I'd like to add a UPS to my system so that at least the circulation pump would work in a blackout. Outages are rare here but usually don't last more than a few hours. I've read about hooking an external marine battery to the UPS to extend the backup time, but I can't seem to find a UPS that will accept a 12 volt external battery hookup. Some of the larger UPSs that I have found in surplus (for cheap) need odd voltages like 32 volts. Can anyone suggest a UPS or brand that will accept a 12 volt external input?