My building has fob-based locks that are battery powered and work when the power is out, which I know because they recently replaced the batteries. Just another anecdote I wanted to contribute.
For buildings with 'heavy duty' entry systems, power to a powered lock can be hooked up through a 'door loop' [1] on the secure side of the door.
Or you can put the electronic lock in the frame of the door, as an electromagnet or an electronic strike [2].
The door to a residence, personal office or hotel room might only need to open 10x per day and so can be battery powered - but the main entrance to an office building might open 1000x per day, so wired power is the right choice there.
A guy I made a website for a looong time ago sold systems like that. The system he sold had some really neat tricks to avoid needing to be connected to a network and the like.
The main entrance had the only networked badge system. What it'd do is when you entered the building it'd write an update list to your fob. When you then open any of the non-networked locks they'd grab the update list from your fob. This way things like revoking access to fobs or adding new fobs to the system would get to all the doors pretty quickly without them needing to be connected to wifi or whatnot.
Of course this also makes it feasible for the locks to run on batteries for a long time. The next trick the locks did was when you unlock a non-networked door it'd write its battery state to your fob. When next operating a networked lock it'd pass the battery state along to the main database. Hey presto you're getting low battery alerts, and there's still no always-on network slurping battery life.
In case of a power outage the reception people could let people in the main door, and all the other locks in the building would keep working as they always have.
