Each source supplies a share si of total energy. The exposure metric is the largest single share, max(s_i) — the biggest hole a single disruption can blow in supply. The resilience score is 100 × (1 − HHI), where HHI = Σ s_i² is the Herfindahl-Hirschman concentration index (0 = perfectly spread across infinite sources, 1 = single source). A shock disrupts one source: supply drops by its share, then recovers gradually as remaining sources partially absorb the gap.
- Concentrated portfolio (1–2 dominant sources) → high exposure, low resilience → a shock produces a deep, slow-recovering supply gap.
- Diversified portfolio (many comparable sources incl. domestic) → low exposure, high resilience → the same shock barely dents total supply.