I just want the pc connected, since is the most delicate but if I can connect the monitor and a 12v modem and router, would be nice... not really necessary.
My monitor is an old LG 19" Flatron W1934S
The PSU will be a Corsair HX1050W wich probably will be running a GTX 580 SLI, I already did some calculations with the online calculator from newegg full specs ~900W
I just need it to turn my pc safely I don't even need 5 minutes, I just want to protect it from outages and unstable power... I'm afraid to buy a low wattage UPS, I read others threads of people with the same SLI and the UPS making noise due the high load (not using the battery)... maybe bad UPS ? I don't know
I'm pretty sure your system (excluding monitor) will be drawing less than 750 Watts from the AC power plug while you're gaming.
The LG Flatron W1934S has a typical power consumption of 36 Watts according to LG's documentation.
Newegg's online power supply calculator tends to overestimate the power requirements because they are trying to sell you a more expensive power supply.
That UPS, that you've linked to, should work just fine with the system and monitor plugged into it.
The power supplies' that have a problem with simulated sine wave UPS' will usually result in the UPS producing characteristic gurgling sounds and shut down in half a minute after switching over to battery operation.
You would then have to consider using this one instead: