Interlock kit
An interlock kit is a sliding metal plate on the breaker panel that makes it mechanically impossible to close the generator backfeed breaker while the main breaker is on. It is the low-cost alternative to a transfer switch for connecting a portable generator.
With an interlock, the generator feeds the whole panel through a dedicated breaker, and you manage load manually by switching individual breakers — flexible, since any circuit can be powered as long as the total stays within the generator’s capacity.
Interlock kits must be listed for the specific panel model to be code-compliant, and some jurisdictions require a transfer switch instead. Use a kit made for your exact panel, installed by an electrician under a permit — generic kits and DIY backfeed cords are dangerous and illegal.