First of all, you should have a flashlight or a kerosene lamp with you, so as not to get stuck in complete darkness. Also, don't forget about spare batteries to avoid being left without light in the end!
A spare mobile battery won't hurt – it will be your reliable assistant in charging devices that are necessary in a critical moment. And most importantly – don't panic. This is a temporary situation that will pass quickly as soon as the electricity is restored.
It's better to over-prepare for possible power supply interruptions than to underprepare – it's better to have extra than to buy at the last moment. Let there be light and tranquility!