Adding a standby generator to a home with solar panels

Why we are writing about this now

We are specifying one. A Dallas-Fort Worth homeowner with grid-tied rooftop solar is adding a Cummins RS30, a 30kW liquid-cooled whole-home generator, with an automatic transfer switch. Choosing the generator took one conversation. The design question was where the solar sits relative to the switch, and it is the question every solar home raises.

Grid-tied inverters are built to follow the grid

A grid-tied inverter synchronizes to the voltage and frequency it sees and exports into them. It is also required to stop when the grid stops. NREL's primer on IEEE 1547-2018 puts that at going offline "within 2 seconds" of the grid failing, followed by a return-to-service delay that "is by default set to 5 minutes" once power comes back.

That design protects line workers. It also carries a catch. The inverter has no way of knowing whether the steady 240 volts at 60 hertz on its terminals comes from the utility or from a generator in your side yard.

What happens with the solar on the generator side

Most residential solar connects through a breaker in the main panel. Install a transfer switch ahead of that panel and the solar is now downstream of the switch, on the generator side. In an outage the generator starts, the switch transfers, the inverters see what looks like a restored grid, and five minutes later they resume exporting. Any time the array makes more than the house is using, the surplus runs backward into the generator.

"If the solar panels and the generator are not electrically isolated, the AC inverter associated with the solar panels may attempt to feed power back to the generator, creating a hazardous and potentially damaging situation for both the generator and the inverter."

Cummins, What is a Transfer Switch? Types and Applications

The inverter makers agree. Enphase's technical brief on AC coupling says: "Do not connect the Enphase Microinverters to the output of an engine generator. This can cause damage to the microinverters and the generator." SolarEdge supports generators only in specific designed configurations, and its alternative power source application note caps solar at 35 percent of generator capacity "to protect the generator(s) and avoid damage." A standard ATS paired with a standard grid-tied inverter has none of those controls.

The layout we install

From the street	Role
Utility meter	Utility metering only. Nothing else goes inside it.
Solar disconnect and supply-side connection	The solar joins the utility conductors here, ahead of the transfer switch.
Service-entrance rated ATS	The home's main disconnect. Selects utility or generator, never both.
Main panel	House circuits, fed by whichever source the switch selects.
Generator	Lands on the ATS generator terminals only.

When the utility fails, the switch disconnects the utility side, and the solar goes with it. The inverters see a dead grid and stay down. The generator carries the house with no second source pushing against it. When the utility restores power, the switch transfers back and the solar resumes after its delay. Tesla uses the same principle for its batteries: its Powerwall and generator guidance has the Powerwall system "installed between the utility meter and the generator transfer switch."

In code terms this is a supply-side source connection. The 2023 National Electrical Code permits it in Section 705.11, which points to Section 230.82(6): a disconnect listed as suitable for service equipment, with overcurrent protection. TDLR adopted the 2026 NEC statewide effective September 1, 2026, and cities adopt editions on their own schedules, so we confirm the governing edition for each permit.

What your utility will and will not allow

"The utility side" does not mean inside the meter, and the rules for the meter are set by the utility that owns it, not by your city. Oncor, which delivers power to most Dallas-Fort Worth addresses, is specific. Its Electric Service Guidelines reserve the meter socket "exclusively for Company metering" and bar meter collars and "other devices" from it. They require "a Visible Lockable Labeled AC Disconnect (VLLD)" between the meter and all distributed generation, generally within 10 feet of the meter. They state that "Molded-case breakers are not considered to be an acceptable VLLD." They also require the generator transfer switch to be "installed on the load side of the meter base." So the solar connection is made in its own enclosure just downstream of the meter, with its own disconnect, and ahead of the ATS.

Other utilities differ in the details. CoServ and TNMP use the same 10-foot, visible, lockable disconnect, and TNMP also rejects molded-case breakers. Denton Municipal Electric wants its disconnect within 4 feet of the meter, allows line-side taps only through connectors in a junction box, and requires sealed engineering drawings. Garland Power & Light caps residential solar at 10 kW. HILCO and Navarro County cooperatives allow a line-side connection after the meter with their pre-approval. Tri-County Electric Cooperative is the exception that changes the design: its distributed generation checklist says it "does not accept line-side taps," so on a Tri-County home the connection method is agreed with Tri-County in writing before we quote. Tri-County also requires a UL 1008 transfer switch within 6 feet of the meter, with no interlock kits or meter-mount switches. Each of our city pages lists the utilities serving that city and what each one publishes. Where a utility publishes nothing on a point, we get its answer in writing rather than borrow another utility's rule.

The honest tradeoff

With this layout the solar is off whenever the generator is running. For most homes that is the right answer: the generator is sized to carry the house by itself, and the solar is back the moment the grid is. If producing solar during an outage matters to you, that is a battery design, not a generator design, and we will quote the two side by side.

Equipment and price

The Cummins RS30 is a 30kW liquid-cooled standby generator at $18,812.69 equipment, the Cummins advertised price, which is the same at every authorized dealer. The transfer switch is sized to your service, not the generator, and is chosen from the Cummins transfer switch range. Why the service-entrance rating matters here is covered in service-entrance transfer switches. Installation on a solar home, including the supply-side solar connection, is quoted in writing after a site survey, because it depends entirely on how the solar connects today.

Questions