You'll need a transfer switch next to your circuit breaker box to switch power source to generator, and designating which circuits you want to keep open
for 12500 watts, I don't know what size home you have, but IMHO that's pretty big and you should be able to run nearly everything in your home.
12500 watts is just 100 amps at 120v. 50A at 240V. Enough to run the outlets and the refrigerator, but not enough for a heat pump or electric stove. The typical house is wired for 200 amps.
my Mig welder alone requires a minimum 8k watt generator. Said welder is only rated to 1/4” metal too. It’s a baby one.
12.5k isn’t a lot.
It’ll run the essentials and not much more though. Depending on the heater set up, it might not manage to run that.