This is actually a real problem due to management rules. Years ago, during the big power outage on the eastern seaboard, I went into a store to buy water (as I lived on a well). The clerk refused to sell me water because he had no electricity and no way to give change. I actually took the water and put the cash on the counter with him accusing me of doing something wrong. I explained to him how to do the whole thing manually.  I didn't need change--I needed water. He locked up when I left. I always think about this when I read about looters after a storm. Sometimes they are looting...sometimes they may not be.....If stores don't have backup systems in place to sell necessities during power failures, we could be in grim trouble indeed.