I want to build a PC with this configuration:
Gigabyte X58A ud3r
Corsair 850W TX
Core i7 930
ATI HD 5850 (crossfire in a near future)
I need an UPS (not a PSU, sorry) because there are regular power cuts at home.
I read in the UPS Faq that the pc usually draws < 300W instead of my PSU's 850W. But what if the power fails while I'm playing a game? Would the pc/ups be damaged if at that moment it's drawing more than 300W or so?
Please, could you recommend me a good and inexpensive PSU?
Your power supply is only going to pull as much power as it needs to.
If your 850w psu only needs 300w, its only going to pull 300w (efficency not included) from the wall.
Yes your power supply is overkill, but that will give it time to age and you room to add more components. The only downfall to keeping the PSU is the money you paid for it.
If you want a new PSU go ahead and keep it in the Corsair family with a 500w+ (650w+ for crossfire). Or go with something from Antec, PC Power & Cooling, Cooler Master, Thermaltake, or SeaSonic. Just make sure it has 80% efficency or better.
nene_mataro - If the local electricity fails while playing a game the only thing that will happen is the UPS will supply power to your computer system for a few minutes so you can save data and shut off the pc. The same is true if you are drawing more than 300 watts when the local electricity fails. Nothing else will happen. The psu and the pc would not be damaged.
Now, about the UPS FAQ - It is generally true that a pc system with one video card will use less than 300 watts most of the time. Surfing the Internet, checking email, and playing some games requires less than 300 watts of power. Some games might require a little more power. A system with two video cards might also require require a little more power.
Here are the official power requirements for the ATI Radeon HD 5850 video cards.
ATI Radeon™ HD5850 System Requirements:
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
500 Watt or greater power supply with two 75 watt, 6-pin, PCI Express® power connectors.
600 Watt or greater power supply with four 75 watt, 6-pin, PCI Express® power connectors for ATI CrossFireX™ technology in dual mode.
The recommendations are for an entire pc system.
As you can see, an 850 watt power supply is a lot more than you will need. Instead, I would recommend the new Corsair, HX650 power supply if it is available in your country:
Ok, the way I'm reading this, your PSU is fine, but you want to buy a UPS.
Here's your problem. The UPS units linked in previous posts are decent for older systems, but may not work for current, top-quality PSUs like your Corsair. Check its documentation; it may say something about using it with a UPS. The reason is that your PSU may insist upon a sine-wave input, and cheaper UPS units only output a stepped approximation. You probably won't damage anything, but your PSU will just shut off if it doesn't like the waveform provided by the UPS. Buy locally so you can avoid shipping charges if you need to return it.
Here is an example of a UPS that provides a true sine wave, the APC SUA-750: http://www.newegg.com/Product/Product.aspx?Item=N82E168... . As you can see, it is more expensive than the others; I had to buy one for my SG-650 because the BE-750G I had (which worked fine for another system) did not provide an acceptable waveform. It is now providing backup power to my cable and DSL modems, router, and an external drive dock, where it works just fine.