Which format uses less battery life? AAC, WMA, MP3?

To be honest none of the formats are going to affect battery life that much if they are encoded at the same bitrate. The battle over whether AAC is better the MP3 sound wise rages on but I will say what I always say: on an ipod which has only decent sound quality at best there isnt going to be much of a difference. Just use the highest bitrate you can in whatever format is the most convenient to use. The differences are negligible.