Glossary
P50 and P90
P50 and P90 are confidence levels on a solar energy yield estimate. P50 is the output a system is expected to exceed in half of all years; P90 is the more conservative figure it should exceed in nine years out of ten. Lenders underwrite on P90, not P50.
A yield simulation produces a distribution, not a single number. Weather varies year to year, so the honest output is a probability: P50 is the median expectation, P90 is the pessimistic case that a project should beat 90% of the time.
The distinction is commercial, not academic. Project finance is sized on P90 because a lender cares about the bad years. A tool that reports only P50 cannot support a financing conversation, however accurate its median is.
In this dataset, loss and uncertainty modelling is a distinct capability from yield simulation, and several tools that simulate yield well do not model P90 at all. Solargis Evaluate, for example, publishes 15-minute TMY P50 simulations; P90 sits outside that tier.
Why it matters
If you will ever need bank finance, a tool that stops at P50 will force you to buy a second one.
Products where this is recorded
- PVsyst — Design & simulation
- DNV SolarFarmer — Design & simulation
- Solargis Evaluate — Design & simulation