Why do i need an 850w PSU to power SLI/CF

[Pandamonia]

Total System draw in games is 450-550w for CF and SLI with a hexacore Sandy.. in many reviews

So why do people say you need 850w PSU?

I have a 620w which i am reluctant to upgrade...

I have a 4770k and one SSD. Why would i need so much PSU? Or its it just internet BS again?

 

[blastingcap]

margin for error especially with bad-quality PSUs that exaggerate their max continuous wattage rating

margin for error if you overclock or overvolt; a strong overvolt in particular can mean over 100 more watts per CPU or GPU

margin for error as PSUs age, they slowly lose max capacity

durability issues and noise because a smaller PSU working hard = more noise and fan wear and tear than a bigger PSU that doesn't need the fan to spin as fast, so it's quieter and less noisy

 

[3DVagabond]

Which cards do you want to Crossfire/SLI? For something like 290X's or 780ti's ~850w is a good recommendation. Although if you don't want to O/C the cards you could use less. Still wouldn't recommend less than 750W.

 

[Pandamonia]

i have 620w Corsair and its been rock solid since i got it.

I want either 290CF or 780 CF for my 1440p

BTW that system power was on an OC Sandy 6 core.

 

[wand3r3r]

290x crossfire here. It was under 700w (650-690) but now I added some fans and it hopped over 700w. Start overclocking and 850w isn't enough. Then again, this is mining so gaming will remain lower.

 

[3DVagabond]

i have 620w Corsair and its been rock solid since i got it.

I want either 290CF or 780 CF for my 1440p

BTW that system power was on an OC Sandy 6 core.

I wouldn't run that system O'C'd on a 620W PSU. Those cards can draw +300W each when O/C'd. No O/C'ing and you can likely squeeze buy. You'll be pushing that PSU pretty hard though. Old caps, etc. might not hold up. It'll likely be spinning the fan up pretty loud trying to keep it cool too.

 

[Pandamonia]

290x crossfire here. It was under 700w (650-690) but now I added some fans and it hopped over 700w. Start overclocking and 850w isn't enough. Then again, this is mining so gaming will remain lower.

How is the CF on those cards? i dont think ill go for the X CF due the price/perf

 

[Puffnstuff]

You don't want to run a ps at near it's maximum capacity so the larger unit provides headroom. I run a 1kw corsair and during gaming I've seen spikes in the 800+ watt range. When I replace it I'll go up to at least a 1250w unit.

 

[Pandamonia]

You don't want to run a ps at near it's maximum capacity so the larger unit provides headroom. I run a 1kw corsair and during gaming I've seen spikes in the 800+ watt range. When I replace it I'll go up to at least a 1250w unit.

you see this makes no sense to me. i would say that running at 620w is exactly what its designed to do or its not a 620w PSU.

It sounds like sales talk to me.

 

[wand3r3r]

How is the CF on those cards? i dont think ill go for the X CF due the price/perf

I've only played BF4 and it was flawless for the little time I've played. These have the XDM crossfire without brackets which is quite refined according to the reviews and is in the same class as SLI imo. I can't see any difference between the GTX 690 and these but I haven't played much yet.

I would go 290 if you can find it closer to retail. There's close to retail prices regularly and certain retailers don't gouge (Microcenter, Amazon - amazon itself, not the sellers there). If you can't 780 is an excellent card but at a $80-100 premium for a similar card (MSRP comparison) although it may have a bit more OC headroom. They mine like champions though which is why I got these. :biggrin:

 

[Pandamonia]

I've only played BF4 and it was flawless for the little time I've played. These have the XDM crossfire without brackets which is quite refined according to the reviews and is in the same class as SLI imo. I can't see any difference between the GTX 690 and these but I haven't played much yet.

I would go 290 if you can find it closer to retail. There's close to retail prices regularly and certain retailers don't gouge (Microcenter, Amazon - amazon itself, not the sellers there). If you can't 780 is an excellent card but at a $80-100 premium for a similar card (MSRP comparison) although it may have a bit more OC headroom. They mine like champions though which is why I got these. :biggrin:

290 Twin Frozr is 358 GBP

780 Twin Frozr is 378 GBP

Thats the choice in CF or SLI i have

 

[BrightCandle]

The 290X cards are basically maximum draw around 300W. They average less than that most of the time but you will come across very different draw amounts in different games. The CPU is 125W TDP but it can pull a peak more like 150W if its fully used for short periods. So why is 850W recommended? Because its basically the minimum supply that can keep everything working even when the machine is maxed out in all areas. The motherboard can pull 50W on its own + drives +any other cards its actually easy to build a scenario that has 850W being a bit close.

In practice most of the time less will work, but when some games occasionally crash you will be left scratching your head as to why.

 

[Pandamonia]

So the question is.

290 Twin Frozr is 358 GBP x2 = 716 GBP

780 Twin Frozr is 378 GBP x2 = 756 GBP

Plus 150 GBP for 860i PSU worth 860-900 GBP?

Or just get the 780ti for 525 GBP?

 

[Lonyo]

You don't want to run a ps at near it's maximum capacity so the larger unit provides headroom. I run a 1kw corsair and during gaming I've seen spikes in the 800+ watt range. When I replace it I'll go up to at least a 1250w unit.

The PSU should be able to spike up to 1100w.

http://www.corsair.com/us/media/cms/manual/corsair-psu-spec-table-091813.pdf
For Corsair RM1000:
Continuous power: 1000w
Peak power: 1100w

If it's a different 1kw Corsair, it will probably still have ~10% headroom for spikes, since that's what most of them seem rated to have.
Power supplies from good manufacturers like Corsair are designed to output what their specs say. They are also supposed to be a minimum level of efficiency at that power output if they are Plat/Gold/etc rated.
That means while running at 80% e.g. 800w might be less efficient than 50%/500w, it's not that much less efficient, and the PSU should be able to handle 80% load, or even higher.

To the OP, that's why people overbuy power supplies. Because they assume all power supplies are badly rated, and don't understand what a quality supply is rated as and designed to do.
A 700w quality power supply is going to be way better than an 800w cheaper power supply in almost every case, for example.

A good way to see what happens when a PSU isn't "properly" rated is to go to http://www.jonnyguru.com/
550w PSU that won't do even 450w:
http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story4&reid=335
550w PSU that will do 550w:
http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story4&reid=333

If I told people I ran an i5-3470 and Geforce 650Ti on a 220w power supply and put gaming loads on it, people probably would think that was a crazy thing to do, because they assume that such a system must need at least 400w but without any real reason for it.

And finally, one other thing to remember:
Whatever load most (not all) websites are listing is often from the wall. That means that due to the efficiency of the PSU, the actual load on the PSU itself is only 80~90% of the number you are seeing.
So if you have an 800w wall load peak, and a 90% efficient power supply, that means your actual PSU load is only 800w*90% = 720w. If you're using a 1kw PSU, that means your PEAK load on that power supply at spike time is 72%.

The guy I quoted is basically saying he doesn't trust his PSU to run at 72% load (or if you assume his peak loads are the same as the manufacturers peak loads, around 66% of rated peak load, 720w/1100w).
He feels a need to get a 1250w PSU so he runs at 57.6% (or 52.4% of peak rating). Starts to seem a little crazy, doesn't it?

 

[Pandamonia]

The PSU should be able to spike up to 1100w.

http://www.corsair.com/us/media/cms/manual/corsair-psu-spec-table-091813.pdf
For Corsair RM1000:
Continuous power: 1000w
Peak power: 1100w

If it's a different 1kw Corsair, it will probably still have ~10% headroom for spikes, since that's what most of them seem rated to have.
Power supplies from good manufacturers like Corsair are designed to output what their specs say. They are also supposed to be a minimum level of efficiency at that power output if they are Plat/Gold/etc rated.
That means while running at 80% e.g. 800w might be less efficient than 50%/500w, it's not that much less efficient, and the PSU should be able to handle 80% load, or even higher.

To the OP, that's why people overbuy power supplies. Because they assume all power supplies are badly rated, and don't understand what a quality supply is rated as and designed to do.
A 700w quality power supply is going to be way better than an 800w cheaper power supply in almost every case, for example.

A good way to see what happens when a PSU isn't "properly" rated is to go to http://www.jonnyguru.com/
550w PSU that won't do even 450w:
http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story4&reid=335
550w PSU that will do 550w:
http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story4&reid=333

If I told people I ran an i5-3470 and Geforce 650Ti on a 220w power supply and put gaming loads on it, people probably would think that was a crazy thing to do, because they assume that such a system must need at least 400w but without any real reason for it.

And finally, one other thing to remember:
Whatever load most (not all) websites are listing is often from the wall. That means that due to the efficiency of the PSU, the actual load on the PSU itself is only 80~90% of the number you are seeing.
So if you have an 800w wall load peak, and a 90% efficient power supply, that means your actual PSU load is only 800w*90% = 720w. If you're using a 1kw PSU, that means your PEAK load on that power supply at spike time is 72%.

The guy I quoted is basically saying he doesn't trust his PSU to run at 72% load (or if you assume his peak loads are the same as the manufacturers peak loads, around 66% of rated peak load, 720w/1100w).
He feels a need to get a 1250w PSU so he runs at 57.6% (or 52.4% of peak rating). Starts to seem a little crazy, doesn't it?

That is crazy. If i buy a PSU rated at 620w and warrantied for 7 years then id expect that PSU to run at 100% for 7 years and not die or cause me issues.

 

[Grooveriding]

Preferable not to run your PSU at or near its max rating, particularly once it starts to get older. A little headroom is a good thing and better for the PSU to run efficiently and keep its fan noise down.

Also if you plan to overclock significantly, GK110 and Hawaii use a ton of power once you start pushing them. I'd never heard the fan on my PSU until I installed a couple overclocked/volted 780s in my rig. Now the fan noise from the PSU is obnoxious with a full load on the GPUs.

I don't have a kill-a-watt, but based on numbers given elsewhere I'm assuming its pulling over 1200W between the overvolted GPUs and CPU.

 

[Pandamonia]

Preferable not to run your PSU at or near its max rating, particularly once it starts to get older. A little headroom is a good thing and better for the PSU to run efficiently and keep its fan noise down.

Also if you plan to overclock significantly, GK110 and Hawaii use a ton of power once you start pushing them. I'd never heard the fan on my PSU until I installed a couple overclocked/volted 780s in my rig. Now the fan noise from the PSU is obnoxious with a full load on the GPUs.

I don't have a kill-a-watt, but based on numbers given elsewhere I'm assuming its pulling over 1200W between the overvolted GPUs and CPU.

i bet you £10 that your system is pulling nowhere near 1200w from the wall.

Power Consumption GeForce GTX 780 Ti 2-way SLI

System in IDLE = 129W
System Wattage with GPU in FULL Stress = 617W
Difference (GPU load) = 488W
Add average IDLE wattage ~20W
Subjective obtained GPU power consumption = ~ 508 Watts

"Our test system is based on a power hungry six-core Intel Core i7-3960X Extreme Edition Sandy Bridge-E based setup on the X79 chipset platform. This setup is overclocked to 4.60 GHz on all cores"

 

[Grooveriding]

i bet you £10 that your system is pulling nowhere near 1200w from the wall.

Power Consumption GeForce GTX 780 Ti 2-way SLI

System in IDLE = 129W
System Wattage with GPU in FULL Stress = 617W
Difference (GPU load) = 488W
Add average IDLE wattage ~20W
Subjective obtained GPU power consumption = ~ 508 Watts

"Our test system is based on a power hungry six-core Intel Core i7-3960X Extreme Edition Sandy Bridge-E based setup on the X79 chipset platform. This setup is overclocked to 4.60 GHz on all cores"

There is a huge difference between GK110 running under ideal conditions with factory mandated lower voltages in the sweet spot of its efficiency vs. pushing 1.35V through it and running it 1300+Mhz.

400W+ per card when overclocked that way.

 

[Pandamonia]

There is a huge difference between GK110 running under ideal conditions with factory mandated lower voltages in the sweet spot of its efficiency vs. pushing 1.35V through it and running it 1300+Mhz.

400W+ per card when overclocked that way.

i highly doubt your cards are pushing 600w more than the setup in that review.

Also you are forgetting that the PSU outputs its rated power and not the wall draw.

 

[Grooveriding]

i highly doubt your cards are pushing 600w more than the setup in that review.

Also you are forgetting that the PSU outputs its rated power and not the wall draw.

It is what it is. They pull huge power over-volted and over-clocked. On OCN people with 1200W PSUs using dual 780 classified type cards or using voltage hacks have had to upgrade their PSUs and shown kill-a-watt results of the cards using over 400W each.

 

[Lonyo]

i highly doubt your cards are pushing 600w more than the setup in that review.

Also you are forgetting that the PSU outputs its rated power and not the wall draw.

Overvolting massively increases power consumption. Massively.
I think AT had an article on it for CPUs, and there have been other things in the CPU forum on it as well from IDC and others regarding voltage vs clockspeed scaling.

Plus it's just general fact that overvolting (+ overclocking) massively increases power use due to exponential scaling.

http://forums.anandtech.com/showthread.php?t=2195927
The relationship in a GPU is essentially the same as in a CPU with regard to power consumption scaling against voltage and clockspeed.
http://forums.anandtech.com/showthread.php?t=2281195

A ~185w system gets to 350w when overclocked and overvolted at the wall. And that's the increase from just the CPU.

 

[Pandamonia]

Overvolting massively increases power consumption. Massively.
I think AT had an article on it for CPUs, and there have been other things in the CPU forum on it as well from IDC and others regarding voltage vs clockspeed scaling.

Plus it's just general fact that overvolting (+ overclocking) massively increases power use due to exponential scaling.

http://forums.anandtech.com/showthread.php?t=2195927
The relationship in a GPU is essentially the same as in a CPU with regard to power consumption scaling against voltage and clockspeed.
http://forums.anandtech.com/showthread.php?t=2281195

A ~185w system gets to 350w when overclocked and overvolted at the wall. And that's the increase from just the CPU.

The AT review shows that there is an increase in 53w of consumption from OC without touching voltage. Im not sure 200 more mhz will add 250w per card.

 

[KaRLiToS]

Power Draw depends on each games. Some games will pull more, some other will pull less. Its always better to have bigger PSU than being on the edge of a shut down. NO?

Especially if you want to bench, you would need 900W PSU.

I have a lepa G 1600 for my Quad Fire 290x with 3930k at 4.6ghz and sometime its pulling 1500w from the wall when the GPUs are at stock.

I use 2600w when benching my rig.

But if you want to use a 620w PSU for CrossFire or SLI, its up to you. But to be honest with you, I wouldn't even want a 620w PSU for an OCed rig with a GTX 780 or R9 290.

 

[Pandamonia]

1304mhz 780ti Whole system pulls 420w on a single card on a 4.5ghz 4770k. Overvolted and Overclocked.

http://www.overclockersclub.com/reviews/nvidia_gtx780ti/16.htm

 

[aaksheytalwar]

Why don't you go for it? If it works then good. If it blows up then good for you.