01
Pre-application report
A paid report telling us what is already on the circuit before we commit to a design.
Circuit capacity understood before design money is spent
Process
Interconnection and permission to operate
Construction runs to plan. The utility queue does not, and it usually decides when your system produces its first kilowatt hour. Here is every gate, who owns it, and what typically goes wrong.
- Governing tariff
- California Rule 21
- Stages
- Eight
- Typical rooftop path
- 26 to 34 weeks
- Filed in parallel
- Permit and application
The path
Eight gates, in order
Each card names the owner, the typical week range and the gate that has to close before the next stage starts.
Eight stages, scroll sideways
02
Rule 21 application filed
The formal interconnection application, filed the same week as the building permit set.
Application deemed complete by the utility
03
Fast track screens
A defined set of pass or fail tests. Pass them all and the project skips the studies entirely.
All screens passed, or routed to supplemental review
04
Supplemental review
A closer look when a screen fails, usually resolving without a full distribution study.
Cleared, conditioned, or escalated to a distribution study
05
AHJ plan check and permit
Building and electrical permits, running alongside the utility process rather than after it.
Permit issued
06
Interconnection agreement
The contract that sets the export arrangement and any conditions from the review.
Agreement executed by both parties
07
Final inspection and witness test
The building inspector signs off, then the utility verifies the protection settings.
Green tag from the AHJ and a passed witness test
08
Permission to operate
The letter that lets you close the breaker. Until it arrives, the system stays off.
PTO letter issued and the system energised
In detail
What happens inside each stage
01
Pre-application report
For a modest fee the utility returns the substation and circuit that serves the site, the existing generation already interconnected, the minimum recorded load on the line and the available capacity.
That one document decides whether the project is a formality or a study. A circuit already carrying significant generation is where fast-track screens start failing.
We run this before the array layout is finalised, because the answer can change the system size we recommend.
Gate: Circuit capacity understood before design money is spent
02
Rule 21 application filed
The application carries the single-line diagram, inverter certification, site plan, equipment specifications and the generating facility's nameplate ratings in both kW AC and kW DC.
Inverters must be certified to the current UL 1741 supplement covering advanced grid-support functions. Uncertified equipment is the most common avoidable rejection we see.
We file the interconnection application and the AHJ plan set in parallel so the two clocks run together instead of end to end.
Gate: Application deemed complete by the utility
03
Fast track screens
The screens test aggregate generation against the line section's minimum load, the generating capacity against the circuit's annual peak, the transformer and conductor ratings, and whether the point of connection is on a spot network.
The most commonly cited threshold is aggregate generation staying under 15 percent of the line section's annual peak load, though the rules also allow other paths for projects that exceed it.
Pass every screen and you go straight to an interconnection agreement. Fail one and the project moves to supplemental review.
Gate: All screens passed, or routed to supplemental review
04
Supplemental review
Supplemental review tests the specific concern that triggered it: penetration against minimum load, voltage and power quality, and safety and reliability on that circuit.
Many projects clear supplemental review with a modest change, such as an inverter power factor setting, an export limit, or a slightly smaller AC rating.
If it does not clear, the project goes to a full distribution study, where scope, cost and calendar all widen considerably.
Gate: Cleared, conditioned, or escalated to a distribution study
05
AHJ plan check and permit
The plan set covers the structural calculations, racking attachment or ballast details, the electrical single line, conductor and overcurrent sizing, labelling and rapid shutdown compliance.
Fire-service access pathways and setbacks are reviewed here, and on commercial roofs they routinely remove usable array area. We design for them from the start.
School sites and other public projects add their own review path, which has a longer and less flexible calendar.
Gate: Permit issued
06
Interconnection agreement
The agreement fixes the approved nameplate capacity, the metering arrangement and the tariff under which exported energy is compensated.
Any conditions from the screens or supplemental review are written in here: export limits, power factor requirements or protective settings.
Nothing may be energised before this is executed, regardless of whether construction is finished.
Gate: Agreement executed by both parties
07
Final inspection and witness test
The AHJ final covers workmanship, labelling, conductor management, rapid shutdown and grounding, and it must be signed before the utility will schedule.
Larger systems get a utility witness test: anti-islanding verified, trip settings confirmed, meter and protective relaying checked.
Failing either inspection is expensive in calendar time because rescheduling depends on someone else's queue, not ours.
Gate: Green tag from the AHJ and a passed witness test
08
Permission to operate
Permission to operate is the utility's written authorisation to energise and export. A finished, inspected, fully commissioned array that runs before it arrives is a serious problem, not a head start.
The meter is exchanged or reprogrammed for the new tariff at this point, and the billing arrangement changes over.
We hand over monitoring credentials, the commissioning report, the as-built set and the O&M calendar in the same week.
Gate: PTO letter issued and the system energised
Schedule strategy
Two clocks, run in parallel
The single most useful thing we do for a project schedule costs nothing: we file the utility interconnection application and the AHJ permit set in the same week, rather than waiting for one to clear before starting the other.
Run sequentially, a 12-week plan check followed by a 16-week utility path is 28 weeks. Run together it is 16. Nothing about the work changes; only the order does.
The second thing is the pre-application report. For a small fee, weeks before any design is finalised, the utility tells us what is already on the circuit. If the answer is that capacity is largely spoken for, we would much rather know in week two than in week fourteen.
The third is monthly written status. Utility queues go quiet for long stretches. A client who knows the application is sitting untouched in a queue is in a completely different position from a client who assumes something is happening.
Answers
Interconnection: questions
Not covered here? Our engineers answer directly, not through a call centre.
Ask a question(661) 555-0183Rule 21 is the California tariff that governs how a generating facility connects to the distribution system of an investor-owned utility. It defines the application, the fast-track screens, supplemental review and the study process, and it is the reason commercial solar schedules are dominated by the utility rather than by construction.
Our typical rooftop project runs about 26 to 34 weeks from contract to permission to operate. Ground-mount with medium-voltage work and a distribution study can run 40 to 60 weeks. Sample ranges, and the variation sits almost entirely in the utility stages.
A paid report from the utility telling you what is already on the circuit that serves your site: existing generation, minimum recorded load and available capacity. It costs a small fee and it is the cheapest risk reduction available, because it tells you before design whether the screens are likely to fail.
The project moves to supplemental review, which is a closer look rather than a full study. Many clear it with a design condition: an export limit, a smaller AC rating, an inverter power factor setting or additional protective relaying. We model what each condition does to production before accepting it.
No. A finished, inspected, commissioned system stays off until the utility issues permission to operate. Running early is a serious compliance problem, not a head start, and we will not do it.
We do. We file the interconnection application, respond to utility questions, run the AHJ plan check in parallel, schedule the inspections and chase the PTO letter. You get a written status every month, including the months when nothing has moved.
We file early and we tell you when nothing moved
Interconnection is the part of a solar project nobody enjoys and everybody underestimates. It is also the part we manage most tightly.
CSLB #1071482 (sample). Prevailing wage on public works. Engineers answer, not a call centre.