The Samsung Galaxy S9 doesn't offer better battery performance over its predecessor
New SoCs usually bring improved efficiency and performance values to the table. The Samsung Galaxy S9 was released a few days ago and featured the new Exynos 9810 and Snapdragon 845 across both of its variants. Both SoCs have been advertised as offering tangible efficiency boosts over their predecessors, the Exynos 8895 and Snapdragon 835 respectively. That boost isn't readily observable, though, as tests on the Samsung Galaxy S9 show it posting similar battery life to the S8.
It's not a case of a downgraded battery, either, as both the S8 and S9 feature 3000 mAh batteries. According to the tests, the S9 records about 13 hours more of MP3 playback over the S8—with the Always on displays of both devices disabled. With Always on enabled, the S9 has a slight 4-hour advantage. Both devices get the exact same scores on the Internet LTE. Internet WiFi, Video Playback, and Internet 3G tests.
Of course, actual usage numbers may differ, as these figures are gotten from Samsung's release specifications. However, if you own an S8 and seek a boost in battery life, you may be better served skipping the S9.