Hey himmatsj,
I found a hidden battery drainer after update to 8.1. I did all my battery saving customs as I had them on WP8 and still the battery was draining faster than before the update. The only thing I did extra three days ago was to disable the features of WiFi Sense. Apparently this was for me a hidden battery drainer. Now my battery is almost at the WP8 with Black FW level, aprox 47-50 hours. I hope it helps your case.