According to some tests, it looks like the iPhone X gets significantly better battery life. I believe this is due to the OLED power savings and one other very important difference I’ve noticed. That major difference is this: iOS 11 is extremely optimized for the iPhone X. I had many strange bugs on my iPhone 7 Plus, to the point of deleting everything and downloading a clean install of iOS 11 from iTunes and restoring my data back on it. It took all afternoon and evening and still bugs persisted, multiple bugs, the entire time in had my 7 plus running iOS 11. I got the iPhone X and started fresh. I haven’t noticed the bugs that were plaguing me since the introduction of iOS 11. Of course they would want their super flagship device to be perfect, and I think they put extra optimization with iOS 11 on iPhone X, since the experience so far has been downright delectable
My battery life is about the same as my 7 plus with the iOS 11 update. So, in other words, it's not the best.