I might need Xperia P for a frnd,he was in search for this phone.
After using desire V,not loving battery at all.22 hours and 33% left.but i have seen this phone on ICS which was worst.but yes cannot compare GN and desire V.
Did a whole battery test which includes
1.75% screen brightness
2.Wifi whole day on
3.Used browser,Tapatalk and facebook whole noon timing
4.tweeked with Poweramp settings for an hour with music ON.
5.about 20sms and 5 calls
In short,battery time is similar to One X.
One thing u will love is UI and animations.Much Much better then touch wiz on S3.i personally found it very much decent and fast,but the speaker on the phone is really a huge downfall.