Today, October 1, is the day CAISO's Cluster 16 interconnection window opens, the first full queue intake run entirely under CAISO's Order 2023 compliance tariff. It closes October 15. Three thousand miles east, ISO-NE's own first cluster-based study, the Transitional Cluster Study, opened a year ago and is now in its final stretch, with a report due in August and executed agreements expected by late October. Between the two, there is now enough happening in real time to say something more concrete than "the rules changed": what a live application under the new regime actually requires, and what the first completed cycle under it actually cost.
CAISO's application window runs exactly two weeks, October 1 through October 15 at 5:00 p.m. Pacific, under the GRIP portal CAISO opened for preparation on September 4. CAISO expects to review submissions for completeness within five business days, and applications submitted or corrected after October 8 will likely not get another chance to cure deficiencies before the window closes, according to RMS Energy. From there, the customer engagement and validation period runs into January 2027, with the Cluster Study itself scheduled to begin that month.
Site control is the requirement that trips up the most applicants, and it is strict from the first day. Under CAISO's Order 2023 compliance tariff, approved by FERC in Docket No. ER24-2042, an interconnection request must demonstrate site control, generally 90 percent of the acreage reasonably necessary for the project, at the time of submission, not at some later study milestone, and that control has to extend through commercial operation. See the FERC order and Zonevex's summary. That threshold climbs to 100 percent by the facilities study agreement and stays there through the interconnection agreement. For battery storage specifically, CAISO tightened the underlying acreage formula this cycle, from 0.06 acre per MW to 0.01 acre per MWh, a change aimed at the energy-capacity ratios storage developers actually build around. The cluster study deadline itself is 150 calendar days from the close of the customer engagement window, a hard deadline with financial consequences for CAISO if it slips, which is the mechanism Order 2023 was built around in the first place.
None of this is abstract about how selective the result will be. Cluster 15, the prior full cycle, drew roughly 347 GW of proposed capacity and accepted only about 68 GW of it into study, a rejection rate near 80 percent, per Zonevex's cluster calendar. Projects that fail viability screening in Cluster 16 are rejected without a refund of the study deposit, and a project withdrawn after filing is out of the queue until Cluster 17 reopens. The two-week window is short by design; the filtering happens almost entirely on the strength of what gets submitted in it.
ISO-NE's Transitional Cluster Study is a year ahead of CAISO's on the calendar, and it is the closest thing available right now to a finished answer on how a cluster-based cycle actually performs. It began October 11, 2025, following FERC orders issued that April and June, as ISO-NE's bridge from serial, first-come-first-served review to the cluster framework Order 2023 requires, per the Massachusetts transitional cluster study presentation. Twenty-three interconnection requests are in it, totaling roughly 4,800 MW: 18 storage projects accounting for 4,053 MW, three wind projects at 494 MW, and two solar projects at 280 MW, concentrated mostly in Massachusetts. See Renewable Energy World and ISO Newswire.
The interim report landed June 7, 2026, with a final report due August 6 and draft interconnection agreements tendered by August 21; interconnection customers then have until October 20 to request execution or go to dispute resolution, with deposit deadlines following in early November. The study's cost results are the most useful number to come out of any reformed cluster cycle so far, and they are not encouraging on predictability: per-project network upgrade costs ranged from $0 to $541 million. Projects combining energy and capacity interconnection service averaged roughly $178 million in upgrade costs, though five of them came in at $65 million or less, while capacity-only projects all stayed under $1 million. That is an enormous spread sitting inside a single, 23-project cycle, and it lands on developers who, under the new rules, have already put real site control and commercial-readiness deposits down before they know which end of that range they are on. ISO-NE's presentation is explicit that withdrawal now carries a penalty once the study has advanced, a deliberate change from the old serial process, where a project could walk away from an energy-only study with no penalty at all.
It is one cycle, so it would be a mistake to read ISO-NE's cost spread as a verdict on cluster studies generally. But it is the first dataset that reflects the thing Order 2023 actually changed, rather than the thing it promised to change: speed is a 150-day deadline on paper, but cost allocation is a function of which projects happen to land in the same cluster and which network upgrades they jointly trigger, and that part of the process was never going to get more predictable just because it moved from serial to parallel review. For a CAISO applicant filing this month, the practical lesson from New England's experience is not about CAISO at all; it is that the 90 percent site control and the escalating deposits CAISO now requires up front are being asked of developers before anyone, including CAISO, can tell them whether their project will land near the bottom of that cost range or the top. Cluster 15's roughly 80 percent rejection rate suggests CAISO's screen will do real work before cost allocation even becomes the question. Whether Cluster 16 behaves anything like ISO-NE's first cycle will not be answerable until CAISO's own cluster study results land, on roughly the same 150-day clock, sometime in the summer of 2027.
