Understanding Tail Risk
P90 means "the value exceeded in only 10% of Monte Carlo draws." Our stochastic model runs 200 simulations of a full year, each with randomly sampled plant outages, demand noise, and weather variability. The P90 blackout hours tell you: in a bad-but-plausible year, how many hours will ERCOT be short on power?
The industry standard is LOLE = 9 hours/year (Loss of Load Expectation). That means a reliable grid should have fewer than 9 hours of expected blackout per year. Our P90 threshold is more conservative — it asks whether even the 90th-percentile bad year stays under that bar.
This is why averages lie. A grid can have a mean of 9 blackout hours and still fail P90 at 39 hours — because the tail of the distribution carries the real risk. The average year is fine. The bad year is catastrophic. Grid planning is about the bad year.
Fidelity Matters
This tool uses the stochastic model — fidelity level 4 of 5 in our progression. A simpler model (hourly dispatch on the best weather year) would tell you 60 GW + 50 GWh is perfectly reliable. The stochastic model reveals it fails catastrophically.
Try it yourself: set gas to 60 GW and storage to 50 GWh. The P90 number shows over 900 blackout hours. That's not a reliability problem — it's a grid collapse. Yet a deterministic model would say "6 hours, ship it."