But is it possible, to be able to use Bitcoin even when we don't have energy?
Today, if I run out of electricity at home and want to go buy some batteries, for a flashlight, I can go to a store with some fiat coins and buy batteries. But that would not be possible if you want to make that purchase in Bitcoins. This remains a very good advantage for fiat coins.
How can Bitcoin overcome this problem?
For now, I don't think we can do something about it because Bitcoin is a digital currency and that means it must have energy for you to operate it so there's no way you can make a transaction with Bitcoin if you run out of electricity. That is the reason why it can never replace fiat in that way, so yeah fiat is much better to use in this kind of circumstances. Besides, I think everything will be a mess if we ran out of energy not only Bitcoin but the majority of the transaction across the world whether it is physical or virtual.