I'm skeptical of any tender that is rendered worthless by something as simple and common as an electrical outage.
Or pay in cash off course, but that only works if you remembered to go to an ATM first and that ATM had power.
Off course you could also write an IOU to the merchant about the amount of government IOUs that you will withdraw later.
I guess that means that your bank notes are worthless in the moments transaction if they are stored in a bank?