CAISO’s Western Energy Imbalance Market has surpassed $8 billion in cumulative economic benefits since its 2014 launch after providing participants with $415.65 million in gross benefits in the fourth quarter of 2025, according to an ISO report.
In a news release accompanying the quarterly report, CAISO noted it has revised the methodology it uses to calculate WEIM benefits to reflect the market behavior of variable energy and battery storage resources.
NV Energy earned the largest share of Q4 benefits, at $83.10 million, followed by PacifiCorp ($66.45 million), Los Angeles Department of Power and Water (LADWP) ($40.71 million), Balancing Authority of Northern California (BANC) ($37.15 million), Public Service Company of New Mexico ($34.78 million) and NorthWestern Energy ($23.41 million).
PacifiCorp, with its East and West balancing authority areas, was the biggest net exporter of energy, at nearly 1.54 million MWh. The next largest exporters were CAISO (720,188 MWh), NV Energy (514,474 MWh), Salt River Project (427,248 MWh) and LADWP (250,431 MWh).
The biggest net importer during the quarter was CAISO, at over 1.02 million MWh, followed by BANC (507,535 MWh), Portland General Electric (433,229 MWh), Powerex (408,684 MWh) and PacifiCorp (391,588 MWh).
In the WEIM, a net export represents the difference between total exports and total imports for a BAA during a particular real-time interval, while a net import represents the inverse, meaning a BAA can be both a heavy exporter and importer over an extended period based on varying momentary needs and trading positions over that period.
CAISO’s BAA facilitated the greatest volume of wheel-through transfers, at 964,219 MWh, followed by PacifiCorp’s two BAAs (501,382 MWH) NV Energy (445,994 MWh) and Arizona Public Service (327,982 MWh).
‘Robust, Transparent, Reflective’
The Q4 report also came with some changes in how CAISO calculates WEIM benefits.
“With significant changes in market resources and operational dynamics across the West, maintaining an accurate picture of market performance is essential,” CAISO said in the release. “Additional time was needed to post this report so that the ISO — working closely with its WEIM partners — could refine the benefits methodology to reflect these evolving market resources and system conditions. This helps to ensure its logic remains robust, transparent and reflective of current conditions.”
The revisions are spelled out in the “Counterfactual Dispatch Cost” section of the updated “EIM Quarterly Benefit Report Methodology.”
In the case of variable energy resources, the revised methodology adjusts the market’s bid range logic for resource base schedules to reflect real-time dispatch (RTD) market data rather than the previous approach of relying on 15-minute market (FMM) data.
“This adjustment offers a more accurate reflection of actual market conditions in two key aspects. Dispatches and transfers from WEIM solution are based on the RTD markets and using bids from RTD market will better align,” CAISO explains in the methodology document.
“Second, the forecasted output for variable energy resources often differs between the FMM and RTD markets. By using the RTD forecast to estimate load imbalance in the benefit calculation, it more accurately reflects actual RTD conditions. It also eliminates imbalances that reflect forecast differences and focus on imbalances from actual market redispatches.”
In describing the impact of the change, CAISO cites the example of a wind resource having 73 MW of energy available based on the FMM forecast but getting reduced to 16 MW in the RTD forecast. Under the new logic, the resource’s bid range would be capped at 16 MW, putting both its base schedule and dispatch-adjusted base schedule at 16 MW heading into the real-time interval, leaving a load imbalance of 0 MW.
“This 0-MW imbalance reflects the scenario where the market is not redispatching the resource down. Instead, it simply accounts for the adjustment in the forecast available in RTD. Therefore, there is no WEIM cost associated with this resource,” CAISO wrote.
Another revision to the methodology deals with the modeling of battery storage resources in the counterfactual dispatch — that is, a theoretical dispatch that would occur without the availability of WEIM transfers.
CAISO explains that, prior to Q4 2025, batteries were modeled like conventional resources, with the model estimating an available dispatch range and determining the counterfactual dispatch based on the resource’s price — an approach that ignored a battery’s limits based on its state of charge. To address that, the updated methodology:
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- Adjusts a battery resource’s maximum bid limit based on its state of charge;
- Enforces a constraint that prevents a battery from being dispatched below a defined minimum state of charge; and
- Recognizes the end-of-hour constraint defined by a battery operator.