P90 is the 90th-percentile annual energy yield estimate — the value the solar plant is forecast to exceed in 90% of years. It is the conservative production benchmark used by lenders to size debt and calculate debt service coverage ratios (DSCR).
Quick Facts
| Field | Detail |
|---|---|
| Term | P90 — 90th Percentile Energy Yield |
| Category | Engineering Math / Bankability |
| Engineering Discipline | Project Finance, Energy Modeling |
| Standard Reference | NREL TP-7A40-60628, IEC 61724-1 |
| Use Case | Senior debt sizing, DSCR calculation |
| Software Used | PVsyst, SAM, Solargis Prospect |
| Difficulty Level | Intermediate |
What is P90?
Formal definition
The P90 annual energy yield is the value Q where the cumulative probability function P(annual energy ≥ Q) = 0.90. In plain English: there’s a 90% chance the plant produces at least P90 in any given year.
Engineering definition
Computed as P50 − z × σ, where z = 1.28 for the 90% confidence interval and σ is the combined annual energy uncertainty (inter-annual weather + modeling).
Industry definition
The standard benchmark for bankable energy yield assessments. Lender due diligence treats P90 as the worst-case-reasonable annual production.
Permitting definition
Not a permit term, but P90 is the basis for project economics submitted with interconnection cost-benefit analyses.
P90 in Project Finance
Debt sizing
A typical project finance structure:
- Year-1 P90 revenue × DSCR target = maximum annual debt service.
- Solving for principal: debt size = annual debt service / financing rate.
- A 100 MW plant with P90 = 230 GWh and PPA = $40/MWh has revenue of $9.2M/yr.
- DSCR target 1.20 → max debt service $7.67M/yr.
- At 6% over 18 years → debt principal capacity = $90M.
Equity returns
Equity IRR is computed at P50, but debt structure is sized at P90. The “uplift” between P90 and P50 cash flow goes to equity, providing the equity returns.
Stress testing
Lenders may also test P95 or P99 for catastrophic scenarios. Resilience to extreme low-yield years is a key due diligence point.
How P90 Is Calculated
Step 1: Mean simulated yield (P50)
PVsyst or SAM hourly simulation produces annual energy μ.
Step 2: Uncertainty quantification
Combine sources:
- Inter-annual weather variability: 3–6% of mean (from multi-year TMY).
- Modeling uncertainty: 3–4% (PVsyst, .PAN/.OND, IAM, soiling).
- Transposition uncertainty: 1–2%.
RSS combination: σ = √(σ_weather² + σ_modeling² + σ_other²) ≈ 4–8% of mean.
Step 3: Apply confidence factor
P90 = μ − 1.28 × σ.
Worked example
- μ = 250 GWh.
- σ_weather = 4% → 10 GWh.
- σ_modeling = 3% → 7.5 GWh.
- σ_other = 1.5% → 3.75 GWh.
- σ_total = √(10² + 7.5² + 3.75²) = √(100 + 56.25 + 14.06) ≈ 13.05 GWh.
- P50 = 250, P90 = 250 − 1.28 × 13.05 = 233.3 GWh.
Year-1 vs. Lifetime P90
| Year | Energy | Factor |
|---|---|---|
| Year-1 P90 | 233 GWh | Baseline |
| Year-5 P90 | 228 GWh | After 4 years degradation |
| Year-25 P90 | 207 GWh | Fully degraded |
| Lifetime avg P90 | ~218 GWh | Linear avg over 25 yrs |
Sensitivity Drivers
| Factor | Effect on P50–P90 Spread |
|---|---|
| Site weather variability | Higher in cloudy regions |
| Length of TMY record | Longer record = lower uncertainty |
| Modeling tool (PVsyst vs. simpler) | PVsyst tighter |
| .PAN/.OND accuracy | Outdated files widen σ |
| Site instrumentation | More pyranometers = tighter |
Common Mistakes
- Assuming P90 = 0.90 × P50 (linear; ignores uncertainty distribution).
- Using year-1 P90 for lifetime debt sizing.
- Ignoring modeling uncertainty.
- Using single-year weather instead of multi-year TMY.
- Mixing real and nominal dollars in DSCR calculation.
- Treating P90 as equity assumption (should be P50).
Best Practices
- Compute P50, P75, P90, P95, P99 together.
- Use 20+ years of weather data.
- Apply Monte Carlo for storage or hybrid projects.
- Quantify each uncertainty source in the EYA.
- Cross-check PVsyst against SAM for high-value projects.
Comparison Tables
Confidence Level Use Cases
| Metric | Confidence | Typical Use |
|---|---|---|
| P50 | 50% | Equity IRR, expected revenue |
| P75 | 75% | Commercial PPA hedges |
| P90 | 90% | Senior debt sizing, EPC warranty |
| P95 | 95% | Aggressive lenders |
| P99 | 99% | Government/pension fund stress test |
Key Takeaways
- P90 is the 90th-percentile annual energy yield — exceeded in 9 of 10 years.
- The standard metric for bankable solar project debt sizing.
- Calculated as P50 − 1.28 × σ where σ combines inter-annual weather and modeling uncertainty.
- Typical P90/P50 ratio: 90–95% depending on site variability.
- Use year-1 P90 for EPC guarantees; lifetime-average P90 for 25-year debt sizing.
Frequently Asked Questions
10 commonly searched questions about P90.
What is P90?
Why do lenders use P90 instead of P50?
What is the ratio of P90 to P50?
What is DSCR and how does P90 affect it?
How is P90 calculated?
What's the difference between Year-1 P90 and lifetime P90?
Is P90 the same as the energy guarantee?
What's a typical P90 for utility-scale solar?
Does P90 differ by site quality?
Why is P99 used sometimes?
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