MrClass wroteit will work, but will not cover the whole 800 watts. Anyway his rig will consume a maximum 500 - 550 watts, this will leave him about a couple of minutes.
if we take your talk , why buying this UPS where he can get more powerfull one specially on a (2500$ +) system ?
or he could drop the psu to a lower one of 600w since his not using them , why putting his system in danger ?
but to go with my trial , i was having a 800 va ups and a 600 w psu , the ups failed to backup the PC for 1 sec , more than that the computer starting turning off and on until i disconnected it .
( by the way the pc was working on desktop - no power load )
now i am having this : MGE Eaton E-Series NV 1000H 1000VA UPS w/ Power Management USB Port ( it is really good but not for him )
and it is for a 600 w psu ,
if you google it also it is written that it is 600 w ready
there is no constant ratio of va per watt , they differ from brand to another, ( for E-Series NV 1 W = 1.66 VA )
it is not about he use these watts or not it is about the output .
