ArsTechnica tested just how much OS X Mountain Lion affects the battery life of some of Apple’s laptops, and their findings were pretty significant.
Reports of OS X Mountain life draining laptop batteries abnormally fast began emerging shortly after the OS’s release filing Apple’s online support forums with complaints. ArsTechnica’s tests show Apple’s latest OS does indeed shorten the battery life of MacBooks, MacBook Pros and MacBook Airs. In some cases the performance drop nears forty percent. ArsTechnica's methods might not be the most "technical", but they do exhibit a reoccurring drop in battery life. Not all of Apple's laptops are affected by the battery drop, and a clear pattern hasn't emerged yet.
Our own testing revealed similar (and significant) drops in battery runtime after installing Mountain Lion. In previous tests, we were able to regularly achieve just over 8 hours of use by relying solely on our Retina MacBook Pro's integrated Intel HD4000 GPU. Performing the same "real-world" test using the same software applications and usage pattern, we never got the Retina MacBook Pro to run for more than a few minutes past 5 hours after a full charge. — ArsTechnica