How much are you willing to pay for backup? Here's the Honda generator output/price sheet:
Honda portable generators provide reliable power for home back up, recreation, and industrial use. From super quiet inverter generators to construction models, Honda has the right generator for you.
powerequipment.honda.com
Starts at a $1,000 for 1000 watts and goes up - and up - and up. . . Then how are you going to use it? You can't use the existing wiring unless you are set up with an external inverter you can plug into (which with solar, you may have). Extension cords won't power the thermostat - no way to plug them in.
I assume that, like a car, you would need to "turn them over" occasionally to keep the part from freezing in place. And you need to get fresh gasoline every month of two.
No. . . .but. . . .central heating and air conditioning requires a thermostat - which requires electricity to run. You could have nat gas and no way to burn it. Of course, if you have a fireplace, and gas logs in the fireplace. . . .and a match. . . .