The misconception that you and most people get is the only way to get better battery life is the physical size of the battery, but thats wrong! what about reducing how much battery the screen uses, or how the phone sips the juice from the battery, or what the phone does when the screen is off? all those things are incorporated in the G3 if you have read or seen their event. so me personally Im thinking probably same or better battery life than the g2 because of innovations.
Not that the battery life is bad, but it isn't class leading or anything. You'll see users posting about 7-8 hours of SOT on M8 and GS5 forums. On this site everyone is just trying their best to get 5-6 hours. I do think 5-6 hours is good, (I got 4 hours on my first discharge, and that included being mostly on LTE and playing around with the camera and taking videos). So i do think I should be able to achieve 5 hours of SOT under normal usage (wifi use and no camera). But its definitively not better than the G2 battery.