What is a Cluster Study? Comparison by ISO
A cluster study is how a grid operator studies a batch of interconnection requests together instead of one at a time. Every project that applies in the same window is modeled as a group, the grid upgrades the group needs are identified at once, and the cost of those upgrades is shared among the projects that cause them.
Six of the seven U.S. ISOs now use cluster studies for new generators: PJM, MISO, SPP, CAISO, NYISO and ISO-NE. ERCOT is the exception. Each ISO runs its cluster differently, with its own windows, deposits, study phases and decision points, and those differences shape how long a project waits and how much it risks along the way.
This guide explains how a cluster study works in general, then walks through how each ISO runs its version. Each section links to LandGate's interconnection guides, which map every step of each ISO's process with its costs and timing.
Why Grid Operators Moved to Cluster Studies
Grid operators moved to cluster studies primarily because the traditional first-come, first-served approach to reviewing interconnection requests became overwhelmed by the massive volume of proposed energy projects - particularly data centers.
Under the older serial approach, interconnection queue requests were studied one at a time in the order they arrived. Each project's study assumed every project ahead of it would be built. When an earlier project dropped out, the projects behind it often had to be restudied, and those restudies rippled down the queue. This process wasn't keeping up with the pace of development, and it was creating delays, uncertainty, and unnecessary costs for both developers and grid operators.
Studying projects in groups fixes much of that. The grid operator models one set of projects against one set of assumptions, identifies upgrades that serve the whole group, and splits the cost. Several ISOs had already moved to clusters, and FERC Order 2023, issued in July 2023, made a "first-ready, first-served" cluster process the national standard. It also required readiness deposits, site control and withdrawal penalties to keep speculative projects out.
How a Cluster Study Works
The details of how cluster studies work varies by ISO, but nearly every cluster follows the same path.
The application window. Projects apply during a set window, often once a year. To get in, a request typically needs technical models of the facility, proof of site control, a study deposit and, under most new rules, a readiness deposit. Projects that miss the window wait for the next one.
The cluster study. Engineers model every project in the cluster together on the grid as it's expected to look. The core analyses are steady-state power flow (do lines and transformers overload?), short-circuit (can equipment handle fault currents?) and stability (does the grid recover from disturbances?). Some ISOs also test deliverability, meaning whether a project's output can reach load when the system is stressed.
Cost sharing. The study identifies the network upgrades the cluster needs and assigns each project a share of the cost, usually based on how much that project contributes to each problem. A project's share can change in later phases as other projects drop out.
Decision points. After each study phase, developers get a short window to review their costs and either post more security to continue or withdraw. Withdrawal penalties usually grow the further a project advances, and some ISOs charge more if a withdrawal raises costs for the projects that remain.
Facilities study and agreement. Projects that stay in get a detailed facilities study of the equipment needed at the point of interconnection, then negotiate and sign an interconnection agreement.
How Each ISO Runs Its Cluster Study
ISO | Cluster structure | What sets it apart |
Cycle process with 3 study phases and 3 decision points | Readiness deposits rise at each decision point | |
Annual cycle with 3 study phases and 2 decision points | A queue cap limits how many MW enter each cycle | |
One annual cluster study with 1 decision point | Projects commit to a set GRID-C rate | |
Annual cluster with scored entry | Only top-scoring requests are studied | |
Two-phase cluster study | Deliverability is added in Phase 2, and costs are reallocated in rounds | |
Annual cluster with flat entry deposits | Costs assigned by proportional impact |
PJM: 3 Phases, 3 Decision Points
PJM's reformed Cycle process starts with an application deadline. Each request needs technical data, a study deposit, Readiness Deposit No. 1, a site plan and evidence of site control. After a review period of at least 90 days, PJM studies the whole Cycle as one cluster across three System Impact Study phases.
Phase I is load flow analysis only, with planning-level upgrade costs. Phases II and III add short-circuit and stability analysis and finalize impacts on neighboring systems. After each phase comes a 30-day decision point. Staying in means posting the next readiness deposit, and at Decision Point III, security for the full cost of the upgrades in the final agreement. PJM refunds readiness deposits to projects that withdraw after their upgrade costs jump past set thresholds, such as a rise of 25% or more and over $10,000/MW between Phase I and Phase II.
MISO: 3 Phases Inside an Annual Cycle
MISO calls its cluster the Definitive Planning Phase (DPP). Applications are due by the cycle deadline with an application fee, a study deposit and a milestone payment, and the generating site needs 100% site control at least 90 days before Phase 1 kickoff. A queue cap limits how many MW enter each cycle, so some valid requests are carried to a later one.
Phase 1 is a preliminary System Impact Study, mainly steady-state power flow. Stability, short-circuit and neighboring-system studies come in Phases 2 and 3. Decision Point 1 gives developers 15 business days to review preliminary upgrade costs and pay the next milestone, or withdraw. Withdrawals carry automatic penalties from Phase 1 kickoff, and MISO can hold more of a project's milestone payment at risk under a "harm test" if leaving raises costs for others.
SPP: 1 Study, 1 Binding Decision
SPP's Consolidated Planning Process, approved by FERC in March 2026, replaced its older multi-phase DISIS process with a single annual Interconnection Cluster Study. Applications must be complete at the window, with 100% site control and financial security. The study covers steady-state, stability, voltage and short-circuit analysis, plus a facilities analysis, and targets a signed agreement 180 days after the window closes. That’s the fastest interconnection target timeline of any ISO.
The biggest change is how costs work. Instead of open-ended upgrade bills, projects commit to a GRID-C, a set rate tied to the grid capacity SPP is already planning. A project at a Planned Interconnection Location generally pays only that rate. A project at an unplanned location can also be assigned upgrade costs below 100 kV. At the single decision point, continuing means committing to the GRID-C with non-refundable security, and the results feed directly into SPP's 10-year transmission plan.
CAISO: Score Your Way In
CAISO accepts requests only during a 15-day window each October, with at least 90% site control, full technical data and all deposits. Incomplete applications get no chance to cure. Each request picks a route (deliverability in a Deliverable Zone, deliverability in a Merchant Zone, or Energy Only), and in Deliverable Zones only enough requests to fill 150% of available deliverability are admitted, ranked by score. Scoring weighs commercial interest, project viability and system need. In Cluster 15, only 145 of 541 applications made it in.
Admitted projects go through a Cluster Study covering short-circuit, steady-state, stability and deliverability. Study costs are split 50% per project and 50% by MW. Withdrawing after the study costs the greater of the study deposit or twice the project's share of study costs, unless the withdrawal doesn't materially affect others.
NYISO: Deliverability Comes Second
NYISO runs a two-phase cluster. Phase 1 estimates each project's local upgrade costs but doesn't include deliverability, so developers decide whether to enter Phase 2 without knowing that outcome. Phase 2 refines the costs after withdrawals and adds deliverability. Developers then accept or reject their cost allocations in final decision rounds, and NYISO reallocates costs and runs another round each time projects drop out.
ISO-NE: Proportional Impact and Enabling Upgrades
ISO-NE moved to a cluster process under Tariff Schedule 22, effective September 1, 2026, and opened its first regular window on October 5, 2026. The cluster study assigns network upgrade costs using a proportional impact method. Where an area needs major new transmission to admit the cluster, ISO-NE can run a Cluster Enabling Transmission Upgrade study and ask developers to post a participation deposit.
The Exception: ERCOT
ERCOT doesn't use cluster studies for generators. Each project is studied individually when it applies, through a screening study and a full interconnection study run by the transmission service provider. ERCOT connects projects and manages congestion in real time rather than building transmission to guarantee each project's output.
Before You Enter a Cluster
A cluster study tells you what your project will cost after you've committed deposits and waited months. Most of the risk can be screened earlier.
Know the deposits and exit costs. Every ISO's deposits, decision points and withdrawal penalties are mapped in LandGate's interconnection guides. Set your project's size and an upgrade cost estimate, and the Costs tab recalculates what's due at each step for your project, so you can compare what it costs to stay in, or to leave, in each region.
Pick a point of interconnection with room. Upgrade costs in a cluster come from overloads near where projects connect. LandGate's available transfer capacity (ATC) data estimates how much more power a substation can take and what upgrades might cost, which helps narrow sites to ones less likely to trigger large upgrade bills.
See who else is nearby. Projects at the same substation in the same cluster share upgrades, and their withdrawals can change your costs. LandGate's platform shows transmission lines, substations and interconnection activity around any parcel, so you can see how crowded a point of interconnection already is.
Check offtake for co-located load. Cluster studies cover generators. If your project pairs generation with a data center or other large load, the load follows a separate path, and offtake capacity, the power available to draw at a substation, matters as much as transfer capacity.
Frequently Asked Questions
What is a cluster study in interconnection?
A cluster study evaluates a group of interconnection requests together rather than one at a time. Projects that apply in the same window are studied as one batch, and the cost of the network upgrades they need is shared among them.
What is the difference between a cluster study and a serial study?
A serial study looks at one request at a time in queue order, so each project's results depend on every project ahead of it. A cluster study looks at a whole group at once, which cuts down on the restudies that happen when earlier projects drop out.
Which ISOs use cluster studies?
PJM, MISO, SPP, CAISO, NYISO and ISO-NE all use cluster studies for new generators. ERCOT studies each generator individually.
What happens if I withdraw from a cluster study?
You usually lose part of your deposits, and the penalty grows the further you advance. Some ISOs, including MISO, can charge more if your withdrawal raises costs for the projects that remain.
