Android has an embedded energy monitor. On my Nexus 7, the screen consistently uses >60% of the whole, even though the tablet spends most of its time in idle (with the screen turned off) and I keep wifi turned on the whole day, including during commute when it connects to dozens of APs.

EDIT: Oh, and I also listen to podcasts and music over bluetooth for hours/day. It hardly seems to make a dent.