This question already has an answer here:
I have recently bought this laptop with preinstalled ubuntu 12.04(64bit). Surprisingly it runs less than two hours in battery mode. But it runs around 4-4.30 hrs in windows7. I had noticed that all the dell laptop has short battery life in linux. I don't have any graphics card and also don't have any fan issue(constantly running). My question is why ubuntu can't give better lifetime for dell where almost all other laptop has better battery life in ubuntu than windows.