We get calls from homeowners who've got solar on the roof and just had a standby generator dropped in the yard. They figure, "More power sources, more backup, right?" Wrong. If those two systems aren't separated properly, you don't have redundancy — you've got a fight on your hands, and your house is the ring.
When your solar inverter is producing and your generator is running, you've got two sources trying to energize the same bus. That's backfeed. And backfeed doesn't play nice.
Solar current pushes back into the generator windings. The alternator wasn't built to take power in — it's built to push power out. Reverse current stresses thrust bearings, heats the windings, and can stall the engine. We've seen units smoke because of this.
Grid-tied inverters want 60 Hz, ±0.7 Hz. Most residential generators can't hold frequency that tight under load swings. The inverter sees dirty power, drops offline, waits five minutes, then tries to re-sync. It does this over and over — hammering the generator, spiking voltage, and frying anything sensitive on your load side.
Generator manufacturers and inverter manufacturers both exclude backfeed damage. You're paying for that repair. Period. No exceptions.
If your setup is wrong and solar backfeeds past your service entrance, that power travels back through the meter, through the transformer, and out onto the primary. Lineworkers think that line is de-energized. That's how people die.
The NEC is clear on this — Article 705 covers supply-side interconnections, and the rule is simple: your solar and your generator never energize the same bus at the same time.
Transfer switch or manual interlock with solar on the utility (line) side and generator on the load side
When the grid drops, the transfer switch isolates the utility feed and brings the generator online — solar is completely disconnected
When grid power returns, the generator drops out, the switch reconnects to utility, and solar re-syncs to clean grid power
No overlap. No paralleling. No backfeed.