Battery life test
The iPhone 6 is equipped with a non-removable Li-Po 1810mAh battery, which is only about 15% bigger than the one of the iPhone 5s. The pixel count on the iPhone 6 is almost 40% higher than the 5s so just by looking at these numbers we guessed that battery life with the screen on won't be as hot. But we turned out wrong.
The standby endurance wasn't that good but it wasn't any better on the iPhone 5s when we last tested it. The manufacturer rates the iPhone 6 batteries for up to 250h of standby but our unit only lasted about 160h. The iPhone 5s lasted way less.
The iPhone 6 scored 9:24h in our own video playback test (10:31h for the iPhone 5s) and 10:29h in our Wi-Fi web browsing test (9:58 h for the iPhone 5s). These are respectable numbers on their own and as you'd see quite similar to the iPhone 5s.
UPDATE: After resetting the iPhone 6 we decided to run a new call test and see what's what. The result turned out to be a better 12 and a half hours, besting the mediocre 8:11 hours and more in line with the 14h stated by Apple themselves.
Our overall endurance rating for the iPhone 6 is 61h, which is how long the battery should last you if you use the phone for an hour of calling, an hour of video playback and an hour of web browsing each daily. In comparison, the iPhone 5s scored 54h, the iPhone 5 - 51h, while the Samsung Galaxy Alpha managed about 52h.