That is - unfortunately - my experience with EVERY single Android update. They are all announced as the best power-saving version of all times, and in real life, the battery life always gets worse. One such invention, "doze", was rated particularly high before it came out in Android 7 - and now that we have it for over a year, we see that, yes, the phone dozes often, but still needs more energy than ever before. Oreo is just the same, but again a bit worse. Announced as power-saving, it is not at all better. Same for MIUI 10. Now I have MIUI 10 under Android 8.1, and the battery life is WAY worse than it was with MIUI 9 under Android 7.