There will be a plug point on the bed side of the window (there is a couch on the other side). It is a great ride if you are two friends/family. Otherwise, I have found foreigners end up being alone in the 2 person deluxe soft sleeper anyway. Certainly take some water, snacks and possibly your favorite tipple along with good book/s to read. That is my favorite mode of travelling between Beijing and Hong Kong. It is non-stop by the way! Customs and immigration now happen at the beginning and end only.
Enjoy!