I used PCMark 7's "Lightweight" test, which simulates typical workloads such as adding music to Media Player, browsing the web, copying files and scanning for malware while leaving a couple of seconds of idle time in between. I looped the test until the battery of the device ran dry. Second, I just let the laptop sit there, do nothing and just "die". While that's not a particular realistic scenario, it demonstrates idle usage which is very important for battery life: Remember, you're not constantly pushing your CPU to its limit -- while you're reading something, sitting in a meeting or just waiting in between presentation slides, your laptop needs to go into idle as fast as possible and stay there until you do something. It is a good basis for judging power management.
Since third party software usually impacts battery life, I installed only the very basic programs such as Office 2010, TeamViewer, SnagIT, Adobe Reader and Flash on both machines. In all cases, I used the "Balanced" power saver mode and cranked up brightness to about 50-70%, while leaving Wi-Fi on.
I repeated each test run exactly three times to avoid discrepancies. In each and every case I got pretty much same results (+/- 5 minutes). Results are measured in minutes:
Microsoft has done its homework. Idle performance of Windows 8 seems to best that of Windows 7 SP1 in every case. This is especially true on the MacBook Air: Here, idle battery life increased by 51 minutes. Even under the PCMark 7 workload, it managed to squeeze 24 minutes of additional battery life out of the machine. Our trusty old Samsung NC 10 ran for 40 more minutes in "idle" while it gave me another 20 minutes under typical workload. While that's not a lot, it may just mean the difference between wrapping up a movie or work on an airplane and having to stare at a blank screen for the rest of the flight.
Overall: While there are no major leaps, these are all solid improvements across the board.