Our standard battery rundown test involves setting the brightness and volume to half, using 4G in a 75 percent or better signal area, enabling GPS and WiFi (not connected) and disabling Bluetooth while looping a video from a full charge until the battery is drained.

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In that test, I set the screen brightness to 75%, leave the wireless on and play locally stored videos back to back till the unit dies.

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In my test, where I use full brightness, disable power-saving software, leave on the Wi-Fi, and play a loop of music, the battery on the XPS 13 lasted just under four hours, the worst I've seen on an ultrabook.

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On our standard Windows battery rundown test, in which we fix the display brightness and loop a video endlessly to exhaustion, the Surface Pro scored just three hours and 46 minutes, despite having a 42.5Wh battery a third larger than the 31.5Wh pack in the Surface RT.

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In my battery test, where I keep the screen at 75% brightness, leave the Internet connection on and email collecting in the background and play videos back to back, the Nexus 7 lasted 10 hours and 44 minutes.

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In my standard battery test, where I play videos back to back with the screen at 75% brightness and both Wi-Fi and cellular connections turned on, the Tablet P lasted just 5 hours and 16 minutes, about half the battery life of an iPad.

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In my harsh tablet battery test, where I play a constant loop of video with the screen set to 75 % brightness and Wi-Fi on to retrieve email in the background, I got almost exactly five hours, which is Asus's official battery life estimate.

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