Do you have a really old wifi router? I remember my n800 battery was drained within a few hours os inactivity when it was connected to that old wifi router of mine (can't remember what model it was). Obviously, there was something in it that stopped wlan energy saving to work on the tablet.

Since I got a new router (long time ago) and replaced the battery with a new nokia bp-5l (recently) battery life is great - several days of moderate use or one day of really heavy use.