The Surface RT tablet computers available now are based upon the low-power type of chips designed for mobile devices, used in Android tablets and iPads (each device has different chips, but they have the same kind of purpose). These devices do pretty well for most tasks, and run on operating systems designed for that environment - Windows 8 RT, Android, and iOS, respectively. The one thing in common is that they all can provide very good battery life; most users talk about using them "all day" without recharging.
The Windows Surface Pro tablets, due in January, will use Intel chips similar to a "normal" notebook. They will therefore have more horsepower to throw at the full Windows 8 operating system, but also have an expected battery life of something like 5 hours - about half that of the Surface RT, and similar to that of a MacBook Air (Apple's featherweight notebook). It's unclear if the perception of the Surface Pro as a tablet, rather than a sort of "convertible notebook" will hurt it in the battery department.