The new Surface Pro 2 is equipped with the Haswell architecture of Intel, meaning it marks strides in battery life compared to its predecessor - 75% improvement, or north of eight hours endurance. The unit has also been made much quieter, though that still means you are likely to see a fan inside. Microsoft didn't stop here, but added an additional battery in a Power Cover keyboard accessory, which prolongs battery life to the whopping 2.5x of Surface Pro, or north of 12 hours when you use them together...
Source: http://www.phonearena.com/news/Microsoft-Surface-Pro-2-is-here-longer-battery-life-better-display-faster-processor_id47676
Source: http://www.phonearena.com/news/Microsoft-Surface-Pro-2-is-here-longer-battery-life-better-display-faster-processor_id47676