Because the UPS has to compensate for both (in)efficiency of the power supply and the AC power factor the VA rating of the UPS should *always* be higher than the load demand or the unit will overheat when it is under full load (when AC power is cut off)

There is a good UPS back up calc here http://www.csgnetwork.com/upssizecalc.html

To get your PC max load demand use this http://extreme.outervision.com/PSUEngine and for your monitor, LCD average is about 40w CRT is about 100w

using 250W guess for your PC demand and 40W for your LCD I get 406VA min and 650VA recommended.

I always recommend a model that has AVR (Automatic Voltage Regulation) it can quiet a noisy power line, and a removable battery is a bonus.

cheers

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