Resource Adequacy: Slice of
Day
Shannon Rivers
Sep 3, 2025
Resource Adequacy
• Resource Adequacy (RA):
o A compliance product to ensure there is a plan for adequate resources to match customer demand with available generation at any hour of the day in the CAISO.
o Resource Adequacy is purchased as available capacity per month. 2 3
Resource Adequacy: Previous
Framework
Resource Adequacy – Monthly Peak Requirement
4 • Prior to 2025, Ava had two types of Resource
Adequacy requirements: System and Flexible
System RA
• Interconnected
to CAISO
• Imports: energy
imported from
outside CAISO
area
Flexible RA
• Determined by
resource ability
to increase
output during
load ramp hours
• Battery storage
resources are
flex
• The volume of RA Ava was required to procure
in each month was based on:
o Ava’s peak customer demand and the
planning reserve margin.
• Load serving entities (LSEs) must
demonstrate compliance both to the CPUC
and CAISO, annually and monthly.
• Local RA and Central Procurement Entity
(CPE): Ava was responsible for a Local RA
requirement in 2022 and years prior. Starting
in 2023, CPE is responsible for Local RA
procurement.
Resource Adequacy – Monthly Peak Supply Stack
5 Source: Gridwell Consulting
Single Peak Monthly Requirement Illustration – CAISO System 6 Resource Adequacy: Current
Framework
Resource Adequacy: Slice of Day
• Starting in 2025, RA requirements changed to a Slice of Day structure. • Intent of change to Slice of Day: Minimize customer cost, meet hourly reliability needs, adapt to a changing grid.
7 Requirement
• RA obligations are based on Ava’s hourly
share of CAISO load for the “worst day” of
each month plus a Planning Reserve
Margin (PRM).
• LSE's must demonstrate sufficient
electricity supply across 24 hours of
each month.
• LSE's must procure to their own load
shape.
• Solar and wind resources are based on
exceedance profiles, which is looking at
historic availability based on region and
technology.
• Thermal resources are based on Net
Qualifying Capacity (NQC), the resource
is assigned a value applied to all hours.
• Storage contributes up the Pmax, but is
limited by the storage efficiency, excess
generation is needed to charge storage.
Counting Rules
Resource Adequacy: Slice of Day Supply Stack
8 24-hour Monthly Requirement Illustration – CAISO System Source: Gridwell Consulting
Resource Adequacy: Slice of Day and Storage
• Storage resources in the Slice of Day framework can shift RA from hours of excess to hours of need. • Excess RA from generating resources is required to charge storage for storage capacity to count toward RA compliance obligation.
• Result: LSEs must procure generation RA in excess of RA compliance obligation to “charge” storage for discharge in other hours.
9 Two hypothetical months under Slice of Day:
Two hypothetical months under Slice of Day: