Process and the grid

Solar interconnection: why it takes so long

The review is not about your house. It is about the transformer your house is attached to, and everyone else attached to it.

Updated September 2026 · Data as of Utility tariffs and federal orders read on 2 September 2026

Written by HyreSolar Research team Research and analysis

Audited by HyreSolar Research team Data audit and fact check

90% of hosting capacity is the pass threshold The main technical screen

+20 business days if you go to supplemental review Plus 10 more for a fault current study

30 kVA below which no extra capacity analysis is run Most residential systems

The short answer

Interconnection is a technical review of the circuit you are joining, and most of what it examines is outside your property. A filed utility tariff runs your system through a series of screens: whether the equipment is certified, whether the local transformer and its conductors can carry what is already attached to them plus you, whether the connection point sits on a networked secondary, and whether the circuit's measured hosting capacity has room. Several of those can fail because of systems your neighbours installed. Fail a screen and you move to a supplemental review that adds a mandated twenty business days, plus another ten if a fault current study is needed. Passing does not even guarantee there is no construction: a separate path exists for requests that pass but still require utility-side upgrades. One useful piece of good news for ordinary homes: in the tariff we read, no additional hosting capacity analysis is performed for projects under 30 kVA, which covers most residential systems.

What is actually being reviewed

The mental model most homeowners have is that interconnection is administrative: forms move between an installer and a utility, someone signs them, and the delay is bureaucratic slack. That model makes the wait feel arbitrary, and it is wrong in a way that matters.

Interconnection is an engineering review of the distribution circuit you are about to start pushing power into. Distribution networks were designed to carry electricity in one direction, from a substation outward to houses. A rooftop system reverses that flow for part of the day. Whether that is fine depends on the transformer you share, the conductors between you and it, how much generation is already attached, and how the local design handles reverse flow.

Almost none of those variables are on your property. That is the single most useful thing to understand about this stage, because it explains why two identical systems on identical roofs in the same city can have completely different experiences, and why nothing your installer does can guarantee a timeline.

The vocabulary the screens are written in

Secondary transformer: The transformer that steps voltage down for your street, typically shared between a handful of houses. Several screens are about this specific piece of equipment and everything already attached to it.
Networked secondary: A distribution design where secondary conductors are meshed together rather than run radially, common in dense urban cores. It handles reverse power flow badly, which makes it a known hard case for interconnection.
Integration Capacity Analysis: The utility’s modelled estimate of how much generation a point on the circuit can absorb, published as a profile of values across hours rather than a single number. The main hosting capacity screen tests against the lowest value in it.
Supplemental review: The second track for a request that fails an initial screen. It is not a rejection. It adds a mandated twenty business days, runs looser versions of some screens, and ends in a determination of what mitigation is required.
Interconnection facilities and distribution upgrades: Physical work on the utility side that your connection requires. A request can pass review and still need these, at which point the timeline becomes a construction schedule rather than a review period.
Export limiting: A mitigation that caps how much power your system may push onto the grid, usually enforced by the inverter. Frequently offered as an alternative to physical upgrades, and it changes what your system is worth to you.

The screens your system is actually run through

Screen	What it asks	Why it can fail
Networked secondary	Is the connection point on a networked secondary system?	A known hard case in dense urban cores, where the distribution design assumes power flows one way into a mesh.
Certified equipment	Is the equipment certified to the standard the tariff requires?	Certified equipment gets no repeat design review. Uncertified equipment does, and certification is a prerequisite for inverters installed after a date in 2017.
Transformer and conductor loading	Do all the generating facilities on this secondary transformer together exceed the transformer or conductor rating?	Your neighbours’ systems count toward this. You can fail on capacity you did not install.
Single-phase imbalance	Does a single-phase generator on the centre tap of a 240 V service unbalance the two legs unacceptably?	A wiring-configuration question about your own service rather than the wider grid.
Hosting capacity	Is the system at or below 90% of the lowest value in the utility’s Integration Capacity Analysis profile?	The big one. It is a measurement of what the circuit can already absorb, and it has nothing to do with your roof.
Penetration	In supplemental review, is the system at or below 100% of the lowest hosting-capacity profile value?	A second, looser pass at the same question for systems that failed the first.

Screens as set out in a filed utility interconnection tariff, read 2 September 2026. Named descriptively here; the tariff labels them by letter.

Read the third column of the transformer row again. The screen asks about the aggregate of every generating facility on your secondary transformer, so a neighbour who installed last year is part of your review.

Hosting capacity, and why the answer can change while you wait

The screen that decides most difficult cases is hosting capacity, and it works differently from how people expect.

The utility maintains an Integration Capacity Analysis: a modelled figure for how much generation a given point on the circuit can absorb, published as a profile of values rather than a single number, because capacity varies by hour and by season. Your system passes if it is at or below 90% of the lowest value in that profile, on more than one measure. Fail either, and the tariff sends the request to supplemental review to determine what mitigation is required.

Two features of this make it frustrating to be on the receiving end of.

It is a measurement of the circuit, not an assessment of you. There is no version of your application that passes a hosting capacity screen a circuit cannot support. It is not about your equipment, your installer or your paperwork.

And the figures can be re-run mid-review. The tariff permits the utility to decide the analysis values at your connection point need updating, re-run the tool, and proceed on the new numbers. It must share the results and explain what changed, and the tariff specifically names "changes to grid conditions or the interconnection queue" as reasons. In other words, other people's pending applications are part of the picture, and the ground can move underneath yours.

The relief for ordinary homes is real and worth knowing. The same section carves out projects under 30 kVA: no additional capacity analysis is performed for them. Most residential systems sit well under that, which is why most residential interconnections are not where the queue pain is.

What happens when a screen fails

Failing a screen is not a rejection. It is a change of track, and the track has its own published clock.

Supplemental review adds a mandated twenty business days. If a fault current study is required, that is another ten. Those are business days, so twenty is around four weeks of calendar time before any holiday.

Supplemental review then runs its own screens, including a looser penetration test at 100% of the lowest hosting capacity profile value rather than 90%. A system that failed the first pass can clear the second.

If it does not, the utility determines what mitigation is required. That can mean export limiting, advanced inverter functions or monitoring rather than physical work; the tariff contemplates all of them as alternatives to construction.

Passing does not mean nothing gets built

There is a step people are frequently surprised by, and it sits after what feels like success.

The tariff provides a separate path for interconnection requests that pass initial review but still require what it calls Interconnection Facilities or Distribution Upgrades. That path triggers cost estimates, a deposit against an agreed cost envelope, and construction.

So "your application passed review" and "no work is needed on the utility side" are different statements, and the first does not imply the second. If upgrades are required, the timeline is no longer about document processing at all. It is about scheduling physical work on a distribution network, which runs on a utility's construction calendar.

There is also a newer constraint worth naming because it is not intuitive. Beyond the classic limits of thermal capacity, voltage and protection coordination, the analysis has added an operational flexibility dimension concerned specifically with reverse flow. Where it binds, the mitigation offered is often operational rather than physical: limiting export, enabling particular inverter behaviours, or adding monitoring.

Some of the clock is yours

Not all of the elapsed time belongs to the utility, and this is worth knowing before you conclude you are being ignored.

The process contains several applicant-side election windows, typically ten to twenty business days, at which you or your installer must respond, choose an option or provide something. Each is extendable. Each is calendar time you experience as waiting, and none of it is the utility taking a long time.

The practical consequence is that "how long has it been" is the wrong question. The right question is whose court the request is currently in, and since when. Ask your installer for the current status and the date it entered that status. If the answer is that a response is due from your side, the delay is fixable this afternoon.

Why there is no national answer to any of this

Everything above comes from one utility's tariff, and that is not a limitation of our research so much as a description of how this is regulated.

The Federal Energy Regulatory Commission has expressly declined jurisdiction here. In its 2020 order on distributed energy resources it stated that it declines "to exercise our jurisdiction over the interconnections of distributed energy resources to distribution facilities" in this context, and that under the long-standing first use test, "such interconnections are governed by the applicable state or local law."

So a rooftop system is governed by a state commission's framework and, operationally, by the tariff your specific utility has filed under it. There are thousands of utilities. The screens described on this page are one utility's implementation of a widely shared engineering logic, and both halves of that sentence matter: the logic generalises, the numbers do not.

The actionable version: find your own utility's interconnection tariff. It is a public document, it is the thing that actually governs your timeline, and almost nobody reads it. Search your utility's name with "interconnection tariff" or "Rule 21". If it publishes hosting capacity maps, which several utilities now do, you can often see the constraint on your own circuit before you sign anything.

How to reduce your exposure to this

1
Ask about your circuit before you sign
Where a utility publishes hosting capacity maps, an installer who works in your area can check your circuit in minutes. A constraint found before contract is a design question. Found after, it is a delay.
2
Ask whether the request is filed before or after installation
The tariff we read advises filing the interconnection request early, and for harder projects up to six months ahead of the target operation date, precisely so the review runs alongside the build rather than after it.
3
Confirm the equipment certification against the current tariff
Certification is a prerequisite in the tariff we read for inverters installed after a date in 2017, and equipment certified to different revisions is handled under different sections. This is a paperwork failure that surfaces at the worst moment.
4
Ask what size threshold applies to you
In the tariff we read, projects under 30 kVA skip the additional capacity analysis. If your design sits just above a threshold like that, it is worth knowing what crossing it costs you in time.
5
Track whose court it is in
Ask for the current stage and the date it entered that stage, not just "how is it going". Applicant-side response windows are part of the elapsed time and are the part you control.
6
Read your own utility’s tariff before escalating
It sets out the screens, the review periods and any processing standards. Escalating with a section number and a date is a different conversation from escalating with a complaint about slowness.

Method and limitations

One tariff, described honestly

Every screen, threshold and review period on this page comes from a single filed utility tariff: PG&E's Electric Rule 21, approved by the California Public Utilities Commission, read directly. It is not California law, it is not how the other large California utilities operate since each files its own, and it is not a national standard.

We use it because it is unusually detailed and publicly available, which makes it the best available illustration of what an interconnection review consists of. Treat the structure as instructive and every number as specific to that utility.

Interconnection rules in New York, Texas, Arizona, Massachusetts and New Jersey were all attempted. Every source refused automated retrieval and the usual archive workaround was unavailable throughout, so no state other than California was verified and this page asserts nothing about any other.

What we do not state

Any electrical code provision. The rapid shutdown and busbar rules commonly cited in solar writing could not be read from any authoritative source; the publisher's free portal returns no text to automated retrieval and the only version we found was a machine-generated summary on a republisher's site. We would rather omit a code number than print one we have not read.

Any meter exchange timeline. A real step that takes real time, and we could not find a tariff provision setting a window for it.

The usual claim about what the 2018 interconnection standard changed. It is widely written that the previous edition required inverters to disconnect on a disturbance while the current one requires them to ride through. We did not read the pre-2018 text and so are not repeating it. What we verified is that ride-through behaviours are mandated and settable under the current regime, with defaults a utility may re-tune where studies require it.

Any typical duration. We have published review periods, which are what should normally happen. We have no measurement of what does happen, and those are different claims.

This may already have changed

A rulemaking opened at the California commission in August 2025 may revise the rule this page draws on, and an expedited dispute-resolution process that once sat alongside it has been suspended following a funding lapse. Read the current tariff rather than this page if a live deadline depends on it.

Questions

Why does solar interconnection take so long?

Because it is an engineering review of the circuit rather than a paperwork exercise. Your system is screened against the shared transformer and its conductors, the local distribution design, and the circuit’s measured hosting capacity. Failing a screen moves the request to a supplemental review that adds a mandated twenty business days, and a further ten if a fault current study is required.

What is hosting capacity?

A utility’s modelled figure for how much generation a given point on the circuit can absorb, published as a profile of values across hours rather than one number. In the tariff we read, a system passes if it is at or below 90% of the lowest value in that profile. It is a measurement of the circuit, so no change to your application makes a constrained circuit pass.

Can my neighbours’ solar affect my application?

Yes, and this surprises people. One screen asks whether the aggregate of all generating facilities on your secondary transformer exceeds the transformer or conductor rating, so capacity installed by neighbours counts against yours. The hosting capacity figures can also be re-run mid-review for reasons the tariff names as including changes to the interconnection queue.

What happens if my system fails a screen?

It is not a rejection. The request moves to supplemental review, which adds twenty business days and runs its own screens, including a looser penetration test at 100% rather than 90%. Systems that fail the first pass often clear the second. If not, the utility determines mitigation, which can be operational, such as export limiting or particular inverter behaviours, rather than physical work.

Does passing review mean no construction is needed?

No. The tariff has a separate path for requests that pass but still require interconnection facilities or distribution upgrades, triggering cost estimates, a deposit and construction. Passing review and requiring no utility-side work are different statements.

Is my residential system subject to all of this?

Some of it, and in the tariff we read there is a real carve-out: no additional hosting capacity analysis is performed for projects under 30 kVA, which covers most residential systems. The other screens still apply. This is one reason ordinary residential interconnections are usually not where queue delays concentrate.

Why does my experience differ from someone in another state?

Because there is no national rule. FERC has expressly declined jurisdiction over these interconnections and stated they are governed by applicable state or local law, so what applies to you is your state commission’s framework and your specific utility’s filed tariff. The engineering logic generalises; none of the numbers do.

How do I find my utility’s interconnection rules?

Search your utility’s name with "interconnection tariff", or with "Rule 21" in California. It is a public document your utility is required to publish. Several utilities also publish hosting capacity maps, which let you or your installer see the constraint on your own circuit before you commit to a design.

Written and audited by

HyreSolar Research

Primary-source research, data analysis and fact checking

We are a research desk, not a sales floor. We read the statute, the tariff, the code section, the federal filing or the manufacturer data sheet ourselves, and we publish the figure with the document it came from and the date we retrieved it. Where a number cannot be traced to a primary source, we publish the shorter page and say what we could not verify. That rule has cost us whole sections, and it is the reason the rest can be trusted.

160: primary sources read and cited
220: figures with a retrieval date
115: federal and state government sources
66: researched pages published

How this desk works

Primary sources only. Statutes from the legislature’s own publishing system, federal data from the agency that collects it, code text from the adopted edition, manufacturer claims from the data sheet. We do not cite an article that cites a source; we go and read the source.
Every figure carries its provenance. A named document and the date we retrieved it, so you can check it and so you know how old it is. Retrieval dates are not decoration: an EIA rate from May is a different fact from an EIA rate from August.
We publish what we could not verify. Every research page carries a section naming the things we tried to establish and could not, and why. A paywalled standard, a state website that refused the request, a manufacturer that publishes no figure at all.
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We do not sell solar, and we take no payment for placement, ranking or a favourable mention. Nobody buys a position on this site.

Data as of Utility tariffs and federal orders read on 2 September 2026. Authorship on this site is organisational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold. Our editorial policy sets out how we source, date and correct what we publish.

Sources & retrieval dates

PG&E Electric Rule No. 21 (Advice 7692-E) — Effective 29 August 2025. Source for the interconnection screens described on this page, the 90% Integration Capacity Analysis threshold and the 100% penetration test, the provision permitting hosting capacity values to be re-run mid-review, the carve-out for projects under 30 kVA, the twenty-business-day supplemental review with ten additional days for a fault current study, the separate path for requests requiring interconnection facilities or distribution upgrades, the applicant-side election windows, and the equipment certification prerequisite. A tariff filed with the California Public Utilities Commission; not a statute and not applicable to other utilities. Retrieved 2 September 2026.
FERC Order No. 2222, 85 FR 67094 — Published 21 October 2020. Source for the Commission declining jurisdiction over interconnections of distributed energy resources to distribution facilities, and for the statement that such interconnections are governed by applicable state or local law under the first use test. Retrieved 2 September 2026.
CPUC, Electric Rule 21 — Source for the description of Rule 21 as a tariff describing interconnection, operating and metering requirements, and for the operational flexibility analysis adding a reverse-flow limitation beyond the classic thermal, voltage and protection constraints. Retrieved 2 September 2026.
IEEE 1547-2018 and IEEE 1547a-2020 — Standard for Interconnection and Interoperability of Distributed Energy Resources, published 6 April 2018 with a 2020 amendment. Source for the edition, title and scope of the standard the tariff harmonises with. Retrieved 2 September 2026.

Waiting on an interconnection with no explanation?

Tell us your utility and where the request has got to. We will point you at the tariff that governs it and what the screens actually require.

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