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.
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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.