I'm going to buy my first UPS for my first unRAID build (Sempron 140, 6 WD Green HDDs, Corsair vx450w PSU) and I'm thinking about APC Back-UPS RS Pro 550 ( http://www.apc.com/products/resource/include/techspec_index.cfm?base_sku=BR550GI&total_watts=300 ) – is it enough for my build? I don't have long blackouts in my neighbourhood but 1-2 seconds long power outages are quite common. So I need it to keep powering my server for about a minute and after that to send shutting-down signal to unRAID. Is 550VA enough for 6 HDDs? If yes, how many additional drives I can safely add?

You should be fine with that. My OCD overbuilding complex would lead me to buy a bigger one, but that's just me. 

 

The number of drives shouldn't matter much. Assuming that you wouldn't be trying to access them during a blackout, they should be idling, which takes very little power.

 

I bought a CyberPower at Costco, which is rated at 1500VA. It got tested for the first time yesterday, as we had a power outage for about fifteen minutes. It still had at least 80% left on the gauge. Oh, and it's hooked to both my unRAID server and my main PC. Not bad for about $100.

You should be fine with that. My OCD overbuilding complex would lead me to buy a bigger one, but that's just me. 

 

The number of drives shouldn't matter much. Assuming that you wouldn't be trying to access them during a blackout, they should be idling, which takes very little power.

 

I bought a CyberPower at Costco, which is rated at 1500VA. It got tested for the first time yesterday, as we had a power outage for about fifteen minutes. It still had at least 80% left on the gauge. Oh, and it's hooked to both my unRAID server and my main PC. Not bad for about $100.

Actually, you are incorrect.  To un-mount the disks they must be spinning, therefore the process of shutting down the server cleanly will spin up all the disks.

 

You should still be OK, as long as you don't expect an extended run-time on battery power.

Oops. Color me embarrassed. Thanks for the correction.