if no electricity, of course nobody could trade with btc.
If you really need the money to trade and no electricity due to natural disaster you can use a generator and trade. Else you can use a car battery and trade or you can go to another place so that you can trade. There are a lot of initiative to do in order to trade and make money to be use for the family.
If natural disasters do happen and there is no electricity, I think I will be more worried about my chance of survival more than trading bitcoin. Although there are some ways to trade with bitcoin during power cut like you said, it would be harder and less convenient. This is one of bitcoin's disadvantage to which there hasn't been any practical and effective solution.