There are times where I just want to watch some videos on my PS4. The system requires you to use a controller to press the PS button and pick a user account. After that point I am able to use a wireless keyboard to navigate the menus to get to Netflix, Youtube, and other services. Is there a way to watch videos without leaving the PS4 controller on the whole time, to help preserve battery life?
There are apps for smart phones (Android and iPhone) that allow you to navigate menus etc, whether they're any good or not, I cannot say as I own an Xbox One and use SmartGlass app (side note: It's brilliant!). More info can be found via: Tom's Hardware
Of course this requires you to have either an Android cell or iPhone/iPad/iWhatever.
You only need some form of controller on to "control" the system. (Press Start/X to select the app and start the video, etc.) You can turn off your controller between every single input if you wanted. So no, your controller does not need to be on to use your PS4.
I have had my controller die in the middle of a show and nothing happens. Simply enough you are able to turn your system on without any controllers present at all. I am not sure if the keyboard piggybacks off of the controller though. But I imagine you can use that from the second the system is on to turning it off. At the least, if you are required to use a controller to select an account that should be all you need to use it for.