Prepared byKevin Coffee, P.E. & Nicole Ramos
Introduction
The 2000-2001 California Energy Crisis led the California Public Utilities Commission (CPUC) and the California Independent System Operator (CAISO) to create the Resource Adequacy (RA) Program. The CPUC, a regulatory agency that oversees utility services in California, and CAISO, the entity responsible for managing the state’s high-voltage electricity transmission system, developed RAP to ensure that Load Serving Entities (LSEs) (utilities or companies that provide electricity to end-use customers) can reliably meet their forecasted customer load demands. The program was designed to guarantee that sufficient energy resources are available to maintain grid reliability, especially during periods of high demand.
Initially, RAP targeted the single peak hour of each month, assuming sufficient supply during peak would cover all other hours. This approach shifted as California transitioned to renewable energy (primarily wind and solar) driven by sustainability goals and incorporated energy storage. This created two reliability challenges: variable supply and the inadequacy of single-hour planning.
Second, because the CAISO’s supply portfolio output was now highly variable, the premise that reliability could be ensured based on comparing supply availability against customer load in a single hour in a month when customer load was highest was no longer valid.
The unintended consequences of the transition to a supply portfolio dominated by variable energy renewable resources were an oversupply of energy in hours when solar energy was highest and undersupply when solar energy was lowest (which the CAISO termed the “Duck Curve”). The RAP could no longer be based on a single hour.
As a result, the CPUC modified its RAP to what is called “Slice of Day” (SOD), which requires the review of supply sufficiency for not just a single hour in a month but the highest 24 hours of the day in a month. The SOD framework divides the day into different periods, or “slices,” reflecting critical moments when the grid is most stressed. LSEs and generator owners must now ensure they have enough resources available during each of these slices rather than just focusing on peak demand periods. This approach addresses the variability of renewable energy, particularly solar and wind, whose generation fluctuates throughout the day.