OP
UPS batteries are working fine with FSP with same system load that shuts down CX550 with UPS overload beep(playing a video file with madvr that loads ~65% of 1050Ti as per gpu-z). So the only explanations left are: a)high inrush current or b)degraded transfer time of UPS(14 months old) less than CX550 but more than FSP Saga II.
@gta5 CX550 worked fine with inverter when I checked it for 2 days but currently I am again using FSP with UPS.Because it is summer so voltage fluctuation is quite high sometimes during day making UPS constantly making clicking sounds at a duration of 1-2 seconds(I then turn off power to UPS for 2-3 minutes & let system run on UPS till fluctuations are gone).I am not sure it is a good idea to directly connect system to inverter without UPS at those times.
@gta5 CX550 worked fine with inverter when I checked it for 2 days but currently I am again using FSP with UPS.Because it is summer so voltage fluctuation is quite high sometimes during day making UPS constantly making clicking sounds at a duration of 1-2 seconds(I then turn off power to UPS for 2-3 minutes & let system run on UPS till fluctuations are gone).I am not sure it is a good idea to directly connect system to inverter without UPS at those times.