ISO New England Proposes Regional Energy Shortfall Threshold Metrics to Assess Risks During Extreme Weather

ISO New England has released its initial proposal for Regional Energy Shortfall Threshold (REST) metrics to help the region assess the grid’s ability to deliver energy during extreme weather conditions.

The New England Power Pool (NEPOOL) Reliability Committee will vote on the ISO’s proposal in September 2025. REST will help New England assess upcoming summer and winter seasons, identifying extreme weather periods when natural gas, oil, or other fuels or resources may be less available to serve demand for electricity. Policymakers and the industry are anticipated to take steps to mitigate the risk.

During periods of extended cold weather, New England depends on oil-fired resources with variable quantities of fuel available on-site to meet the combined peak demand from heating customers and power generation.

REST focuses on 0.25% of extreme conditions, which in practical terms means conditions expected to occur, on average, once every 90 summers or winters. The ISO’s proposal defines magnitude and duration limits for the energy shortfall that is predicted to result from such an event:

• Magnitude—If the shortfall occurs, the region can tolerate up to either about 3% of customers being without power for 72 hours, or approximately every customer being without power for 3% of 72 hours (2.16 hours).
• Duration—If the shortfall occurs, the region can tolerate up to about 18 hours of cumulative shortfall over a 21-day period.

REST is violated if both magnitude and duration exceed their thresholds.

The metrics were cited by the US Department of Energy as a useful example of reliability risk evaluation. They were developed through the ISO’s collaboration with stakeholders, and relied on the Probabilistic Energy Adequacy Tool (PEAT), a modeling technique developed with the Electric Power Research Institute (EPRI).

The region is expected to determine the duration of the year to evaluate potential REST violations. Summer assessments are expected to be released in June, while winter and long-term assessments will be released in November.