"Adding an extra battery pack DOES NOT increase maximum current
the inverter or UPS can deliver. This current limit is based on the
inverter
internal design and electronic parts (transistors, fuses) used to build
it...
Adding extra battery pack will increase TIME the inverter will work
from
the battery with the same load, but will not increase the maximum
load."

I am sorry if my words were mis-spoken and should have been in Watts
not VA,( =0 sorry i respond to this stuff early in the morning).

What I meant that a UPS is a short run item and should not be used for
sensative equipment unless wet or dry cells are being utilized. Then
there is the whole hydrogen buildup issue. The long and short is a UPS
can come with a bad battery out of the box and will degrade over time
just like any battery source. I will not protect my valuable investment
with a 150$ UPS. Even if you dont have a power bump the UPS will still
charge and recharge the battery every day and you will have no idea how
long it will last unless you do a run test every month or so. Even if a
UPS is rated for 1500VA more than likely you should not run more than
80% of full load.

I installed 35 of the 1500VA models at Genentech and I tested each one
for runtime based on a 100VA draw. Each one was different and two even
had bad batteries. After 6 months the times changes, some dramatically
some not so, but the load never did! I will work if it is all you want
to deal with but I dont recommed it unless you get a sreaming dealon a
decent model.