"It's a whole system," said Wiens in an interview this week, talking about power-saving design.
Among the factors that will affect battery life for the Surface and Surface Pro, said Wiens, are how successful Microsoft is at integrating power management features; how many cores the processors have and how well those cores are managed; the screen resolution; how many applications can run simultaneously; and how Windows RT and Windows 8 handle power consumption by those active apps.
Microsoft has disclosed some power consumption details about Windows RT and Windows 8.
In a blog post last February, two program managers on the operating system's Fundamentals and User Experience teams described the power handling, including something they called "Connected Standby." It mimics a smartphone-like, deep-sleep power mode on ARM, the processor used by the Surface.
Their post followed one the previous November by a program manager on the Windows Kernel team that discussed Windows 8 power management in more general terms.
The more recent missive spelled out the various power states of Metro apps, including one that suspends an app that has been opened, but is not currently in use.
"Since the operating system is not scheduling the app, the app is not using the CPU, and it is possible for the CPU to drop into lower power states," said the two program managers, Sharif Farag and Ben Srour. "Getting the CPU into low power states can be critical to achieving better battery life."
Although what the Microsoft officials discussed may have been new to Windows, such steps are commonly used by other mobile operating systems, including Apple's iOS and Google's Android, on smartphones and tablets.
Wiens was eager to get his hands on a Surface to tear it down, look inside and try to scout out the tablet's internal organs and get a better feel for its battery life.
iFixit will do a teardown of the Surface when Microsoft launches the tablet. The company has said it will debut the Windows RT-powered model at the same time Windows 8 goes on sale. Most analysts expect that in September or October.
Wiens also pointed out that, at least by the design drawings Microsoft has published of the Surface, the tablet may be easier to get into, perhaps for battery replacement by the owner. Those renderings show what appear to be Torx screws fastening the case.
Last week, iFixit called the new lighter, thinner MacBook Pro -- the first Apple notebook to feature a "Retina"-style high resolution display -- "the least-repairable laptop we've taken apart."
"Let's hope Microsoft will be better at letting users repair or upgrade their own devices," said Wiens.
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed. His email address is firstname.lastname@example.org.
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This story, "Microsoft Surface tablets may not match iPad battery life" was originally published by Computerworld.