I picked up an older APC 1500 XS with an external 47lb. APC XS/RS battery pack for $20 as-is from a surplus place ($1500+ retail). I let the unit charge overnight and with a PC pulling 200W and my screens, speakers, modem and airport extreme pulling less than another 100W I could get 120 min of backup power! I knew though, it wouldn't be in surplus if it was working like it should. Under a 700W load the UPS would fail it's self-test, but at my barebones (387W) it passed just fine even with batteries last replaced in 2005 according to the powerchute software. Everytime the UPS went onto battery, I would hear a constant electrical noise/hum, I thought it was the UPS inverters. I then tried to do the real test - unplug the UPS and see how long the batteries really last under a 387W load. The 6 12V batteries lost 10% every few seconds; batteries are dead. Near the end of the test I bent down and I could hear that the Jacob's Ladder arcing sound/hum was coming from my PC's power supply not the UPS! So yesterday 2 9Ah 12V batteries (Tempest) replacement batteries arrived. I installed them and removed the dead external APC battery pack. Now the estimated time is about 25 min, but it will actually last that long. I thought new batteries would have solved the problem. I have an APC 350 between my APC 1500 and my PC, every time I switch to battery power the 350 kicks in and my PC sounds fine for the brief time it's on. Here are my Kill-a-Watt readings: AC Utility Power: 120-121V, Power Factor: .97-.98 Battery Power: 116V, Power Factor: .79 Here is the sound:http://www.filedropper.com/powersupply It actually sounds more like a large hair trimmer... From my research, I found that PSUs don't like the square or sine-wave the older APC's put out, and the noise is the PSU's Power Factor correction? Especially if I can expand this UPS unit, I want to make sure it's not doing harm to the devices connected! Thanks for the help guys!