We kept a back-up copper line for about 5 years. In the end we decided that our cell phones were fine since 90% of calls go through them anyway. If cell service is unavailable, my UPS seems to work fine for the sorts of minimal power outages we get here. Don't see why a low draw wouldn't last about what calcs would show given the battery size you work with, the UPS only sees the draw and that is normally pretty linear effect on the battery.
Doug Ritter
Equipped To Survive®
Chairman & Executive Director
Equipped To Survive Foundation