You can buy dual fuel generators.

I have a 10,000 watt portable generator that runs on gasoline or propane. Gasoline has slightly higher output, but running on propane means not having to worry about winterizing fuel every year. Changing fuel type can be done in a few minutes.

Install a generator interlock kit on the panel.

Small concrete pad, enclose it with a locking door. Install a 120v and a 240v plug, 120v is on the main panel and has constant power to keep a battery tender hooked up. 240v is on the interlock breaker for backfeed.

10k is enough to run most items in an effecient house. I can run my well pump and water heater for showers, and keep the fridge/freezer going. Realistically, it's all I need since we use wood heat.

For a fraction of the cost of a Generac.