Remember that incredibly specific rumor from July that talked about the iPhone 7's battery being around 14% bigger than the one in the iPhone 6s? Well, it turns out it was right on the money. The iPhone 7's battery is 1,960 mAh, exactly as rumored. The figure comes from TENAA, the Chinese authority equivalent to the FCC in the US, so it's as official as possible - especially considering the fact that Apple itself never publishes such information.
For comparison, know that the iPhone 6s had a 1,715 mAh cell. Moving on to the bigger iPhone 7 Plus, this employs a 2,900 mAh battery, which is 150 mAh more than what was found inside its predecessor, the iPhone 6s Plus. Here we have a more modest 5% increase in capacity from the previous generation, not as big a step forward as we've seen for the iPhone 7.
While you may jump to the conclusion that the iPhone 7 should last 14% more on a charge than the iPhone 6s, keep in mind that the new device comes with a different chipset and a brighter screen, both things that can influence battery life a lot. So it's best to simply wait for our extensive battery testing to conclude for Apple's new iPhones, and then you'll have a good idea of what to expect.