P90

P90 meaning in solar: it is the yield figure a plant should beat 9 years out of 10. This entry is for solar developers, EPCs, analysts and lenders who read energy yield assessments and need to understand how P90 is calculated and why project finance relies on it instead of P50.

Quick Facts

What does P90 mean in solar?

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 is P90 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

Sensitivity Drivers

Common Mistakes

1. Assuming P90 = 0.90 × P50 (linear; ignores uncertainty distribution).
2. Using year-1 P90 for lifetime debt sizing.
3. Ignoring modeling uncertainty.
4. Using single-year weather instead of multi-year TMY.
5. Mixing real and nominal dollars in DSCR calculation.
6. 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

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.

Related Reading

P90 numbers don’t mean much on their own, they only carry weight once the underlying energy yield assessment has been engineered to a standard lenders will accept, which is why the P50-to-P90 uncertainty band gets scrutinized alongside every other input feeding project economics.

On the revenue side, the same discipline shows up in tariff bidding: SECI Tariff Math, How Engineering Inputs Move Bids by 4 Paise/kWh walks through how small changes in engineering assumptions move the numbers lenders and bidders both depend on.

For EPCs and developers assembling the broader design package that supports a bankable EYA, the Complete Guide to Solar Design Services in India covers the scope of engineering deliverables, permit sets, CEIG drawings, PMC, that sit alongside the yield model in a lender’s due diligence file.