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)
Thats impossible my friend you need to atleast take some money out of the Bitcoin before any kind of disaster happens buy it with foods and flashlights and other stuff that could save a life or in crisis. In case some disastrous situation happens for the time being in your place Bitcoin is useless.