Right UK user here I feel your pain.
My 920 went from 100% to 32% in 7 hours, I didn't use the phone in this period of time. A hard reset with the same conditions gave me an increase in performance, The battery went from 100% to 68% in 7 hours. I then tried a mixture of solutions from forums where people had the same issue. As for now the battery has stayed at 100% for an hour (phone was left idle) so I think things are improving. Here's what I did:
In email + accounts change the download new items period to an hour or later intervals. I only changed Google + Yahoo times and left my Live account to update as items arrived. Apparently Google is the issue according to most people.
I made sure things like NFC, bluetooth etc are switched off. My location was left ON.
Then I made sure no app is allowed to run in the background, this can be done in the battery saver settings, in settings.
There is a new option called "Sync my settings" turned everything off. It includes themes, internet explorer, passwords and settings(i think) but yeah turned all of those off. Most people said it was just the themes that caused the problem but I didn't want the other things to sync either.
All these changes are giving me an improved performance. I will let my battery drain today, then charge it up to the max and do the 7 hour test. Let's see how much of a difference it makes.
Please let me know if this works for you, other readers will find it useful.