How big of a power supply do I need?

[foggyb]

I think bitcoin mining is going to quickly separate the men from the boys when it comes to PSU brands.

We are going to see a lot of blown up power supplies! 

Thermaltake wasn't banking (pun intended) on bitcoin taking off.

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[polishwarrior]

If you only plan on using (2) 5870's, a psu in the neighborhood of 650-750W would be plenty. You can get a 650w corsair tx for $80 after MIR, or the 750W for $95 after MIR from newegg.

[bcpokey]

I am running 2x 5870s overclocked to 975MHz and a slightly overclocked sapphire 5850 (I've downclocked the memory to 300Mhz on all of them) in my big box. It puts out about 1.1GHash/sec and pulls 555Watts from the wall. Assuming an 87% efficiency rate that's 482Watts. So 2 5870s should be able to run on a 500W PSU.

[allinvain]

My dual 5870 rig eats up 440 Watts at the wall and I have a 600W power supply, so I'd say 600W is a good number to have. This way you have a fair bit of headroom and you aren't maxing out or coming close to maxing out the power supply.

[BitcoinRigs.com]

Thermaltake W0133RU ToughPower 1200W Modular Power Supply

This is why you really can't go by the overall wattage ratings. Wattage ratings include all sorts of voltages that don't matter to video cards. You need to look at how the +12V power is distributed.

Look at the TP 1200: +12V1@20A, +12V2@20A, +12V3@36A, +12V4@36A. You're sort of limited on what you can do with several uneven rails like that. If you put a 30A card on that 36A rail, you're left with 6A of unusable power. Put in a second 30A card and you've lost another 6A. Now your 1200W PSU is really only 1056W.

Now look at the Cooler Master Silent Pro Gold 1200W: +12V@98A. One big fat 12V rail means each video card will get what it needs, and there won't be any rails with wasted power.

[bernd]

i recently blew up my corsair 550 W PSU with 2* 5850 and athlon X4 cpu in my computer. now i have a 4 * 5850 mining rig with 950 W PSU and there are no problems whatsoever.