A generator is the obvious answer.

Or a battery. It's weird that everyone that replied forgot about batteries.

How do you recharge your batteries?

With bigger batteries.

In terms of complete house power - Batteries are fantastic for a small size use case, but economically difficult in large size. I have 42kwh of battery storage (Enphase), plus 20kw of Solar generation, but that combined is not enough to cover a complete power outage longer than 24-36 hours in stormy weather. The cost of that system is well more than 10 times the cost to get a generator that can do the entire house (22kw Generac for example).