EYA (Energy Yield Assessment)

Quick Facts

What’s in an EYA

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

EYA Stakeholders

Statistical Confidence Bands

Major IE Firms

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

Common Mistakes

1. Outdated weather data (10-year TMY when 20+ available).
2. Wrong .PAN/.OND files for the specific equipment ordered.
3. Optimistic soiling assumptions for dusty climates.
4. Single-year simulation instead of statistical analysis.
5. 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.

Related Reading

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.