If the power is out for long enough, I think you won't be able to pay with cash, too. Stores need power for all kinds of things like registers, lighting and refrigerators. I imagine they would close pretty early if not immediately.

Additionally, long-term power outages are emergency situations if you live in a region that depends a lot on power. There will be looting. You won't think about your bitcoin in such a situation¹. You will care about food, shelter and being safe from your biggest threat: other humans in survival mode.