beeping Back-UPS Pro 1000

[BillP]

We have a Back-UPS Pro 1000 with one server, a NAS, a switch, a KVM, and a monitor on it.  The load on the UPS says around 400 W.  The UPS is rated at 600 W.  We get (usually in the evening) beeps from the UPS.  One day an employee (whose office is just across from the room where this is) said that it beeped at 6:05, 7:05, 8:05, and 9:05.  We thought it might be the power coming into the UPS.  We had the building management people check the power and everything seems fine.  The room where it is was supposed to have it's own circuit.  However, we found that another office (shipping) is also on the circuit.  We are going to correct that.  We happened to have another UPS (same model), so I put the server on it and left everything else on the other one.  I also added a desktop computer to the second UPS.  So, there is 200 W on each UPS.  That solved the beeping problem.  We had a power outage and the server powered off as soon as the UPS went into battery mode.  Kind of defeats the purpose.  I understand that when the UPS is in battery mode, it isn't providing 120 V - more like 90-100 V.  Is it possible that the server won't run on 90-100 V?  This is a development setup.  The main rack is on a Smart-UPS X (SMX3000RMLV2UNC) with 2 auxiliary batteries.  It showed 120 V for the output voltage during the time it was in battery mode.  It has 7 servers, 2 NASs, and about a dozen misc. network hardware on it.  The UPS shows about 1900 W load.  It lasted an hour on battery and everything stayed up.

What's wrong with the Back-UPS Pro 1000?  Why can't we get closer to the 600 W of capacity?

Is there a product in that load range that will give us a full 120 V while on battery?