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?
How can btc overcome this problem when it is regarded as digital currency, and by digital it means something that process numbers or digits which are not represented physically like you have in fiat, which like you said is a disadvantage to using cryptocurrency as against fiat currency,
The only way I see this can be overcome is by having physical btc (if it will be possible) otherwise in a situation where there is total electricity outage there will be serious difficulties to access crypto.