I agree with EVO that a backup natural gas generator is more practical than an EV for backup power. However if there's a concern as healy pointed out about a backup generator being too loud, then a battery backup would work for a while. For example, in the past 365 days, 83% of our power has been free from solar and battery backup. Some of that 17% of the power I pulled from the grid through the year was me preparing for upcoming storms by making my batteries charge from the grid to be fully charged in case the grid went down (at the times tornado warnings or snow warnings weren't preceded by sunny weather to fully charge the batteries with solar).
But that's works well only if it's a large battery storage (in my case 92kWh) and if your home doesn't need much power (i.e. I have a variable speed heat pump for cooling and heating the home, a hybrid water heater that runs at only 300W, I duct the cold-air output from the water heater so that it's drawn in by my HVAC to spread through the house to save on A/C from then home heat pump, etc.)
Looking back in hind sight, I should have done those kind of energy improvements on the home long before I sat down and did the math on what it would take to make us more energy self-reliant with solar so that the left has less control over my family with their energy policies. I recommend those kind of energy saving steps for someone interested in being able to power his home with backup power -- they're a two-fer in that an energy efficient home helps a generator run the whole house more efficiently if the house doesn't need much power anyway, and you save in power bills in the long run. Stated another way: spending the money on a small backup generator + home energy improvements might be a better investment than spending money on a large backup generator with no home improvements.
The Generac generators are about as noisy as an AC condenser unit. Noise certainly could be a bigger issue for the portable units..