I upgraded from lion to mountain lion on my Macbook air 2012. This is how I fixed my mountain lion battery issues on a 2012 macbook air. As soon as I installed mountain lion I instantly noticed that the battery life was terrible. Instead of 8-9 hours I was getting 2-3 hours. Under the same load.

I looked round the apple forums, and didn’t find anything.
I found a post somewhere [sorry, can’t remember where] that suggested I delete the desktop preferences plist file and restart the Dock.

It seems to have worked for me.
I now have 8 hours battery life again.

To apply this fix:
open terminal

rm ~/Library/Preferences/com.apple.desktop.plist
killall Dock

You will lose your wallpaper settings, but your battery life should be fixed.

Please let me know in the comments if it works, and I might be able to get some evidence to send to apple for a fix!

Some nice people from Reddit came along, and some of them said that it isn’t a fix.

I thought id put my reasoning in here incase you need convincing.

my guess is that its something to do with mission control or launchpad, though I did notice in the plist file itself that there is a
Change = Never;
ChangeTime = 1800;
if the change = never is getting ignored, and the ChangeTime is seconds, then every 30 mins each of your desktops refresh. probably not enough to have an impact on your battery, but if its ms, then every 1.8 seconds its refreshing.
that would be enough to kill your battery.

deleting the file resets it to have pretty much nothing in it. which seems to fit with my fix

Update 2: Apple have released a couple of firmware updates for Macbook air and Macbook pro that might help