather data details.

EYA Stakeholders

Role	Function
Developer	Initial EYA, design optimization
Independent Engineer (IE)	Review + independent EYA for lender
Lender	Uses P90 for debt sizing
Equity investor	Uses P50 for IRR projections
EPC	References EYA for performance guarantees

Statistical Confidence Bands

Metric	Use
P50	Expected/mean, equity returns, revenue projections
P75	Some commercial PPA hedges
P90	Senior debt sizing, standard bankable
P95	Aggressive lenders
P99	Stress test, government loans

Major IE Firms

DNV, UL Solutions, AECOM, Black & Veatch, R.W. Beck, Sterling and Wilson, Mahindra Susten, Tata Consulting Engineers, CRISIL.

Common Mistakes

Outdated weather data (10-year TMY when 20+ available).
Wrong.PAN/.OND files for the specific equipment ordered.
Optimistic soiling assumptions for dusty climates.
Single-year simulation instead of statistical analysis.
Missing reviewer commentary.

Best Practices

Use 20+ years of weather data.
Cross-check PVsyst with SAM on high-value projects.
Document every input override.
Quantify each uncertainty source separately.
Engage the IE early to align methodology.

Standards & Certifications

IEC 61724-1, PV system performance methodology.
NREL TP-7A40-60628, PV performance modeling best practices.
IEC TS 61724-2/3/4, Capacity evaluation, energy assessment methods.

Key Takeaways

An EYA is the bankable energy production projection for a solar plant, including P50/P75/P90/P99 confidence bands.
Required for utility-scale and most commercial debt-financed projects.
Independent Engineer review produces the lender-acceptable version after developer first draft.
PVsyst is the industry-standard simulation tool; SAM is the common cross-check.
Uncertainty analysis is the critical differentiator from a basic PVsyst report.

An EYA rarely stands alone, it’s one deliverable inside a larger engineering and finance workflow. EPCs and developers assembling a full package of bankable documentation often source it as part of a broader solar design services engagement, and the choice of firm matters enough that it’s worth reading through our notes on how to choose a solar design partner before committing a project’s yield modeling to one.

Since the report is only as credible as the simulation behind it, teams evaluating PVsyst and other yield-modeling software should treat that choice as inseparable from EYA quality. And because P50/P90 assumptions flow straight into project economics, our breakdown of SECI tariff math shows concretely how small changes in engineering inputs move bid pricing.

Disclosure: SurgePV is a Heaven Designs group company.

Frequently Asked Questions

4 commonly searched questions about EYA (Energy Yield Assessment).

What is the difference between P50 and P90 in an EYA?

P50 is the median estimate, with a 50% chance that actual annual energy will be higher. P90 is a conservative estimate that should be exceeded in 90% of years or cases. Lenders often size debt on P90 or P99 values.

What inputs does an energy yield assessment need?

Typical inputs include long-term solar resource data, site layout, module and inverter datasheets, tilt or tracker settings, shading obstacles, soiling and albedo assumptions, cable losses, availability and degradation. The quality and uncertainty of the resource data usually drives most of the P90 spread.

Who prepares a bankable EYA?

Lenders usually expect an independent engineer or a recognized technical consultant, separate from the developer, to prepare or review the assessment. Their report documents assumptions, data sources and uncertainty so financiers can rely on it during due diligence.

How is an EYA different from a simple PV simulation?

A basic simulation gives one production estimate. An EYA adds long-term resource analysis, a documented loss chain, uncertainty quantification and exceedance probabilities, and is written for financial decision makers. The added rigor is what makes the numbers acceptable to lenders.

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