Definition
An Energy Yield Assessment (EYA) is the formal engineering report projecting a solar plant's annual energy production over its 25-year operating life, with statistical confidence bands (P50, P75, P90, P99) used by lenders and investors for project finance decisions.
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- Outdated weather data (10-year TMY when 20+ available).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.
Frequently Asked Questions
10 commonly searched questions about EYA (Energy Yield Assessment).
What is an EYA?
Energy Yield Assessment — a formal engineering report projecting a solar plant's annual energy production with statistical confidence intervals (P50, P75, P90, P99). Required for bankable project finance, EPC contracts, and PPA negotiations.
Who writes an EYA?
An independent engineer (IE) or bankable consulting firm. The developer's design team produces a 'developer EYA' first; the lender's IE reviews and produces an 'independent EYA' for due diligence.
What does an EYA include?
Site weather data analysis (multi-year), PVsyst simulation, loss breakdown, monthly yield profile, statistical confidence intervals (P50/P75/P90), uncertainty analysis, 25-year degradation projection, sensitivity analysis, and reviewer commentary.
What software is used?
PVsyst is the bankable industry standard. SAM (NREL) for cross-check. Meteonorm 8 or Solargis for weather inputs. Helioscope occasionally for early-stage designs.
How long does an EYA take?
Initial developer EYA: 2–4 weeks. Independent EYA + lender review: 4–8 weeks. Total IE engagement: 6–12 weeks.
Is an EYA different from a PVsyst report?
Yes. A PVsyst report is the raw simulation output. An EYA adds weather analysis, uncertainty quantification, statistical confidence bands, reviewer commentary, and conformity to industry-standard reporting (e.g., IEC 61724-1).
Do all solar projects need an EYA?
No. Residential: not required. Small commercial: not required for non-leveraged projects. Commercial >500 kW with debt financing: typically required. Utility-scale: always required.
What weather data should be used?
Meteonorm 8 (typical bankable default), Solargis (premium tropical/desert), NSRDB (US), or site-measured TMY from on-site pyranometers (gold standard for >50 MW).
How is uncertainty calculated?
Inter-annual weather variability (3–6%) + modeling uncertainty (3–4%) + transposition + soiling + other → combined RSS → σ = 5–8% of mean. P90 = P50 − 1.28 × σ.
Why does the lender need an independent EYA?
Avoid bias from developer optimism. IE provides an arms-length validation that the energy projections meet bankability criteria. Standard in project finance.
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