Altair, I am not on bipap (yet) but we just purchased a generator for our house. We don't have many power failures but we were worried about not having any way to get water as we are on an acreage and have a well pump. I really struggle if I dont have a tap with running water to wash the gunk out of my mouth so I wanted to make sure the we had a back up. Because the well pump is 240V and 6A we decided on a 4000 watt unit but you could probably get a 1000 or 2000 watt Honda or Yamaha generator that are very portable, very quiet and could provide lots of power to run a bipap for hours.
I am not sure how long a battery backup would work for, what is the wattage or amperage draw of the bipap?