Agreed but if it's in the middle of a city centre like some of my customers, poor road access, no 'outside' other than the street, or office between one above and below etc...
If it's an industrial estate or the like seems a very sensible option.
Agreed, if you really wanted to do this route than I see no other sensible option than a couple of large leisure type batteries.
As far as using them vs. not using them, I also thought many maglocks went into 'power reduction' once the magnet had been attached?