If you are a California developer looking for the 2026 Transmission Plan Deliverability allocation window, stop looking. There isn't one.
That is not a scheduling rumour. CAISO's Interconnection Process Enhancements 2023 Track 3 Updated Final Proposal sets out the revised calendar in one paragraph: "The 2025 seeking and retention affidavit due date for cluster 14 and prior clusters will be delayed from March 15 to September 1, 2025. The TPD study will begin on November 1, 2025, and the TPD study results will be published on March 31, 2026. With the date of the 2025 TPD allocation results occurring in 2026, no additional TPD allocations will be performed in 2026. The 2027 TPD allocation cycle will occur on the standard schedule with the affidavits due March 15, 2027."
So the calendar that actually governs is: affidavits were due 1 September 2025, the study ran from 1 November 2025, results published 31 March 2026, and the next chance to file is 15 March 2027. Anyone planning around a "2026 window" is planning around a cycle that was collapsed into the 2025 one.
What TPD actually is
CAISO's own definition, from its transmission plan deliverability page, is worth reading slowly: TPD is "the capability, measured in megawatts (MWs), of the California ISO Controlled Grid as modified by transmission upgrades and additions modeled or identified in the annual Transmission Plan to support the interconnection with full or partial capacity deliverability status of additional generating facilities in a specified geographic or electrical area of the ISO Controlled Grid."
Three consequences follow from that sentence.
TPD is a finite quantity in a place. It is created by transmission the ISO has planned, and it exists in a study area, not on the system as a whole. Deliverability at Windhub does nothing for a project behind Lugo.
It is allocated, not earned. Projects compete for it annually against every other candidate in the same area.
It is the gate to Full or Partial Capacity Deliverability Status, and therefore to counting as resource adequacy capacity in California. An Energy Only project generates and sells; it does not carry RA value. That is the commercial difference, and it is the reason the affidavit matters more than almost any other date in a California project's calendar.
The affidavit is the whole ballgame
The 2025 TPD Allocation Report is blunt about what happens if you miss it: "TPD candidates that did not submit TPD allocation affidavit are not eligible for the allocation. TPD candidates that have submitted valid TPD allocation affidavit are placed in the proper allocation group. TPD candidates not belonging to any of the allocation group are not eligible for allocation."
No affidavit, no group. No group, no allocation — regardless of how good your project is, how far along the queue it sits, or how much deliverability happens to be free in your area. The affidavit is a filing deadline that behaves like a qualification round.
The document set is not exotic. CAISO's standing notices for the process point at GIDAP Appendix DD sections 8.9.1, 8.9.2 and 8.9.3 — for Cluster 4 and earlier projects, for projects seeking an allocation, and for projects retaining one previously allocated — plus Appendix DD 6.7.4 for the annual Commercial Viability Criteria demonstration, with the corresponding business practice manual sections at BPM 6.2.9.3 through 6.2.9.5. Documents go to [email protected].
What trips teams up is not the form. It is that retention is annual. A project that won an allocation and then went quiet still has to demonstrate continued satisfaction of the Commercial Viability Criteria every year, and the affidavit is where that demonstration lands.
The four groups, and why group order beats project quality
The 2025 report sets out the allocation groups verbatim:
(A) Interconnection Customers that have executed power purchase agreements, and Interconnection Customers in the current Queue Cluster that are Load Serving Entities serving their own Load. (B) Interconnection Customers that are actively negotiating a power purchase agreement or on an active short list to receive a power purchase agreement. (C) Interconnection Customers that have achieved Commercial Operation for the capacity seeking TP Deliverability. (D) Interconnection Customers electing to be subject to Section 8.9.2.3.
Group A is allocated first, then B, then C, then D. Within each group, candidates are ranked by TPD score. And the allocation mechanic in constrained areas is brutal: "For each area deliverability constraint, the capacity of the TPD candidates is reduced one by one in the ascending order of the deliverability priority until the overloads are eliminated."
Reduced one by one until the overloads are eliminated. In a tight area, the group boundary is the whole outcome. A group B project behind a binding constraint does not get a smaller allocation than a group A project; it frequently gets nothing. The 2025 report records exactly that in several study areas — for the North of Lugo area, it states that "no deliverability could be allocated to generators located in the North of Lugo area that were seeking deliverability in the 2025 TPD allocation cycle."
The scale of the contest is in the report's own candidate table. Summing it: 84 parked Queue Cluster 14 projects sought 22,627 MW of interconnection service capacity, and a further 49 Energy Only projects sought 5,768 MW — PG&E alone accounting for 34 parked candidates at 8,988 MW. That is what your project is queuing behind.
Group B is being abolished
The change most likely to catch a 2027 filer off guard is in the Track 3 proposal: "The proposed changes for cluster 14 and prior projects would begin with the 2027 TPD allocation cycle. Those changes are the result of the proposed elimination of allocation group B (the shortlist group) in 2027. No project will be able to seek an allocation based on being shortlisted or actively negotiating a PPA in 2027."
The shortlist route disappears. From the 2027 cycle, the practical routes are an executed PPA, being an LSE serving its own load, or having achieved commercial operation. The proposal further notes that projects that received a group D allocation in 2025 "will only be able to retain a group D allocation in 2027 by demonstrating a PPA."
Translate that into a development calendar: if your project is relying on TPD and does not have an executed PPA by roughly the turn of 2027, the March 2027 affidavit is not a filing problem, it is a commercial one. The ISO has moved the proof point from "someone is interested" to "someone has signed."
What this does to Cluster 15
Cluster 15 is the cohort that pays for the calendar slip. The Track 3 proposal is explicit: "It will not be possible to have a TPD opportunity for cluster 15 projects to seek an allocation during their facility study that occurs from May into November, 2026. Thus, all cluster 15 projects, including those that have a PPA during the facility study will have to wait until the March 2027 cycle to seek TPD."
And again, flatly: "Cluster 15 would not be eligible to seek allocations of TPD until March 2027."
The ISO's answer to the objection is a structural one it makes in the same document — that under FERC Order No. 2023, "the required GIA execution date is always before the results from the first of three TPD allocation cycles are available." In other words, in the post-Order-2023 world, every cluster signs its interconnection agreement before it knows its deliverability outcome. That is not a CAISO quirk; it is what first-ready-first-served cluster processing does to the sequencing.
After a cluster's facilities study completes, its projects get "three consecutive opportunities to seek an allocation of TPD," and the proposal notes that parking "will no longer be associated with this process." Three shots, on a fixed annual clock, with no parking to buy time.
Things to do before March 2027
Diary 15 March 2027, and work backwards from it. Both the seeking affidavit and the retention affidavit land on that date. Retention failures are unforced errors.
Convert shortlist to signature. With group B eliminated in 2027, a shortlist is worth nothing at the affidavit. If your PPA negotiation is real, the schedule pressure is now regulatory, not just commercial.
Know your study area, not just your county. TPD is allocated against area deliverability constraints with names like Midway–Harlan 500 kV, North of Magunden and Windhub. Read the 2025 report's area sections for the constraint your point of interconnection sits behind, and look at what was allocated there last cycle before you underwrite anything.
Treat Energy Only as a real option, not a failure. An Energy Only project with a good node can be financeable on energy and ancillary revenue. It is a different business than an RA-contracted project, and it should be underwritten as one from the start rather than as a fallback after a failed allocation.
Do not model deliverability transfers you cannot make. The Track 3 board decision memo records that "deliverability transfers from cluster 15 and later clusters to earlier queued projects will be prohibited," while transfers from Cluster 14 and earlier to Cluster 15 are permitted with the receiving customer inheriting the transferring party's obligations. The direction of travel matters.
Watch the second financial security posting. The ER25-2044 tariff amendment moved the second Interconnection Financial Security posting for parked Cluster 14 projects to "the later of (a) May 29, 2026, or (b) sixty (60) days after the CAISO publishes the results of the TP Deliverability" allocation — the point being, as the Track 3 proposal explains, that customers should not have to post without knowing the allocation results.
The honest summary
CAISO did not tighten the deliverability rules for 2026. It removed 2026 from the calendar, folded that year's opportunity into a 2025 cycle whose results landed on 31 March 2026, and set the next affidavit for 15 March 2027 — by which point the shortlist route is gone and an executed PPA is the price of entry to the front of the queue.
If you are building in California and your revenue case needs Full Capacity Deliverability Status, the deadline you are working to is 15 March 2027, and the deliverable is a signature.
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