The Intex 600 VA UPS produces a simulated sine wave when operating on battery power.
Read the Compatibility with UPSes
section of X-bit Labs review of the Seasonic S12-II 520W here:http://www.xbitlabs.com/articles/cases/display/seasonic-psu-roundup_2.html
The APC SmartUPS SC 620
that was used during the test is a simulated sine wave UPS
(i.e. stepped approximation to a sine wave).
Most PSU review sites do not test for UPS compatibility.