CAISO Cluster 15 Results: How 347 GW Became 68 GW and What It Means for Cluster 16

The California Independent System Operator published Cluster 15 scoring results in March 2026, and the numbers tell a stark story. Developers submitted 541 interconnection requests representing approximately 347 GW of proposed generation capacity. After CAISO applied its multi-factor scoring system, only 145 projects — roughly 68 GW — cleared the threshold to enter the cluster study phase. That is a 73% reduction in application count and an 80% reduction in proposed capacity, the most aggressive front-door filtering of any FERC-jurisdictional RTO.

To put the scale in context: CAISO's entire installed generation capacity is approximately 85 GW. The 347 GW of applications in Cluster 15 represented more than four times the system's total existing capacity. Even the 68 GW that survived scoring is still nearly the entire installed base. The oversubscription problem in CAISO is not an abstraction — it is a structural condition that the scoring system is designed to manage.

For developers planning to file Cluster 16 applications when the window opens in October 2026, the Cluster 15 results are the clearest signal yet of what CAISO values, what it penalizes, and where your preparation efforts should focus. This article breaks down the scoring mechanics, the role of site control in the filtering process, the upcoming IPE 5.0 reforms, the Williamson Act complication, hybrid project considerations, and a practical six-month preparation timeline for Cluster 16.

Cluster 15 by the numbers

The raw statistics from Cluster 15 deserve close examination because they reveal the scale of the filtering challenge CAISO faces and the intensity of competition developers are up against.

The average project size that survived scoring was smaller than the average submitted project. This is consistent with CAISO's scoring system favoring projects that demonstrate concrete readiness over projects that propose massive capacity on speculative land positions. A 300 MW project with executed leases, completed permitting milestones, and strong financial commitments will outscore a 1,200 MW project that has filed option-to-lease agreements on half its footprint and has no permits in hand.

The 80% capacity reduction is unprecedented among FERC-jurisdictional RTOs. For comparison, PJM's Cycle 1 process requires 100% site control at application but does not use a scoring system to rank and filter — every qualifying application enters the study. MISO's Definitive Planning Phase requires 50% site control at application and ramps to higher thresholds at later stages but similarly does not score applications against each other. CAISO is the only major RTO that uses competitive scoring to determine which projects even enter the study phase.

This distinction is fundamental. In PJM or MISO, meeting the threshold is binary: you either qualify or you do not. In CAISO, meeting the minimum threshold is necessary but not sufficient. You are scored against every other application in the cluster, and if your score is not competitive, you are filtered out regardless of whether you technically meet the minimum requirements. The implication for developers is clear: in CAISO, good enough is not good enough. You need to be better than the majority of your competitors.

How CAISO's scoring system works

CAISO's interconnection scoring system evaluates applications across multiple weighted factors designed to identify projects with the highest probability of reaching commercial operation. Unlike the binary pass/fail systems used by other RTOs, CAISO assigns a composite score to each application and uses that score to rank projects within the cluster. Projects below the scoring threshold are filtered out before the cluster study begins.

The scoring factors fall into four primary categories:

1. Site control readiness

Site control is one of the most heavily weighted scoring factors. CAISO evaluates both the type and completeness of site control documentation. Projects with executed leases on 100% of their footprint score significantly higher than projects relying on options-to-lease or options-to-purchase. Projects with partial coverage — even if they meet the minimum threshold — are penalized relative to projects with complete coverage.

The scoring system also evaluates the quality of site control documentation. Instruments that clearly identify all parcels, include complete legal descriptions, and demonstrate signatory authority from all required owners receive full marks. Instruments with ambiguous parcel identification, missing signatures, or unclear legal descriptions are scored lower even if they technically demonstrate control.

2. Permitting milestones

CAISO scores projects based on the permitting milestones they have achieved prior to application. Projects that have obtained conditional use permits, completed CEQA (California Environmental Quality Act) review, or secured county land use approvals score higher than projects that have not begun the permitting process. In California, where permitting timelines can stretch 18 to 36 months for utility-scale solar projects, having permits in hand at the time of application is a powerful differentiator.

The permitting factor also accounts for environmental compliance. Projects that have completed biological surveys, cultural resource assessments, and Phase I environmental site assessments demonstrate a level of development maturity that speculative projects cannot match.

3. Financial commitments

CAISO evaluates the financial commitments a developer has made to the project beyond the required interconnection deposits. Evidence of power purchase agreement negotiations, tax equity interest, development financing, or other financial commitments signals that the project has commercial momentum. Projects backed by established developers with track records of completed projects in CAISO territory are scored higher than first-time applicants with no operating assets.

4. Commercial readiness

The commercial readiness factor captures a range of project maturity indicators: whether the developer has secured a point of interconnection, whether the project has an offtake strategy, whether engineering and procurement activities have begun, and whether the project has a realistic commercial operation date. Projects proposing commercial operation dates that are inconsistent with their permitting and construction timelines are penalized.

The composite score is not a simple average of these factors. CAISO weights each factor based on its predictive value for project completion. Site control and permitting milestones carry the highest weights because they are the factors most correlated with a project actually reaching commercial operation.

Site control's role in the score: why marginal control means elimination

The Cluster 15 results confirm what experienced CAISO developers have known for several cycles: site control is the single most important factor in the scoring system, and the difference between a competitive score and an eliminated application often comes down to the details of the land package.

CAISO's scoring system creates a tiered hierarchy of site control quality:

• Tier 1 — Fee simple ownership or executed leases on 100% of the project footprint. This is the gold standard. Projects with fully executed leases on every parcel, signed by all required owners, with no encumbrances, receive the maximum site control score. In a cluster with 4:1 oversubscription, this tier is effectively the minimum for competitive scoring.
• Tier 2 — Mix of executed leases and options-to-lease, with 100% coverage. Projects that have executed leases on the majority of their footprint with options covering the remainder receive a lower but still competitive score. The scoring penalty increases as the proportion of option-covered acreage increases.
• Tier 3 — Options-to-lease on the majority of the footprint. Projects relying primarily on options receive a substantially lower site control score. In a 4:1 oversubscription environment, Tier 3 projects are almost always filtered out unless they score exceptionally high on permitting and commercial readiness factors.
• Tier 4 — Partial coverage or documentation gaps. Projects with less than 100% coverage, missing signatures, ambiguous legal descriptions, or unresolved encumbrances receive the lowest site control scores. These projects are eliminated in every competitive cluster.

The practical takeaway from Cluster 15 is unambiguous: in a 4:1 oversubscription environment, marginal site control equals elimination. The 396 applications that did not survive scoring were not all speculative or half-baked. Many had legitimate site positions but could not differentiate themselves from the 145 projects that had stronger, more complete, more thoroughly documented land packages.

For developers, this means the site control package is not just a compliance checkbox — it is a competitive weapon. Every unconverted option, every missing signature, every unresolved encumbrance, and every ambiguous parcel description reduces the composite score and increases the probability of elimination. In CAISO, the site control work that happens before the application window is the most valuable development activity you can perform.

CAISO site control thresholds by stage

While the scoring system determines which projects enter the cluster study, CAISO also maintains stage-specific site control thresholds that must be met throughout the study process. The following table reflects CAISO's current requirements under the FERC Order 2023 compliance tariff.

The critical difference between CAISO and other RTOs is the application stage. While the formal minimum threshold at application is not 100%, the competitive scoring dynamic means that practical entry requires coverage approaching or at 100% — and preferably with executed instruments rather than options. A project that enters with 80% coverage on options will technically meet CAISO's minimum but will almost certainly be scored out of the cluster.

For the full cross-RTO comparison of how FERC Order 2023 changed site control thresholds across all seven markets, see FERC Order 2023 Site Control Coverage Thresholds by RTO and Stage.

IPE 5.0 reforms ahead of Cluster 16

CAISO's Interconnection Process Enhancements (IPE) 5.0 initiative introduces further refinements to the interconnection process ahead of the Cluster 16 application window in October 2026. The draft final proposal signals several changes that developers must account for in their Cluster 16 planning.

Tighter scoring criteria

IPE 5.0 proposes refining the scoring methodology to increase the weight of factors most correlated with project completion. Based on historical data from Clusters 12 through 15, CAISO has identified that site control completeness and permitting milestones are the strongest predictors of whether a project ultimately reaches commercial operation. IPE 5.0 is expected to increase the scoring weight of these factors, further disadvantaging projects that rely on options or that have not begun permitting.

Potentially higher financial deposits

IPE 5.0 includes a proposal to increase interconnection study deposits and introduce additional financial commitment requirements. The logic is consistent with FERC Order 2023's overall direction: higher financial barriers filter out speculative projects that lack committed capital. Developers should budget for deposit increases of 25% to 50% above current levels when modeling Cluster 16 financial exposure.

Enhanced site control documentation standards

The proposal includes provisions for more rigorous site control documentation review at the application stage. CAISO has observed that some projects in prior clusters submitted site control packages with ambiguous parcel identification, incomplete legal descriptions, or instruments that did not clearly cover the proposed project footprint. IPE 5.0 would establish more explicit documentation standards and potentially require third-party verification of site control packages.

Zonal scoring adjustments

One of the more significant IPE 5.0 proposals is the introduction of zonal considerations in the scoring process. CAISO may adjust scoring to favor projects in transmission zones with available capacity, effectively penalizing projects that propose to interconnect in already-congested areas. This change would have significant implications for project siting strategy and could shift development activity toward less congested zones in the Central Valley, Inland Empire, and desert regions.

For a detailed analysis of the IPE 5.0 proposals and their expected impact on Cluster 16, see the Law of Renewable Energy analysis.

The Williamson Act complication

No discussion of CAISO site control is complete without addressing the Williamson Act — California's agricultural land preservation statute that directly impacts site control for utility-scale solar and wind projects across some of the state's most developable land.

The California Land Conservation Act of 1965, universally known as the Williamson Act, places approximately 16 million acres of California farmland and open space under agricultural preserve contracts. These contracts restrict land use to agricultural and compatible uses in exchange for reduced property tax assessments. The contracts are automatically renewed annually, creating a rolling restriction that can only be terminated through a formal non-renewal process.

The problem for solar developers is straightforward: utility-scale solar development is generally not a "compatible use" under Williamson Act contracts. A landowner who has entered a Williamson Act contract cannot simply execute a solar lease on that parcel — the contract restricts the use of the land, and the lease may be unenforceable or subject to penalty clawback if the development proceeds without proper non-renewal or cancellation.

Non-renewal takes 9+ years

The standard path to exit a Williamson Act contract is non-renewal. When a landowner files a notice of non-renewal, the contract begins a phase-out period that lasts for the remaining term of the contract — typically 9 to 10 years. During this phase-out period, the land remains under contract restrictions, and property tax assessments gradually increase to full market value.

For a developer filing a Cluster 16 application in October 2026, a Williamson Act parcel whose non-renewal was filed in 2026 would not be free of contract restrictions until approximately 2035 or 2036. This timeline is entirely incompatible with CAISO interconnection milestones, which expect commercial operation within 5 to 7 years of cluster entry.

Cancellation is faster but costly and uncertain

The alternative to non-renewal is cancellation, which requires county board of supervisors approval and payment of a cancellation fee equal to 12.5% of the property's unrestricted fair market value. Cancellations are discretionary — the county can deny them — and they are politically contentious in agricultural counties where preservation is a core policy objective. Even when approved, cancellations take 6 to 18 months to process and may face legal challenges from preservation advocates.

Impact on CAISO scoring

A project that includes Williamson Act parcels in its footprint faces a severe scoring penalty. If the parcels are still under contract, the site control instruments covering those parcels are effectively unenforceable for solar development purposes. CAISO's scoring system recognizes this: parcels with unresolved Williamson Act restrictions are treated as having deficient site control, which reduces the composite score and increases the probability of elimination.

For a comprehensive analysis of how Williamson Act preserves interact with CAISO interconnection requirements, including a California solar developer checklist, see our dedicated article: Williamson Act and CAISO Interconnection Queue.

Best practice: Before including any parcel in a CAISO site control package, verify its Williamson Act status through the county assessor's office. If the parcel is under contract, either exclude it from your project footprint or ensure that the non-renewal or cancellation timeline is compatible with your interconnection milestones. Do not assume that an option-to-lease on a Williamson Act parcel will be executable.

Hybrid project dominance: the solar+storage site control challenge

One of the defining characteristics of CAISO's interconnection queue is the overwhelming dominance of hybrid solar+storage projects. According to Berkeley Lab's Queued Up 2025 report, approximately 92% of proposed solar capacity in CAISO is paired with battery storage. This is far higher than any other RTO — PJM is approximately 45% hybrid, MISO is approximately 38%, and ISO-NE is approximately 30%.

The near-universal pairing of storage with solar in CAISO is driven by California's aggressive clean energy mandates, the duck curve pricing dynamics that make standalone solar increasingly uneconomic during midday hours, and the CPUC's preference for dispatchable clean energy resources. For developers, solar+storage is not optional in CAISO — it is the baseline expectation.

Hybrid projects introduce additional site control complexity that standalone generation projects do not face:

• Battery footprint requires dedicated site control. Battery energy storage systems (BESS) require dedicated land area for battery containers, inverters, transformers, fire suppression systems, and safety setback zones. A 200 MW / 800 MWh BESS facility can require 15 to 25 acres of dedicated land area, depending on battery chemistry and configuration. This land must be under site control and included in the project footprint.
• Fire setback requirements. California fire codes (Title 24) and local fire authority requirements impose setback distances around BESS facilities that can significantly increase the total land area required. Setback distances of 50 to 100 feet from property lines and structures are common, and some jurisdictions require additional buffer zones for large-scale lithium-ion installations.
• Shared interconnection considerations. Hybrid projects that share a single point of interconnection for both solar and storage must ensure that the site control package covers the entire co-located facility footprint, including any transmission infrastructure connecting the solar array to the battery system. Parcels that fall between the solar array and the battery facility must be under control even if no generating equipment is placed on them.
• Conditional use permit complexity. Battery storage facilities often require separate or additional conditional use permits beyond what is required for the solar array alone. These permits may impose additional land use restrictions that affect site control requirements, including vegetative screening buffers, noise attenuation zones, and emergency access corridors.

For CAISO scoring purposes, incomplete site control over the battery footprint is treated the same as incomplete site control over the solar array. A project that has 100% coverage on its solar parcels but only 80% coverage on the parcels needed for the BESS facility will receive a combined coverage score that reflects the gap. In a competitive cluster, this gap can be the difference between entry and elimination.

Practical guidance: six months to Cluster 16

The Cluster 16 application window opens in October 2026. That gives developers approximately six months from the date of this article to prepare their applications. Based on the Cluster 15 scoring results and the expected IPE 5.0 refinements, the following timeline represents the minimum preparation path for a competitive Cluster 16 submission.

April – May 2026: Site control audit and gap analysis

1. Convert every option-to-lease to an executed lease. This is the single highest-impact action. In a competitive scoring environment, options are scored lower than executed instruments. Begin conversion negotiations immediately with every landowner where you hold an option. Budget 60 to 120 days for straightforward conversions and 120 to 180 days for parcels with multiple owners or entity counterparties.
2. Run title searches on every parcel. Identify all owners of record, encumbrances, Williamson Act status, and ownership structure (individuals, LLCs, trusts, estates). Flag any parcel with unresolved issues for immediate legal review.
3. Map your complete footprint including BESS parcels. If your project is solar+storage, ensure that your site control package covers the battery footprint, fire setback zones, shared interconnection corridor, and any access parcels. Identify coverage gaps now, not in September.
4. Check Williamson Act status on every parcel. Any parcel under an active Williamson Act contract must either be excluded from the project footprint or have a non-renewal/cancellation process underway with a timeline compatible with your interconnection milestones.

June – July 2026: Resolve encumbrances and ownership issues

1. Negotiate SNDAs for every encumbered parcel. Mortgages and liens on leased parcels must be cured through Subordination, Non-Disturbance, and Attornment agreements. Begin SNDA requests at least 90 days before the application deadline.
2. Resolve entity ownership chains. For every LLC, LP, or trust counterparty, obtain operating agreements, articles of organization, trust documents, and signing authority resolutions. CAISO may require documentation showing that the signatory was authorized to execute the instrument on behalf of the entity.
3. Advance permitting milestones. File conditional use permit applications, initiate CEQA review, and begin biological and cultural resource surveys. Permitting milestones achieved before the application window significantly improve the commercial readiness score.
4. Engage county assessors. Verify APN numbers, parcel boundaries, and acreage figures against county records. Discrepancies between GIS-derived boundaries and assessor records can cause documentation issues during CAISO review.

August – September 2026: Package assembly and quality assurance

1. Assemble the complete site control package. Every instrument, every parcel, every signature, every legal description, every encumbrance disclosure. Cross-check every data point against county records and title reports.
2. Verify all instrument expiration dates. Ensure that every lease and option will remain active through the projected study timeline. An instrument that expires three months after application weakens the scoring position.
3. Model your financial exposure. Calculate study deposits, readiness deposits, and potential withdrawal penalties under multiple network upgrade cost scenarios. Ensure that capital is committed and available.
4. Conduct a mock scoring review. Evaluate your application against the Cluster 15 scoring criteria. Identify any factor where your project would score below the competitive threshold and address it before the window opens.

The developers who survived Cluster 15 scoring did not start their preparation six months before the application window. They started 12 to 18 months before. For Cluster 16, the time to begin preparing is now.

What Cluster 15 tells us about the future of CAISO interconnection

Cluster 15 is not an anomaly. It is the new normal. CAISO's queue will remain massively oversubscribed for the foreseeable future, driven by California's aggressive clean energy targets (100% carbon-free electricity by 2045), the retirement of natural gas generation assets, the electrification of transportation and buildings, and the federal IRA incentives that continue to drive renewable energy investment.

The scoring system is CAISO's mechanism for managing this oversubscription, and it is working as designed: filtering out projects with lower readiness in favor of projects with higher probability of completion. The filtering ratio may fluctuate from cluster to cluster — Cluster 14 saw approximately 65% filtering, Cluster 15 saw 73% — but the trajectory is toward more aggressive filtering as oversubscription continues to grow.

For developers, the strategic implications are clear:

• Site control is the differentiator. In a scoring-based system, the quality and completeness of your site control package is the primary lever you control. Permitting timelines and financial commitments are important, but they take longer to influence. Site control quality can be improved in 3 to 6 months with focused effort.
• Early preparation is non-negotiable. The six-month timeline above is the minimum. Developers who are serious about Cluster 16 should have begun site acquisition 12+ months ago and should be in the lease conversion and encumbrance resolution phase now.
• Speculative applications are dead. The era of filing a CAISO interconnection request to hold a queue position on speculative land is over. The scoring system is specifically designed to identify and eliminate these projects. Filing a speculative application in Cluster 16 is not just a waste of application fees — it is a waste of the 6 to 12 months of development resources spent preparing the submission.
• Withdrawal penalties matter more than ever. For projects that enter the cluster study and subsequently discover site control deficiencies, the financial consequences of withdrawal are substantial. For a detailed breakdown of withdrawal penalties across all RTOs, see Interconnection Queue Withdrawal Penalties by RTO.

Sources

• FERC Order 2023 — Interconnection Final Rule Explainer
• Berkeley Lab Queued Up 2025 — Characteristics of Power Plants Seeking Transmission Interconnection
• Stoel Rives — Refining California's Grid Access Framework: CAISO's Draft Final Proposal for IPE 5.0
• Law of Renewable Energy — CAISO Proposes Key Refinements Ahead of Cluster 16

Related articles

• Williamson Act and CAISO Interconnection Queue — How 16 million acres of agricultural preserves affect CAISO site control and what developers need to do about it.
• FERC Order 2023 Site Control Coverage Thresholds by RTO and Stage — The cross-RTO reference table for all seven markets.
• Interconnection Queue Withdrawal Penalties by RTO — Financial consequences of queue withdrawal across PJM, MISO, CAISO, ISO-NE, NYISO, and SPP.