Standalone circuit for emergency power
Drew there are panels that will do exactly what you want. They use the existing wiring and have a limited number of circuits. These manual transfer of 1/2 dozen or more circuits is really expensive. See generac, reliance 6 circuit transfer switches.
These systems cost in the range of 350.00-450.00 I have seen them used with a 2kw generator.
Old school lightbulbs that is 200 not just 1.
The house I am thinking of ran there gas furnace.
Lighting they needed and refrigerator.
When they wanted to run the well they turned off the refrigerator circuit and turned on the well pump.
So it can be done even using your existing wiring. I would suggest a generator interlock kit this locks out your main breaker while turning on the breaker to feed the home from another source whatever it is.
You turn off all the circuit breakers you don’t need or cannot power with the new source. A lot cheaper and works similarly for under 100$.