Don't do this. Install a transfer switch. A simple plug and another power source feeds power back into the grid. It's absolutely against code. If you remember to flip the furnace breaker off before you do this, then it's not going to backfeed, but this isn't something you should rely on remembering to do. It doesn't cost much more to do it right.

It's not against code in all locations. You're essentially adding a power cord to your furnace and turning it's existing panel connection into an outlet.

Eg, in North Dakota

"If manufacturer instructions allow the use of not more than six feet of flexible appliance cord, it shall be permitted on permanently installed one hundred twenty volt twenty amp or less fuel-fired furnaces in residential dwellings."

https://www.law.cornell.edu/regulations/north-dakota/N-D-A-C...

I think they're proposing switching your furnace from hard wired to plugged, so you have the option to power it either from an outlet (grid) or solar/generator/etc. This can't backfeed the grid, and is very safe.

It looks like when it's against code it's "by default": if an appliance is not explicitly listed as acceptable for a plug it has to be hard wired. ND and OK have passed state level amendments allowing it, some places have done this locally, and in many places building inspectors allow it with their discretion.