If there's a natural disaster cutting off electricity for days, trading bitcoin should be least of your worries really. Priorities man.
sorry I meant to say, how does one trade with BTC
ie. how does one buy food/water with BTC in such circumstances if BTC is the most preferred currency for trade (like Venezuela)
The simple answer is: you don't.
For a transaction to be validated you need to send it over the network. And this needs electricity and connection.
Normally at least the sender and recipient's phone would work (again: electricity and connection)
If this minimum requirement is not met, you're out of luck.
A different approach would be to have with yourself a number of paper wallets and had over the actual wallet. But that needs a super-high level of trust between you and the other part, since you can hand out empty paper wallets or just empty them at a short time later. Or give the same wallet to many others... This idea has far too many issues to be a good solution...
if someone ask me that question I would answer the same as you and ask this questions. Can you use a device without electricity? If there's an electricity second question will be ask if you can use a device with an electricity, Is there an internet connection? Why would bother asking that question that even a kid can answer the first and second question. The answer is You can't without electricity and internet connection.