Connecting your xperia to 3G data networks will dry your battery very soon. I have experienced the battery went down in just 2 hours after unplugging from the charger! Ericsson themself, suggest to switch off wireless, 3G etc when not in use. If you have wireless as well as 3G option, go for wireless. Here is the way how to disable your 3G network when not in use.
Settings > Wireless Controls > Mobile Networks > Network Mode
select GSM Only
This will disable your data transfer!