Battery life comes and goes. Sometimes its really pretty good, other times you can tell the system is stuck doing something and it tanks. For having just upgraded today, give it a few days time - just like a new phone. Also, since this is still preview software, it has generally been recommended to hard reset your phone after the upgrade, if you haven't done that already. You'll lose anything not backed up/sync'd somewhere, but it does generally smooth things out if you just did an in place upgrade.