2 days sounds nuts to me. I’ve installed a large complex solar system that ran into issues, later installed high-voltage run for electric car charger, and then later installed whole-house backup batteries. Never was I down for more than a couple hours, during switch over.
I would directly ask the installers how long you will be without power, and tell them about how your tank can’t go for more than a couple hours without power.
In addition, I’d follow the advice from others above. Also I wouldn’t worry to much about running the generator after hours if you can get permission from your neighbors. One of those things that’s only enforced when there’s a complaint.