It depends on how you make them of course... but no matter what you do, you must ensure the main circuit is open or you are feeding power back onto overhead lines that are thought to be dead... a danger for linemen. It is also a danger for you if the main breaker is not open and you are generating power and the line power comes on.

Frankly, if this sort of thing "happens all the time" for you, I would have backup power properly wired in, with a cut over switch.

This is not the sort of thing you want to jury rig on a regular basis.