Auto brightness uses the sensors and the cpu as far as I know. That means it consumes more power. I also set brightness around 20-30% indoor and also outdoor... No sunshine in Scotland...
If you don't have the status bar brightness mod (or cm brightness mod) this is a great app. I use it to replace auto brightness, and I set an almost invisible slider on the left side of my screen to leave brightness as a manual setting and be able to bump it up on the fly without leaving an app.