Adding for future reference, I had the misfortune of dealing with an unplanned winter time outage during a blizzard in Ohio. I was much less experienced at the time, and was fish sitting several dozen freshwater tanks for my cousin's boyfriend. What started as some simple feeding obviously became more complicated during a several day power outage with the apartment quickly going below freezing. Fortunately the water heater was gas and continued to operate.
Short term a warm home and battery powered air stone get the job done. If there if a large refugium or live rock are in the sump I like to put an air stone there too.
Longer term even when generators are out of stock you can usually find a 700ish watt car inverter. Larger ones need to be connected to the battery, but usually come with cheap clamps for that. Small ones generally work in the cigarette lighter. Hook it to the car idling out front and run an extension cord inside to power the tank. AC needle wheel pumps don't run well on cheap inverters, but DC ones do fine and everything else is normally fine as well.
If it's cold out and the house isn't warm get a lid on the tank even if that "lid" is a clean plastic trash bag. Cutting down on evaporation can cut your heating needs in half.
If the power outage goes on more than a half day I usually pull the foam media from filters that aren't getting flow. I store it in humid but not wet areas (like a plastic bag), and give it a quick rinse before returning it once the power comes back on.
It doesn't get all that cold in the Bay Area, but I'll keep going for reference.
Next is a blanket around the tank, or foam, clothing, whatever. After this if things get too cold too long water changes with warm water are a big help. In a freshwater tank it's easy, in a reef if can be harder. If you have gas heat from a stove you can just heat water in a pot. If not, consider a grill, camp stove, or really anything you can get to. That car that's idling out front, blast the heater at the foot well and keep a bunch of water there that you periodically swap out.
If your water heater is the only source it will be fresh water, Put it in bottles and float them in the tank. Be really careful putting really hot water directly in to a tank. I've actually watched fish swim in to the stream of hot water, which I was moving to keep away from them. Really hot water for a few seconds too long and you have a dead fish.
In general, I consider backup power to be an insurance policy. If you spend $1000/year insuring a 10,000 car, I don't consider it unreasonable to spend $100/year providing backup power to a $2000 reef. I have a backup battery for my MP60, and I test it annually. I keep spare batteries for my battery powered air stones, and I have an arrangement with my neighbor across the street who usually works from home. I was about to add a generator, as I now have a larger tank with more to lose, but now that I'm moving that will wait until I have a new permanent address.