Good and affordable UPS for Corsair CX450

I've built a "basic" PC desktop, I mean 150-200w max for "basic" including my monitor, with a Corsair CX450 PSU (OEM CWT RPS0053) with active PFC. Now I need a UPS but my budget isn't high to get the best (eg. an online UPS) so I'd like to buy a UPS with the best quality/price relationship for my goals and equipment within 180USD. AFAIK I need a UPS that generates a pure sine wave during a blackout because of my PSU's active PFC, now let's define my goals.

My worst scenario is when it's raining hard because blackouts occurs randomly, eg. last time it rained there were 3 blackouts within 30 minutes and then no more ones though it kept on raining for hours. My "limit" is 2 close blackouts, I mean I've to be able to shut my pc off correctly during the 2nd blackout then... I'll just wait for better weather time Well my electrical system suffers brownouts oftenly too but I guess a line-interactive UPS, I mean with AVR, will be a solution for that.

That said please tell me how much minimum power (VA / W) has to generate an UPS to get my goal, I mean the worst scenario I've described above. Then please tell me if an UPS must have other specifications. Finally please suggest me some UPS models to buy within 180USD that fit my personal case.