A major ice storm a couple years ago knocked down most of the antennas on top of buildings that let them connect to the payment systems, so you could walk into Walgreens and they had power but no connection to verify payment card info.
There was a line of people begging to buy items from cigarettes to cold medicine. I held up a $20 bill, said I can pay for what I have, and was ushered to the front of the line. I paid for diapers and stuff for a sick toddler and walked out. Only after I did that did others even seem to think of trying cash.
If the outage was to last more than a few days I wouldn’t hold up a $20 for nothing. It would be like hanging a pork chop around your neck and stepping into the lion’s cage.