Correlated uncertainty propagation enables multi-impact decision support for electrical system decarbonization

Decarbonization planning requires comparing electricity pathways across economic, ecological and health outcomes under uncertainty. We present PHASED (Probabilistic Hourly Assessment of Scenarios for Electrical Decarbonization), which simulates hourly electricity supply and compares prescribed pathways using the same sampled inputs. At a 2% discount rate, mean monetized costs across expansion pathways and costing methods range from $448.3 to $509.7 billion (2024 USD, 2025–2050). For eight New England pathways over 2025–2050, we compare uncertainty in absolute costs with uncertainty in paired differences across 1,000 simulations. The standard deviation of estimates of total cost differences across pathways is roughly 1/3 that of absolute costs for a given pathway. A pathway incorporating small modular nuclear reactors lowers total monetized costs relative to the “All Options” pathway retained as the baseline planning scenario by utilities and governments. Pathways with similar mean monetized costs also differ widely in air emissions and other ecological impacts known to be of interest to stakeholders including avian mortality and water withdrawals.