Shortly after general availability of the Surface Pro 2, Microsoft pushed out a firmware update that allowed the Marvell WiFi solution to drive down to even lower power states.



From Anandtech:

I spoke with Microsoft after the update went live and immediately re-ran both of our battery life benchmarks on the Surface Pro 2. The improvement is significant. As a recap, both of our battery life tests are run with the displays calibrated to 200 nits and running a fixed workload. A faster system doesn't mean more repetitions of a workload, just that the system enjoys more time idle. The web browsing benchmark cycles through a series of desktop web pages at an aggressive interval until the battery dies.

  Microsoft Surface Pro 2 Firmware Update Improves Battery Life