I’ve had a gas furnace keep me and the water heated multiple times in a cold weather power outage.
The major manufacturers have systems that will use the heat pump when the temperatures are not 'crazy', and kick in fossil at a certain point:
* https://www.trane.com/residential/en/resources/glossary/dual...
* https://www.carrier.com/residential/en/ca/homeowner-resource...
* https://www.lennox.com/residential/buyers-guide/guide-to-hva...
Depending on the cost of power and fossil fuels, you can program it to switch over once the COP becomes too low to justify running up kWh on your meter.
But whereas in the past heat pumps would have their COP drop around 40F/5C, modern systems can be fairly efficient at much lower temperature nowadays:
* https://neep.org/heating-electrification/ccashp-specificatio...
You have heat pumps running in Alaska:
* https://www.adn.com/business-economy/energy/2024/09/01/energ...
* https://alaskarenewableenergy.org/wp-content/uploads/2024/10...
If you have a backup generator, is it too small for your AC in the summer?
I would never use the generator in the summer though, doesn’t get that hot in the Pacific Northwest.