PSU - How much do i need?

teradoom

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Apr 9, 2012
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Hello,

So im thinking of putting together a 2x Hd6850 GPu with either i5 2500k or the i7 26(7)00k with a Asus P8Z68-V LX.

I checked new egg and they recomend 600w PSU but others say a lot more. So how much do i need if i was to have all the 16gb 1600rams and a ssd plus HDD fans etc the whole lot?
 
Solution
For a system using two Radeon HD 6850 graphics cards in 2-way CrossFireX mode AMD specifies a minimum of an 600 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 36 Amps or greater and have at least two 6-pin PCI Express supplementary power connectors.

Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Continuous Amperage Available on the +12V Rail(s) is the most important factor.

A Seasonic X-Series 560W (SS-560KM) is more than capable with its combined +12 Volt continuous current rating of 46 Amps and with two (6+2)-pin PCI Express supplementary power connectors.

A Corsair...

cyansnow

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Jan 20, 2012
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A high quality 750w will suffice, I mean like 80 Plus silver rating. Like Brett said, you must get a GOOD 750w PSU not some budget back of the store crap. As I have said in many threads good PSU makers include: Antec, Seasonic, corsair, Silverstone, and XFX.
 
For a system using two Radeon HD 6850 graphics cards in 2-way CrossFireX mode AMD specifies a minimum of an 600 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 36 Amps or greater and have at least two 6-pin PCI Express supplementary power connectors.

Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Continuous Amperage Available on the +12V Rail(s) is the most important factor.

A Seasonic X-Series 560W (SS-560KM) is more than capable with its combined +12 Volt continuous current rating of 46 Amps and with two (6+2)-pin PCI Express supplementary power connectors.

A Corsair Gaming Series GS600 (CMPSU-600G) is also more than capable with its combined +12 Volt continuous current rating of 45 Amps and with two (6+2)-pin PCI Express supplementary power connectors

The Enthusiast Series Modular TX550M (CMPSU-550TXM) is also more than capable with its combined +12 Volt continuous current rating of 48 Amps and with two (6+2)-pin PCI Express supplementary power connectors.
 
Solution

ram1009

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The bigger the better but only the highest quality. Don't listen to people telling you how small a PSU you can get away with. It's false economics.
 

ram1009

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My boss would probably debate that with you since I've been specifying power supplies for him (among other things) for almost 18 years without a single failure.
 

I'm not telling you how to do your job, but PCs consume significantly less power than most people think. http://www.silentpcreview.com/article1245-page3.html
Significantly overspecifying PSUs costs extra money for parts with unneeded operational capacity. It can also drive the PSU to operate outside its peak efficiency spread, 40-70% of operational capacity, and that leads to higher electricity costs. The additional electricity cost per PC will be low, but the costs add up with multiple PCs over an extended period of time.

To the OP: Buy a high quality PSU that meets your system requirements; ko888 gave some great PSU suggestions. Power draw from a single 6850 is ~121W. http://www.guru3d.com/article/radeon-hd-6850-6870-review/10 Crossfire does not double power draw, but let's assume it does and increase power to 242W. Add 160W for the rest of the system at full load (that's more than enough) and you hit 400W. For all of the PSUs listed by ko888, hat would put a max load at around 72% of operational capacity - right in the efficiency sweet spot.
 

ram1009

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You're exposing your ignorance. A 1000W PSU supplying 500W isn't consuming any more power than a 500W PSU doing the same thing. The difference is that the 500W supply is maxed out and over stressed. The 1000W supply is only cruising. It doesn't take a rocket scientist to figure out which PSU will last longer and be more reliable along the way.
 

sk1939

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Not quite, as any EE will tell you. A power supply operates with efficiency on a bell curve, which means that near it's peak (about 75% utilized) is when it's most efficient. While the wear on electronics is greater toward the max capacity, it is far more energy efficient, and a well designed power supply takes that into account.


900W Power Supply Efficiency

eff-comparison.png



As you can see, peak efficiency is between 450 and 800W.
 

You are incorrect, sir.

I agree that either a 500W or a 1000W PSU will provide the same amount of power to the PC. The PSU will only provide as much power as the PC requires. The issue is how much power is drawn from the outlet to provide that power. The power drawn from the wall will vary by the efficiency of the PSU in converting the AC from the wall and into the DC that will be used by the PC. PSU efficiency in converting AC-to-DC power varies at different loads on the PSU. The variable efficiency is usually charted on a bell curve. Modern PSUs typically have peak efficiencies in the 40-70% load range and have made significant progress in flattening the curves to provide similar efficiency across a wider range of loads.

Let's take a 500W and a 1000W PSU and assume that both are similar build quality and 80+ Bronze certified. Let's say that the PC requires a constant 300W of power. Both PSUs would provide the same 300W of power that the PC required. 300W represents a 60% load for the 500W PSU and let's say that results in 88% efficiency. That same 300W is a 30% load for a 1000W PSU; let's say that results in a 83% efficiency. In this case, the 1000W PSU will draw 361W from the wall to provide 300W to the PC (300W/.83). The 500W will draw 341W watts from the wall to provide 300W to the PC (300W/.88). Running 6 hours/day for 365 days a year; that is an additional 43.8 kWh.
 

ram1009

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You guys must have a lot of time on your hands for splitting hairs. I won't even bother challenging your numbers. I'll only say that using your numbers the extra electricity consumed costs $.18 per day in the US at the rate of $.112 per KWH according to Wikipedia. If you're too cheap to spend $.18 per day for the peace of mind provided by an unsteressed power supply then have at it. I never said it was free, but only that it was the cheapest insurance policy you could buy for the well being of your entire PC.
 
I agree that a good quality PSU is a critical part of a PC and gives great peace of mind. I just don't believe that you have to significantly overspecify for extra peace. It costs more than the electricity. You can get a good quality 550W PSU for around $60-70. A good quality 1000W PSU costs $180. That's a significant amount of money that could be invested in other parts of the PC.
 

ram1009

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Well, we finally agree on one thing. Invest is the right word. I'll put my $100 into something that improves my entire box rather than something that gets me a few extra FPS that I'll never notice.
 

ram1009

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A $100 boot drive is a waste of money. That will only buy a 60GB which isn't nearly enough. I have a 256GB M4 and wouldn't consider anything smaller. I also consider an SSD a good investment but not at the expense of a proper PSU.
 

sk1939

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I could buy a 850W psu, but what would be the point when my PC only pulls 300W from the wall at 100% load? There is the law of diminishing returns to consider as well.
 

ram1009

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Very nice article. Thanks for the link.