Glossary
What do P50, P90 and P99 mean for a solar project?
These are exceedance probabilities for annual production. P50 is the energy met or exceeded with 50% probability, P90 with 90% and P99 with 99%. The higher the number, the more conservative the figure and the lower the energy.
Who uses which?
| Percentile | Question it answers | Who looks at it |
|---|---|---|
| P50 | What does a typical year produce? | Developer, equity investor |
| P90 | What does a 1-in-10 bad year produce? | Lender, to size debt |
| P99 | Does the project survive a very bad year? | Lender, as a stress test |
Where does the gap come from?
From total uncertainty: each year’s weather, the resource dataset, the simulation model, soiling and availability. Under a normal distribution, P90 = P50 · (1 − 1.282 · σ) and P99 = P50 · (1 − 2.326 · σ). Since σ differs from project to project, subtracting a flat percentage doesn’t hold up.
There are two common ways to compute it:
- Analytical: combine the uncertainties in quadrature and apply a distribution shape. It’s correct when the sources are independent.
- Monte Carlo: simulate many combinations of errors. It separates what averages out over the years from what doesn’t, and it lets sources be correlated.
Two nuances
- Horizon matters. A one-year P90 isn’t the same as the P90 of a 10-year average. See 1-year vs. 10-year P90.
- P90 drives debt. Each year’s cash flow in the P90 case sets the DSCR, and with it how much debt the project supports.
Limits
- The gap is project-specific. How far P90 and P99 sit below P50 depends on each project’s uncertainty: there is no percentage that carries over from one plant to another.
- Uncertainties are assumptions. The starting profiles aren’t measurements of your site and need to be justified project by project.
- Ten weather years bound the analysis. With few years, the result depends on which years are included.
- Validated against measured data at one plant. The energy chain was checked against the NIST Ground Array (−1.1% against 371 MWh measured): one plant, not a fleet. No track record with lenders yet.
→ Full methodology: P50 vs P90: the number your debt is sized against.