I had the same issue with battery life after IOS 7 update but managed to solve the problem.
Try this steps:
- As suggested by Linda Swenberg, go to Setting-General-Usage and scroll all the way down to read Usage and Standby time. If they are the same even though the phone has been on standby for some time since last full charge, it means phone was still working in the background.
- Go to Setting-Cellular-System Services and find out which service was comsuming most data. Most likely that's the one that is causing the battery drain. In my case, it was a profile I installed to access the Corporate Exchange.
- Delete the profile (if you can live without it) that is causing the phone to work even on standby.
- Put the phone on standby for a few minutes. Check if Standby time increases more than usage time. If it does, problem is solved.
After deleting the profile that caused the problem, my 4S battery life is now better than when it was on IOS 6.