The iPhone 6 features an 1810 mAh, 3.82 V Lithium-ion Polymer battery with an energy rating of 6.91 Wh. Turning it over, the rating improves to 7.01 Wh! Perhaps the engineers were able to squeeze an extra 0.1 Wh in the battery after confirming the text on the front.
Apple preaches that this 28-gram (3.75" x 1.5" x 0.13") power pack will let you talk for up to 14 hours on 3G and offers 250 hours of standby time.
This is a fair bit smaller than the iPhone 6 Plus's 11.1 Wh, 2915 mAh battery—but still a notable bump from the 1560 mAh unit in the iPhone 5s.