I was having some serious battery issues yesterday after updating my Lumia 925, however, after re-training the battery things seems to be a lot better!
After updating to 8.1, my battery stated it had 55% charge. I tried plugging it in and the phone started to charge rather slowly. I then rebooted the phone and it listed itself as having 38%; that's a huge difference and a loss of battery after charging it. I then recharged the phone to 100% from the 38%, but it discharged from 100% to 20% in like 3 hours. I let the phone die, plugged it back in, let it charge to 100% and then unplugged it. Since that charge last night at around midnight, my phone is currently sitting at 85% after some pretty heavy usage this morning; I get up around 6am.
I honestly thing that it needs to just be retrained after this update, and it's also probably syncing a lot of initial data after the update, which isn't good for battery life. :winktongue:
Hope this helps someone else!