Search “An event at my grocery store” for your answer.
BTW, about power backups, IIRC, Mom worked at K-Mart from 1973 to 1976 and she told me that if the power ever went out, they had 4 “special registers” hooked up to a battery backup system that would work, but in today’s world, if the comm lines were down, the EBT and debit cards would be useless although cash and check transactions can still work.