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I use CORSAIR CX430V2
im using my pc for 3 years without ups. (AMARON iACE)
and my pc doesnt reboot.
you should put inverter in UPS MODE and changeover will be fast
in UPS MODE the input for the inverter will be limited to 180-265V. beyond/less than this will go to battery mode immediately.
in my office pc runs off directly from sinewave inverter and no reboots there with 2 computers, printers and lights...
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APC has
Stepped approximation to a sinewave
APC HAS 2x9Ah 12v Batteries
but with a inverter u can use a 1x 80 or 100 or 150 or 200 Ah battery
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get
inverter 800va=640w------------ 4.6k
exide battery 100ah-------------10k(approximate)
go to local shop and enquire.
total 14.6k no any other wiring necessary, just like ups.
if you want fans and lights to run when pc is off then you need to modify wiring.
UTL 810 VA Pure Sine Wave Inverter for Home/Office/PC | eBay
old review last summer of this inverter on a famous inverter discussion site
Buy UTL 800 VA Inverter Online on Ebay for Rs.3700/-