Resource Adequacy Slice of
Day 101
Shannon Rivers
May 6, 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. o Capacity is the ability to generate not the generation itself and does not show up on the power content label.
2 Resource Adequacy and Reliability in CA
3 CAISO historical and projected annual peak load and energy (2014-2030) Source: CAISO 2025 Summer Loads and Resources Assessment All-time peak
Ava in comparison to 51,061 MW
CAISO:
• Share of the total CA
load is about 3% (2025)
• Peak load is about
2,000 MW
• Peak load share is
about 10%
4 Resource Adequacy: Slice of
Day
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.
5 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 looks at historic availability each
hour of each month based on region and technology.
• Thermal resources are based on Net Qualifying
Capacity (NQC), a value that accounts for normal
plant operation and is applied to all hours.
• Excess generation is needed to charge
batteries/storage. Once charged,
storage contributes up to its "nameplate" value, but
is limited by "storage efficiency" of ~80%.
Counting Rules
Resource Adequacy: Slice of Day Supply Stack
6 24-hour Monthly Requirement Illustration –
CAISO System
Source: Gridwell Consulting
Supply Tech Shape Capacity
(MW)*
Capacity (
% of total
supply)
Wind Intermittent 6,360 7
Solar Intermittent 22,510 25
Large Hydro Baseload 12,281 14
Nuclear Baseload 2,280 2.7
Geothermal Baseload 2,715 3
Natural Gas Baseload 38,576 43
Sampling of Supply Stack
*California has about 61,962 MW of RA net qualifying
capacity installed (as of 10/08/25).
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.
7 Two hypothetical months under Slice of Day:
charging charging
8 Transactability
The Challenge/Problem with Slice of Day:
9 • Unable to transact at the same granularity as
the requirement
o LSEs must purchase more RA than they
need to meet their obligations.
o Creating artificial RA market scarcity by
increasing demand for RA and increasing
prices.
o As a System there is sufficient capacity,
but individual load serving entities (LSEs)
are deficient.
o If an LSE contracts for a resource, it must
do so for the resource's NQC in all 24
hours of the month even if the LSE only has
an open position in one hour.
o Limiting compliance and cost-effective
procurement.
Compliance is hourly; the market is monthly
Source: CPUC
The Solution: Load obligation trading
• Concept:
o LSEs transact load obligations on an hourly basis under the SOD framework.
o LSEs with open positions in some hours to pay another LSE with long positions to cover these positions.
o Trading a load obligation would not relieve an LSE of its obligation to serve load.
o Promote affordability by reducing RA shortages and minimizing over-procurement by allowing LSEs to procure to meet load shapes. o Studies by CalCCA and CCPower show that hourly load obligation trading would have increased compliance with year-ahead RA requirements.
10 (note: not the same as purchasing or trading capacity) Load obligation trading: An example
11 Source: CalCCA
Load obligation trading: An example
12 Source: CalCCA
Load obligation trading: An example
13 Source: CalCCA
Load obligation trading: An example
14 Source: CalCCA
Appendix (companion to slide 9)
16 Source: CPUC