Microsoft said during its massive Windows 10 hardware event earlier this week that the “ultimate” Surface Book laptop will last 12 hours on a single charge. But that’s in laptop mode when the tablet part is attached to the keyboard dock. The more important question is how much battery life you’d get in tablet mode when it’s unplugged from the dock?

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After all, the Surface Book has some impressive specs, including a large 13.5-inch display with high resolution (3000 x 2000 pixels), which means it’s likely to consume plenty of power during tablet use. Microsoft told Mashable recently that the battery is split between the tablet and the dock, with the tablet’s pack offering up to four hours of life, and the dock ready to provide eight hours of life.

However, a video from TechCrunch says that battery life is more around three hours for the Surface Book when used in tablet mode. That should offer plenty of autonomy depending on the type of usage, but don’t expect it to last more than that if you leave the dock at home.

As Busines Insider notes, a Microsoft employee said that “three-quarters of the battery is in the base, one-quarter is in the top, so you get about three hours of runtime in ‘clipboard’ mode.”

More interestingly though, when in laptop mode and not connected to a power source, the Surface Book first uses the battery in the dock, so that the user can still detach the top and use it as a tablet.

“It appears to use the battery of the dock first, since it’s much bigger, then it switches and uses the very last part of the battery in the tablet as it winds closer to zero,” a Microsoft employee said on Reddit.

TechCrunch’s video follows below.

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