Quick answer
P50 and P90 are energy-yield exceedance estimates under a specified probability model and time horizon. P50 has a 50% probability of being exceeded; P90 has a 90% probability and is normally lower. The gap depends on uncertainty assumptions. Neither value is a production guarantee or a fixed percentage of the other.
What are P50 and P90?
PVsyst’s documentation defines P50 and P90 as exceedance values. P90 corresponds to the lower 10th percentile of the modeled energy distribution. Specify whether the estimate concerns one year or a multiyear average.
The P-number is a probability label. It is not an equipment-efficiency rating, an accuracy score or a promise about a particular sequence of future years.
How does the gap depend on uncertainty?
For a normal model whose mean equals P50:
P90 ≈ P50 − 1.2816 × standard deviationUse energy units consistently. An illustrative P50 of 10,000 MWh/year and standard deviation of 500 MWh/year gives P90 of about 9,359 MWh/year. These are assumed calculation inputs, not a measured project or a regional benchmark.
Different uncertainty inputs produce a different gap. A narrower gap alone does not establish better data or a more accurate model: it can also reflect uncertainty omitted from the assessment.
What does the estimate need to disclose?
| Item | Why it matters |
|---|---|
| Energy boundary and units | Inverter output and electricity delivered at the meter may differ |
| Time horizon | A single-year value and a multiyear average address different events |
| Weather and equipment inputs | The probability assessment is built on a production model |
| Uncertainty components | Their source, dependencies and aggregation affect the result |
| Method and distribution | A normal model is an assumption, not a universal requirement |
NREL’s P50/P90 analysis provides a methodology reference. Retain the chosen method with the report rather than quoting only the final P-values.
Use in proposals and financial review
Use the energy scenario required by the actual project’s reviewer or contract. A P90 number does not automatically establish lender approval, debt-service coverage or customer savings. Those assessments also require the applicable financial and operating assumptions.
Read the P50 versus P90 energy-yield guide for a worked example, uncertainty checklist and comparison method. Confirm the available report and released scope when evaluating solar simulation software; do not presume that every product uses the same statistical method.
Related Glossary Terms
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CEO & Co-Founder · SurgePV
Keyur Rakholiya is identified by SurgePV as its CEO and a company co-founder. His SurgePV author page lists only role information that can be tied to the public profile below; credentials, project totals, testing claims, media appearances, and speaking engagements are not asserted without retained evidence.
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