Since upgrading to iOS 8, even iOS 8.0.2, my iPhone 5 has been draining battery like CRAZY. I used to be able to start at 100% in the morning, go to work, come home, and be around 20%. But after "upgrading" my battery only lasted half a day at most!
The only thing keeping my phone going was my Mophie Juice Pack Powerstation Duo which I HIGHLY recommend (I can charge my iPhone 5 AND iPad Mini at the same time with this thing).
This weekend though I figured out the cause and the solution, which I now present to you and hope you find it helpful:
Check your battery usage by going to Settings > General > Usage > Battery Usage. Look at "Usage" time vs "Standby". If you've had your phone screen off in your pocket all day, Usage should be low compared to Standby. In my case though, Usage and Standby were almost the same (check out that image to the left)! If your Usage and Standby are pretty different, you might just be using your phone a lot.
Look at the app list to figure out what's using a lot of battery. If you use the phone a lot, maybe that's at the top. For me, Calendar was at the top (see the picture above again) which doesn't make a lot of sense, I don't sit there and mess with my calendar for hours at a time! That told me there's some kind of issue with my calendar.
The fix is to delete and restore my Outlook account that had the calendar. I simply removed it from my iPhone, restarted the phone, and re-created the account on my phone. It took maybe 3 minutes for my iPhone to re-load my emails and calendars. But after a while I checked my battery usage, and lo and behold: fixed! (see image to the right).
If your iPhone battery is performing poorly after upgrading to iOS 8, maybe you have a runaway email/calendar account. Delete and recreate the account on your phone and see if that fixes it. And don't be without power on the go -- pick up a mobile battery recharger like the Mophie Juice Pack Powerstation Duo!