Adding a Standby Generator to a Home With Solar Panels: Where the Solar Has to Connect

If your home has grid-tied solar panels and you add a whole-home standby generator, the solar has to connect on the utility side of the automatic transfer switch, on the conductors between the utility meter and the switch. It must not connect on the side of the switch the generator powers. When the solar is on the generator side, the inverters can mistake the running generator for the utility grid and push power back into it. Cummins, the generator's manufacturer, describes that as "a hazardous and potentially damaging situation for both the generator and the inverter." This post explains why it happens, how we wire around it, and what Oncor allows.

The job behind this post

We are designing this installation right now. A Dallas-Fort Worth homeowner with a grid-tied rooftop solar system wants a Cummins RS30, a 30kW liquid-cooled whole-home standby generator, with an automatic transfer switch (ATS). The generator is the easy part. The question that decides the design is the one every solar home raises: which side of the transfer switch does the solar connect to? Get that wrong and the two most expensive systems on the house end up working against each other in the one situation they were bought for.

What grid-tied solar does when the power goes out

Most residential solar is grid-tied, or "interactive" in code language. The inverters convert the panels' DC output into AC that matches the utility's voltage and frequency. They are also required to stop when the utility stops. It is the reason a lineman working on a dead line is not hit by power from your roof.

• Inverters certified to UL 1741 and IEEE 1547 have to detect a loss of the grid. The National Renewable Energy Laboratory's primer on IEEE 1547-2018 says they must "detect the island and go offline within 2 seconds."
• Once the grid comes back, they wait before exporting again. The same NREL primer says this "return-to-service delay is by default set to 5 minutes."
• The National Electrical Code says the same thing from the building side. Section 705.40 (2023 edition) requires a power production source to disconnect automatically when the utility is lost and not reconnect until the utility is restored.

That is why a solar-only home goes dark in an outage even on a bright afternoon. It is working exactly as designed.

What goes wrong when the solar is on the generator side

In many solar homes, the solar connects inside the main panel through a breaker, which the NEC calls a load-side connection (Section 705.12). If a transfer switch is installed ahead of that panel, the solar ends up on the generator side of the switch. In an outage, the sequence runs like this:

1. The utility fails. The inverters detect it and go offline within 2 seconds.
2. The generator starts and the transfer switch moves the house onto it.
3. The inverters now see steady 240-volt, 60-hertz power on their terminals, and to them it looks like the grid came back.
4. After their return-to-service delay, they begin exporting again, this time into a system whose only source is your generator.
5. Whenever the panels produce more than the house is using, the surplus has nowhere to go but back into the generator. A generator is built to deliver power, not to absorb it.

The manufacturers are blunt about the result. On transfer switches, Cummins writes: "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." Enphase's technical brief on AC coupling of its microinverters is shorter: "Do not connect the Enphase Microinverters to the output of an engine generator. This can cause damage to the microinverters and the generator." It then gives the remedy: "Use a transfer switch to ensure that the generator is always isolated from the Enphase Microinverters."

Solar manufacturers that do support generators do it with dedicated controls. SolarEdge's generator application note, for example, caps solar at 35 percent of generator capacity "to protect the generator(s) and avoid damage." A standard ATS and a standard grid-tied inverter have no such controls between them. The dependable answer is to keep the two apart.

The fix: connect the solar on the utility side of the transfer switch

We move, or design, the solar connection so it lands between the meter and the transfer switch. That is on the utility side of the switch, where the generator can never reach it. Reading from the street to the house, the layout is:

In an outage, the switch opens the utility side and closes onto the generator. The solar is on the utility side, so the switch cuts it off along with the grid. The inverters see a dead utility and stay offline, exactly as the anti-islanding rules intend. The generator carries the house with no second source pushing back at it. When the utility restores power, the switch transfers the house back, the inverters wait out their 5-minute delay, and the solar resumes as normal.

The NEC provides for this connection. Section 705.11 of the 2023 edition allows a power source to connect to a service on the supply side of the service disconnecting means, in accordance with Section 230.82(6). That section requires a disconnect listed as suitable for service equipment, plus proper overcurrent protection. Texas adopted the 2026 NEC as the state electrical code effective September 1, 2026, and the 2026 edition renumbers parts of Section 230.82. Cities adopt editions on their own schedule, so we confirm which edition governs your permit before we design around it.

What your utility allows at the meter

It would be convenient to make the connection at the meter itself. Whether you can, and what disconnect you need, is set by the utility that owns the meter, not by the city. Oncor delivers power to most Dallas-Fort Worth addresses, and its Electric Service Guidelines (February 2025) are specific:

• Meter sockets "are to be used exclusively for Company metering."
• "Non-utility meter collars or other devices are not allowed to be installed between the utility meter and meter socket or inside the meter socket."
• "A Visible Lockable Labeled AC Disconnect (VLLD) must be located between the Company Meter and all sources of Distributed Generation." It must be within 10 feet of the meter unless special placards are used, and "Molded-case breakers are not considered to be an acceptable VLLD."
• "A customer generator transfer switch is to be installed on the load side of the meter base," and never "integrated or placed in series with the meter and the base."

The NEC defers to the utility here as well. Section 705.11(C)(3) allows connections in equipment under the utility's exclusive control only where the utility approves them, and Oncor has said it does not. So the supply-side connection is made downstream of the meter socket in its own enclosure, with the disconnect Oncor requires. Meter-collar generator adapters and solar adapters are off the table in Oncor territory.

Other utilities in the Metroplex publish their own rules, and they are not identical:

One of those rows changes the design itself. Tri-County Electric Cooperative, which serves parts of Keller, Southlake, Grapevine, Trophy Club and Haslet, states in its distributed generation checklist that it "does not accept line-side taps." On a Tri-County home, the usual supply-side solar connection is not available, so the way the solar is kept on the utility side of the transfer switch has to be agreed with Tri-County in writing before we quote. Tri-County also requires the transfer switch itself to be UL 1008 listed and within 6 feet of the meter, and does not accept interlock kits or meter-mount transfer switches.

Where a utility does not publish a rule, we get it in writing before the job rather than assume another utility's rule applies. The utility on your electric bill is the one whose rules govern, and territory does not follow city limits. Each of our service area pages lists the utilities serving that city and what each one requires.

The tradeoff: your solar will not run during an outage

This design keeps the solar off while the generator runs. For most homeowners that is the right trade. The generator carries the whole house, and the solar is back the moment the grid is. If you want your solar producing during an outage, that takes a battery system designed for it, and it is a different design. Tesla's guidance on combining Powerwall with a generator, for example, has the "Powerwall system installed between the utility meter and the generator transfer switch." The principle is the same: the solar side stays upstream of the generator's transfer switch. Enphase handles it through its IQ System Controller, which its generator integration brief says "ensures that the generator is never connected to the home when the system is on-grid."

Which transfer switch, and how it is sized

The utility-side solar connection works most cleanly with a service-entrance rated automatic transfer switch, because the switch becomes the home's main disconnect and there is one clear boundary between the utility side and the house side. The switch is sized to the electrical service, not to the generator: a 200-amp service takes a 200-amp switch whether the generator behind it is 20kW or 30kW. Cummins offers service-entrance rated switches in its RA family, and the differences are laid out in the ElectricPros guide to service-entrance transfer switches. Every Cummins spec sheet carries the same warning, which is the reason the switch exists: "Back feed to a utility system can cause electrocution and/or property damage."

What we check on a solar home before quoting

• Where the solar connects today: a breaker in the main panel, a tap ahead of the main breaker, or a separate disconnect. We inspect it rather than assume.
• The inverter type (microinverters or a string inverter) and the system's size, from the nameplate and the interconnection paperwork.
• Whether the home has a battery, and whether that battery already includes its own transfer device.
• The service size and meter arrangement, which decide the transfer switch rating and where the solar disconnect can go within the utility's distance rule: 10 feet from the meter for Oncor, CoServ and TNMP, 4 feet for Denton Municipal Electric.
• The gas meter and line, because a 30kW liquid-cooled generator is the largest gas appliance most homes will ever have. See how to size a standby generator.
• Who installed the solar. Moving its point of connection touches their work, so we coordinate with them and with your utility rather than surprise either one.

Permits and inspections

A generator, a transfer switch and a solar reconnection are permitted electrical work. Texas Senate Bill 1202 limits what cities may require for residential backup power systems, and we cover what that does and does not change in SB 1202 and generator permits. Utilities have their own generator paperwork too. CoServ and TNMP each require an application for the generator itself and an open-transition transfer switch, and Oncor asks, on its residential project requirements, that when a generator is connected the installer confirm the transfer switch's mode of operation, open or closed transition. We handle the permit, the inspection and the utility coordination.

What it costs

A typical complete 20kW whole-house installation in Dallas-Fort Worth runs $13,500–$17,000 all in. That is an estimate, and a solar home with a liquid-cooled 30kW unit is quoted in writing after an on-site survey, because the supply-side solar connection, the transfer switch rating and the gas service all depend on what is already on the house. Equipment prices for every Cummins model, the RS30 included, are published at utselectricpros.com. Clements Electric installs Cummins as the electrical contracting arm of UTS ElectricPros, LLC, an authorized Cummins dealer.