More recently I began using an 11" MacBook Air, to save a little weight in the backpack and to make it fit a little better on the airplane for those times when the person in front of you decides to recline their seat at full speed without warning, and thus crunching your laptop screen. (Karma would put such people in front of a 3 year old on their next flight who likes to kick the seat and open and close the tray the entire flight).
Now, this is not an apples to apples comparison because I have wifi on in flight versus not, but the 11" MacBook Air has a substantially weaker battery than the 13". At a 97% charge, I'm getting a 2 hour and 40 minute time, which I've found to be pretty accurate after a couple flights.
Dimming the keyboard after 5 seconds of inactivity gets me another 27 minutes. This gets me about what I need for a Eastbound flight from the West coast to Chicago, but not a Westbound flight where it's sometimes an hour longer.
Here's hoping that airlines that are good enough to have in-flight wifi will also consider providing power outlets. Actually, I need to read a few books an relax for the last hour anyway...