rule of thumb.. never load your ups beyond 50-60% of its max capacity.. meaning donot put CONTINUOUS loads of over 500W on a 1000W ups.. ocassionaly you may but not constant,,.
better to buy an APC 1400VA or 1500VA model.. both are 980W, have upto 14Amps charging, and wont restart your PC ..for cyberpower, check their website, they have all models listed,call their office for prices..check VA vs Watts for power calculations..
as for batteries. if going for APC, buy a one time expense of AGS 165AH x 2 batteries.. it will be costly, but you will have long backups for almost over 2 years.. AGS and APC is a tried and tested combo in most of my installations, and works best.. Exides are average.. pehonix batteries are great but difficult to find them and get warranty claims.. complany is very strict.
if price much of a problem, volta batteries do a very good job at decent price.