Well, you don't need to use the device to realise that:
The HTC One M9 has a 2840mah battery. It powers 2073600 when the display is on. Let the CPU efficiency = 100%. Therefore, each 1mah of battery would have to power approximately 730 at a time. Comparatively, the S6 will need to power 3686400 pixels with a 2550mah battery. According to Samsung's claims, the efficiency should = ~130%. Due to it having an AMOLED display, let's say only ~3.4 million pixels are lit a time. Unlike the HTC, each milliampere-hour must power approximately 1025 pixels at a time. If my calculations are correct, each mah from the HTC's battery should last 40% longer than that of the S6. (BTW, based on those calculations, the larger battery on the S6 Edge will change it from 40% to ~37.7%). Not to mention it's inclusion of the much less battery friendly TouchWiz...
Of course, this is only based on the display and battery - two of the many components that make up a phone. Real world usage will be very different. I figured that since the display is usually the most power draining aspect of a phone, it would be most accurate to base it on that. Also, if the HTC One m8 - with it's smaller battery - can last a day, the One M9 will (unless, of course, there is an issue with the new processor or Sense 7 etc.) And, I also don't believe Samsung is yet to confirm that the US and other countries will be getting the supposedly more efficient Exynos processor. If not, and the battery remains the same, it'll be even worse.