dalmvern is right about the power rating. additionally a UPS does the same thing but uses batteries to maintain a voltage, but only while they are charged. any extended duration low voltage events and your batteres drain and the UPS powers down. how long this takes will depend on how low the voltage gets and how much power your using . the lower the voltage, and more power used, the less time it can regulate.
for a 1000w PSU i would recommend a minimum 2000VA rated UPS, with voltage regualtion (not all will have this). this will give you 5-10minutes in a power failure event, 10-20minutes of voltage regulation... and could cost more than $1000US (numbers from APC, would vary by manufacturer)