Microsoft's new Surface tablets may not match Apple's iPad on battery life, according to estimates made by Computerworld based on comparable devices.
On Monday, when Microsoft unveiled its Surface tablet line-up, it revealed the watt-hour (Wh) capacity of the batteries, but made no claims about how long those batteries would keep each device running under various conditions.
The omission was noticed by virtually every observer, blogger and analyst alike.
"They didn't talk about battery life, which is very important to decisions about [which] tablet to buy," said Michael Cherry of Directions on Microsoft, a Wash.-based research firm that focuses solely on Microsoft moves.
Unlike laptops such as Apple's MacBook Air and Windows "ultrabooks," which are often used for long stretches at a desk or near an outlet, tablets are meant to be more mobile. That makes battery lifespan critical to their operation -- and market success.
It's possible to estimate the battery life of the two Surface tablets by comparing their watt-hour ratings with existing products that have made between-charges claims. (A 30Wh rating means the battery can produce one watt of power for 30 hours, or, say, 6 watts of power for 5 hours.)
Microsoft assigned a 31.4Wh rating to the Windows RT Surface -- the tablet powered by Windows RT, an offshoot of Windows 8 designed to run on the power-miserly ARM processor architecture -- and gave the Windows 8 Pro Surface a rating of 42Wh.
The Windows 8 Pro Surface (or just "Surface Pro") runs on Intel processors, and is the same operating system that will power traditional laptops and even desktop PCs when it launches later this year.
Apple's current iPad, which leads the tablet market in sales, includes a 42.5Wh battery that Apple claims lasts 10 hours while browsing the Web, watching video or listening to music.
The Windows RT Surface (or "Surface") battery capacity is 25% less than the iPad's, putting its lifespan at 7.5 hours, or 25% less than the iPad's 10 hours.
A comparison with the iPad 2, which Apple continues to sell for $399, gives a longer lifespan estimate, however. The iPad 2's screen resolution is nearer to that likely used by the Surface; unfortunately, display resolution was another missing piece of information.
The iPad 2 features a 25Wh battery, approximately 25% less than the Surface; that would result in a battery life estimate of about 12.5 hours for the Surface based on the iPad 2's 10 hours. The average of the two numbers for the Surface -- 7.5 and 12.5 hours -- is 10 hours, or exactly the same as the iPad.
The Surface Pro is a different story.
The power-hungry Intel processor in the Surface Pro and the full-fledged Windows 8 operating system -- which will likely require more memory so users can run several applications at once -- makes it more like an ultrabook than a tablet, a fact Microsoft tacitly admitted when it said the price of the Surface Pro would be "comparable ... [with an] Intel ultrabook-class PC."
Apple's 11.6-in. MacBook Air -- considered the benchmark that Windows computer makers are shooting for -- is similar enough to the Surface Pro in specs, albeit with a slightly larger screen than the 10.6-in. used in Microsoft's tablets, for comparison purposes.
That MacBook Air sports a 35Wh battery rated to last 5 hours of browsing via Wi-Fi.
But the Surface Pro's battery has 20% more battery capacity, according to Microsoft, putting that device's lifespan at 6 hours, near but not equal to the seven-to-nine hours that numerous Windows ultrabooks boast. Admittedly, those laptops are significantly thicker and bigger than the Surface Pro, and have more room for larger batteries.
Hardware guru Kyle Wiens, the CEO of iFixit, a website that tears apart devices to create do-it-yourself repair guides, cautioned against reading too much into the estimates: There are simply too many unknowns that will determine Surface battery life.
"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.
Read more about hardware in Computerworld's Hardware Topic Center.