Chris Foresman at Ars:
Users claim that battery life reported by their systems is significantly lower after installing Mountain Lion—in some cases, it's nearly half of what was reported under Lion. The batteries themselves also seem to drain noticeably faster, with more than one user claiming that it was possible to watch the battery indicator drop in capacity.
I’ve noticed this (albeit not to such a major extent) on my 13” Sandy Bridge MacBook Pro. Previously with my usual set of apps open (Writer, Messages, TweetBot, Safari, Terminal, Steam and Mail) my machine would report five hours available from a full charge. Under Mountain Lion, it tends to report three.