P50

P50 and P90 are the two energy yield numbers that appear in every bankable solar report. This page is for solar developers, energy analysts and project finance teams who need the P50 meaning, the P50 and P90 difference and how P50 is calculated.

Quick Facts

What does P50 mean in solar?

Formal definition

The P50 annual energy yield is the value Q where the cumulative probability function P(annual energy ≥ Q) = 0.50. In plain English: there’s a 50% chance the plant produces at least P50 in any given year.

Engineering definition

Calculated by combining hourly time-series simulation (PVsyst, SAM) with statistical analysis of:

1. Inter-annual weather variability (multi-year TMY).
2. Modeling uncertainty (PVsyst ±3%, IAM, soiling, thermal).
3. Long-term degradation (typically modeled separately for year-N P50).

Industry definition

P50 is the headline “expected” yield reported in every Energy Yield Assessment (EYA).

Permitting definition

Not a permit term, but P50 appears in interconnection cost-benefit analyses and PPA prices indexed to expected production.

How P50 is Used

Equity investors

Equity returns (IRR, NPV) are projected against P50, the central expectation.

Lenders

Debt service coverage ratio (DSCR) is calculated against P90 or P95, not P50. Lenders want assurance that worst-case revenue still covers debt.

Developers

Use P50 for project pricing, PPA negotiation, and revenue forecasting.

EPCs

EPC performance guarantees are typically set to P90 or P95 of year-1 production.

How do you calculate P50?

Step 1: Hourly simulation

PVsyst or SAM produces 8,760 hourly energy values for a typical year using TMY weather.

Step 2: Inter-annual variability

Multi-year weather records (typically 20+ years from Meteonorm or Solargis) provide the standard deviation of annual GHI/POA.

Step 3: Modeling uncertainty

PVsyst published uncertainty is ±3% for well-instrumented inputs. IAM, soiling, and thermal models add another 2-4%.

Step 4: Combined uncertainty

Combine inter-annual variability + modeling uncertainty as root-sum-square (RSS):

σ_total = √(σ_weather² + σ_modeling²)

Typically σ_total = 4-8% of mean.

Step 5: Confidence interval

Assuming approximately normal distribution:

• P50 = μ (mean)
• P75 = μ − 0.67 × σ
• P90 = μ − 1.28 × σ
• P99 = μ − 2.33 × σ

Worked example

Mean simulated yield μ = 250 GWh/yr. Combined uncertainty σ = 6% of mean = 15 GWh.

• P50 = 250 GWh
• P75 = 250 − 0.67 × 15 = 240 GWh
• P90 = 250 − 1.28 × 15 = 231 GWh
• P99 = 250 − 2.33 × 15 = 215 GWh

Sources of Uncertainty

Year-1 vs. Lifetime P50

Year-1 P50 reflects fresh modules. Year-25 P50 applies cumulative degradation:

Energy_yr25 = Energy_yr1 × (1 − annual_degradation)^24
            ≈ Energy_yr1 × 0.88 (at 0.5%/yr degradation)

Lifetime average P50 ≈ 0.94 × Year-1 P50 (with linear degradation).

Common Mistakes

1. Reporting P50 without specifying year-1 vs. lifetime average.
2. Ignoring inter-annual variability, P90 = P50 is wrong.
3. Combining uncertainties additively instead of in RSS.
4. Using assumed σ instead of measured weather variability.
5. Not differentiating P50 from energy guarantee in contracts.

Best Practices

• Always report P50, P90, and P99 together.
• Specify year-1 and 25-year average for both metrics.
• Document the uncertainty sources used.
• Use 20+ years of weather data for inter-annual variability.
• Apply Monte Carlo simulation for complex/storage projects.

Comparison Tables

Confidence Levels in Use

Key Takeaways

• P50 is the median expected annual energy yield, equal probability of beating or missing.
• Calculated from hourly simulation + inter-annual variability + modeling uncertainty.
• Equity investors use P50; lenders use P90; EPC guarantees often P95.
• Year-1 P50 vs. 25-year average P50 differ by ~6% due to degradation.
• Always report P50, P90, and P99 together with uncertainty breakdown for full transparency.

Related Reading

P50 numbers don’t stay on the EYA cover page, they flow straight into bid pricing and revenue models further down the pipeline. SECI tariff math shows how small swings in the engineering inputs feeding an energy yield assessment move a bid by paise per kWh, which is the same P50 output this page walks through.

EPCs deciding how much of that yield-modeling and EYA workload to keep in-house versus hand off to a third-party design partner can see how that scope is typically structured in the complete guide to solar design services in India.

Since the accuracy of any P50 figure depends heavily on the simulation engine behind it, SurgePV’s comparison of the leading solar design software platforms is a useful reference when evaluating PVsyst, SAM, or other tools used to generate these yield estimates.

Disclosure: SurgePV is a Heaven Designs group company.