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gtx 780 ti sli power consumption

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  • Gtx
  • Power Supplies
  • Power Consumption
  • SLI
  • Graphics
Last response: in Graphics Cards
November 12, 2013 10:04:20 AM

i have an 875watt power supply and was wondering if it can handle 2 gtx 780 ti in sli?
in case u need to know i have a 3960x cpu

More about : gtx 780 sli power consumption

a b ) Power supply
November 12, 2013 10:07:32 AM

What PSU do you have?
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November 12, 2013 6:25:20 PM

lp231 said:
What PSU do you have?


I dont know what psu i have i got it from alienware but in their site you can choose to have two gtx 780s sli but no option for 780ti thats why in asking
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Best solution

a b ) Power supply
November 12, 2013 6:37:50 PM

Just open up your case and look at the label.
If it can support GTX 780 in SLI, then there should be no problems running a pair of GTX 780Ti in SLI.
This should be the PSU used in Alienware (from Dell forums)
Share
November 12, 2013 6:51:59 PM

lp231 said:
Just open up your case and look at the label.
If it can support GTX 780 in SLI, then there should be no problems running a pair of GTX 780Ti in SLI.
This should be the PSU used in Alienware (from Dell forums)
yes thats my psu. Thanks alot for your help ill go ahead and buy myself a pair of gtx 780ti right now :D 

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a c 878 ) Power supply
November 12, 2013 8:19:30 PM

Yes with 825watts on the combined +12 volt rail it should handle it fine.
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January 2, 2014 5:44:57 AM

When loading PSU with multiple GPU's (on a gaming rig) one of the two most important (and most often missed) "power" factors when picking out a power supply are 1. AMPs and 2. How many Amps on each 12Volt rail , or to be precise, getting the maximum amount of amperage on a (preferred) single volt rail. Multiple rail PSUs, (IMO) are more for servers or for those that wish redundant power across multiple rails incase one or two rails fails. Also, multiple PSUs (IMO) I are cheaper and cheaper to manufacture. Not to mention, that if power across multi rails is not all used it's wasted. For gaming (and especially SLI/Xfire) I ALWAYS recommend one single twelve volt + rail. Now some may say well if that rail dies then the PSU is dead and that's correct, but gaming at max with 2 cards on a multi-rail system that's lost a rail will most likely fail/crash anyways.

Now, as far as "enough power" I tend to go by AMPERAGE not wattage. All high end video cards have a recommended (amperage) to them, it may be hard to find in the specs but they do. This is more important of a number to go by because you can have two 500 watt power supplies and one have 22amps and one have 30amps. If the card you have requires a 25amp minimum, you're more likely to crash at extended periods of max gaming on the 22amp PSU. And, it's tricky because sometimes a manufacturer will say "minimum 500w" but one has less amperage (less current strength) than the other and may not support the max load, especially in SLI/Xfire. Side note, most "GOOD" power supplies, will hold steady a little past there posted values and have some room to push it, GOOD ONES. Crappy ones are probably over rated and preform less than posted values or not at a constant rate.

A handy (but general) formula for figuring out how much amps are in a unit is Total Number of Watts/12 (Volts for the rail) = Amps... so a 875Watt PSU would have 72 amps... NOW, for a GEFORCE 780 I believe the minimum recommended amount of Amps for that card is 40... now, the second card won't run at exactly the same TDP as the first card but at 72 amps you are REALLY close to the limit. To be safe, I'd give yourself some more head room, if it's not too late, and go with an 80 or 85 amp unit with a single +12v rail and if you want to really be safe go with Silver or Gold rated because they more efficient with more constant values and without as much minor current loss as lower rated PSUs.

If you REALLY want to learn a WHOLE LOT about power supplies go here and read this > http://www.hardwaresecrets.com/article/Anatomy-of-Switc...
it's my favorite site with the best info IMO about power supplies and reviews. EXCELLENT SITE.

Hope it helps.

At least that's my jist of it.
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a c 878 ) Power supply
January 2, 2014 6:23:45 AM

RigRebel said:
When loading PSU with multiple GPU's (on a gaming rig) one of the two most important (and most often missed) "power" factors when picking out a power supply are 1. AMPs and 2. How many Amps on each 12Volt rail , or to be precise, getting the maximum amount of amperage on a (preferred) single volt rail. Multiple rail PSUs, (IMO) are more for servers or for those that wish redundant power across multiple rails incase one or two rails fails. Also, multiple PSUs (IMO) I are cheaper and cheaper to manufacture. Not to mention, that if power across multi rails is not all used it's wasted. For gaming (and especially SLI/Xfire) I ALWAYS recommend one single twelve volt + rail. Now some may say well if that rail dies then the PSU is dead and that's correct, but gaming at max with 2 cards on a multi-rail system that's lost a rail will most likely fail/crash anyways.

Now, as far as "enough power" I tend to go by AMPERAGE not wattage. All high end video cards have a recommended (amperage) to them, it may be hard to find in the specs but they do. This is more important of a number to go by because you can have two 500 watt power supplies and one have 22amps and one have 30amps. If the card you have requires a 25amp minimum, you're more likely to crash at extended periods of max gaming on the 22amp PSU. And, it's tricky because sometimes a manufacturer will say "minimum 500w" but one has less amperage (less current strength) than the other and may not support the max load, especially in SLI/Xfire. Side note, most "GOOD" power supplies, will hold steady a little past there posted values and have some room to push it, GOOD ONES. Crappy ones are probably over rated and preform less than posted values or not at a constant rate.

A handy (but general) formula for figuring out how much amps are in a unit is Total Number of Watts/12 (Volts for the rail) = Amps... so a 875Watt PSU would have 72 amps... NOW, for a GEFORCE 780 I believe the minimum recommended amount of Amps for that card is 40... now, the second card won't run at exactly the same TDP as the first card but at 72 amps you are REALLY close to the limit. To be safe, I'd give yourself some more head room, if it's not too late, and go with an 80 or 85 amp unit with a single +12v rail and if you want to really be safe go with Silver or Gold rated because they more efficient with more constant values and without as much minor current loss as lower rated PSUs.

If you REALLY want to learn a WHOLE LOT about power supplies go here and read this > http://www.hardwaresecrets.com/article/Anatomy-of-Switc...
it's my favorite site with the best info IMO about power supplies and reviews. EXCELLENT SITE.

Hope it helps.

At least that's my jist of it.

The 40amp is for the whole system so the second card needs a lot less or each card peaks at 328watts http://www.techpowerup.com/reviews/ASUS/GTX_780_Ti_Dire... which equals 27.33 amps wich makes 2 cards consume only 54.66amps under stress test. Here is actually total system power consumption measured with 2 x 780Ti in SLI maxing at 654watts so OP is fine http://anandtech.com/show/7492/the-geforce-gtx-780-ti-r...
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January 3, 2014 12:57:43 PM

rolli59 said:
RigRebel said:
When loading PSU with multiple GPU's (on a gaming rig) one of the two most important (and most often missed) "power" factors when picking out a power supply are 1. AMPs and 2. How many Amps on each 12Volt rail , or to be precise, getting the maximum amount of amperage on a (preferred) single volt rail. Multiple rail PSUs, (IMO) are more for servers or for those that wish redundant power across multiple rails incase one or two rails fails. Also, multiple PSUs (IMO) I are cheaper and cheaper to manufacture. Not to mention, that if power across multi rails is not all used it's wasted. For gaming (and especially SLI/Xfire) I ALWAYS recommend one single twelve volt + rail. Now some may say well if that rail dies then the PSU is dead and that's correct, but gaming at max with 2 cards on a multi-rail system that's lost a rail will most likely fail/crash anyways.

Now, as far as "enough power" I tend to go by AMPERAGE not wattage. All high end video cards have a recommended (amperage) to them, it may be hard to find in the specs but they do. This is more important of a number to go by because you can have two 500 watt power supplies and one have 22amps and one have 30amps. If the card you have requires a 25amp minimum, you're more likely to crash at extended periods of max gaming on the 22amp PSU. And, it's tricky because sometimes a manufacturer will say "minimum 500w" but one has less amperage (less current strength) than the other and may not support the max load, especially in SLI/Xfire. Side note, most "GOOD" power supplies, will hold steady a little past there posted values and have some room to push it, GOOD ONES. Crappy ones are probably over rated and preform less than posted values or not at a constant rate.

A handy (but general) formula for figuring out how much amps are in a unit is Total Number of Watts/12 (Volts for the rail) = Amps... so a 875Watt PSU would have 72 amps... NOW, for a GEFORCE 780 I believe the minimum recommended amount of Amps for that card is 40... now, the second card won't run at exactly the same TDP as the first card but at 72 amps you are REALLY close to the limit. To be safe, I'd give yourself some more head room, if it's not too late, and go with an 80 or 85 amp unit with a single +12v rail and if you want to really be safe go with Silver or Gold rated because they more efficient with more constant values and without as much minor current loss as lower rated PSUs.

If you REALLY want to learn a WHOLE LOT about power supplies go here and read this > http://www.hardwaresecrets.com/article/Anatomy-of-Switc...
it's my favorite site with the best info IMO about power supplies and reviews. EXCELLENT SITE.

Hope it helps.

At least that's my jist of it.

The 40amp is for the whole system so the second card needs a lot less or each card peaks at 328watts http://www.techpowerup.com/reviews/ASUS/GTX_780_Ti_Dire... which equals 27.33 amps wich makes 2 cards consume only 54.66amps under stress test. Here is actually total system power consumption measured with 2 x 780Ti in SLI maxing at 654watts so OP is fine http://anandtech.com/show/7492/the-geforce-gtx-780-ti-r...


I am confused... It does not appear to 40amps for total system according to this statement on the specs for an EVGA GTX 780 on new egg. Here's the quote >

"600 watt or greater power supply with a minimum of 42 amps on the +12 volt rail.
One 8-pin PCI Express power connector or two available 6-pin PCI Express power connectors and one 6-pin PCI Express power connector or two available hard disk power connector."

Here's the link > http://www.newegg.com/Product/Product.aspx?Item=N82E168...

You'll notice it says "42amps on the +12V" also...the entire system is not fed by +12V rail. If I remember correctly the +3v or +5V feeds the cpu and some other components but it's been a while since I researched it can't remember and in a hurry so can't look atm. However, it would appear that Newegg add is stating the card needs a minimum of 42amps so I am not clear about what you mean by 40amps for total system ? Did you mean that with a single card running the entire system will Draw 40amps ? Well if so, doesn't that still mean you would need 40amps (or in the GTX 780 link above 42amps) to run the card since the card can't run without the system ? lol. I do agree with you that the second card in SLI does not require as much amps as the first card. In fact, I did say "now, the second card won't run at exactly the same TDP " in my post.

I think I get what you mean though and I do see by this article http://www.guru3d.com/articles_pages/gtx_780_ti_sli_gef...

Both GTX 780 cards running in SLI caused a TDP for the CARDS THEMSELVES at 508 watts and the Total system draw was 617Watts. I know test vary from review to review but the 508watts is even less amperage required than the 54.6 amps you mentioned below. But good to know and thank you for expanding on my quantitative values.

My main point to the readers is that going by Wattage alone is a more general approach to picking a proper PSU and if you want to dig deeper or you're going to do SLI, taking a closer look at amperage might be worth the consideration; especially, if you're dumping gobs of $$$ on a high end SLI/Xfire system and have multiple PSU candidates to consider, wouldn't you want the most amperage for the $$$ ? I know I do, that's why when I spend money on a PSU I also look at which one is rated better on amperage and I love a single +12V rail personally.

But your info is good to know and thank you for clarify what the manufacture's sales statements and expanding on my general (and apparently incorrect) quantitative values.
Sorry my reply sloppy in a hurry.
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a c 878 ) Power supply
January 3, 2014 2:51:49 PM

RigRebel said:
rolli59 said:
RigRebel said:
When loading PSU with multiple GPU's (on a gaming rig) one of the two most important (and most often missed) "power" factors when picking out a power supply are 1. AMPs and 2. How many Amps on each 12Volt rail , or to be precise, getting the maximum amount of amperage on a (preferred) single volt rail. Multiple rail PSUs, (IMO) are more for servers or for those that wish redundant power across multiple rails incase one or two rails fails. Also, multiple PSUs (IMO) I are cheaper and cheaper to manufacture. Not to mention, that if power across multi rails is not all used it's wasted. For gaming (and especially SLI/Xfire) I ALWAYS recommend one single twelve volt + rail. Now some may say well if that rail dies then the PSU is dead and that's correct, but gaming at max with 2 cards on a multi-rail system that's lost a rail will most likely fail/crash anyways.

Now, as far as "enough power" I tend to go by AMPERAGE not wattage. All high end video cards have a recommended (amperage) to them, it may be hard to find in the specs but they do. This is more important of a number to go by because you can have two 500 watt power supplies and one have 22amps and one have 30amps. If the card you have requires a 25amp minimum, you're more likely to crash at extended periods of max gaming on the 22amp PSU. And, it's tricky because sometimes a manufacturer will say "minimum 500w" but one has less amperage (less current strength) than the other and may not support the max load, especially in SLI/Xfire. Side note, most "GOOD" power supplies, will hold steady a little past there posted values and have some room to push it, GOOD ONES. Crappy ones are probably over rated and preform less than posted values or not at a constant rate.

A handy (but general) formula for figuring out how much amps are in a unit is Total Number of Watts/12 (Volts for the rail) = Amps... so a 875Watt PSU would have 72 amps... NOW, for a GEFORCE 780 I believe the minimum recommended amount of Amps for that card is 40... now, the second card won't run at exactly the same TDP as the first card but at 72 amps you are REALLY close to the limit. To be safe, I'd give yourself some more head room, if it's not too late, and go with an 80 or 85 amp unit with a single +12v rail and if you want to really be safe go with Silver or Gold rated because they more efficient with more constant values and without as much minor current loss as lower rated PSUs.

If you REALLY want to learn a WHOLE LOT about power supplies go here and read this > http://www.hardwaresecrets.com/article/Anatomy-of-Switc...
it's my favorite site with the best info IMO about power supplies and reviews. EXCELLENT SITE.

Hope it helps.

At least that's my jist of it.

The 40amp is for the whole system so the second card needs a lot less or each card peaks at 328watts http://www.techpowerup.com/reviews/ASUS/GTX_780_Ti_Dire... which equals 27.33 amps wich makes 2 cards consume only 54.66amps under stress test. Here is actually total system power consumption measured with 2 x 780Ti in SLI maxing at 654watts so OP is fine http://anandtech.com/show/7492/the-geforce-gtx-780-ti-r...


I am confused... It does not appear to 40amps for total system according to this statement on the specs for an EVGA GTX 780 on new egg. Here's the quote >

"600 watt or greater power supply with a minimum of 42 amps on the +12 volt rail.
One 8-pin PCI Express power connector or two available 6-pin PCI Express power connectors and one 6-pin PCI Express power connector or two available hard disk power connector."

Here's the link > http://www.newegg.com/Product/Product.aspx?Item=N82E168...

You'll notice it says "42amps on the +12V" also...the entire system is not fed by +12V rail. If I remember correctly the +3v or +5V feeds the cpu and some other components but it's been a while since I researched it can't remember and in a hurry so can't look atm. However, it would appear that Newegg add is stating the card needs a minimum of 42amps so I am not clear about what you mean by 40amps for total system ? Did you mean that with a single card running the entire system will Draw 40amps ? Well if so, doesn't that still mean you would need 40amps (or in the GTX 780 link above 42amps) to run the card since the card can't run without the system ? lol. I do agree with you that the second card in SLI does not require as much amps as the first card. In fact, I did say "now, the second card won't run at exactly the same TDP " in my post.

I think I get what you mean though and I do see by this article http://www.guru3d.com/articles_pages/gtx_780_ti_sli_gef...

Both GTX 780 cards running in SLI caused a TDP for the CARDS THEMSELVES at 508 watts and the Total system draw was 617Watts. I know test vary from review to review but the 508watts is even less amperage required than the 54.6 amps you mentioned below. But good to know and thank you for expanding on my quantitative values.

My main point to the readers is that going by Wattage alone is a more general approach to picking a proper PSU and if you want to dig deeper or you're going to do SLI, taking a closer look at amperage might be worth the consideration; especially, if you're dumping gobs of $$$ on a high end SLI/Xfire system and have multiple PSU candidates to consider, wouldn't you want the most amperage for the $$$ ? I know I do, that's why when I spend money on a PSU I also look at which one is rated better on amperage and I love a single +12V rail personally.

But your info is good to know and thank you for clarify what the manufacture's sales statements and expanding on my general (and apparently incorrect) quantitative values.
Sorry my reply sloppy in a hurry.


600watt with 42 amp on the +12 volt actually means for the total system! If you think of it this way the card gets 75 watts through the slot, 75 watts through a 6 pin connector and 150 watts through a 8 pin connector it maxes out at 300watt.
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January 3, 2014 5:14:33 PM

rolli59 said:
RigRebel said:
rolli59 said:
RigRebel said:
When loading PSU with multiple GPU's (on a gaming rig) one of the two most important (and most often missed) "power" factors when picking out a power supply are 1. AMPs and 2. How many Amps on each 12Volt rail , or to be precise, getting the maximum amount of amperage on a (preferred) single volt rail. Multiple rail PSUs, (IMO) are more for servers or for those that wish redundant power across multiple rails incase one or two rails fails. Also, multiple PSUs (IMO) I are cheaper and cheaper to manufacture. Not to mention, that if power across multi rails is not all used it's wasted. For gaming (and especially SLI/Xfire) I ALWAYS recommend one single twelve volt + rail. Now some may say well if that rail dies then the PSU is dead and that's correct, but gaming at max with 2 cards on a multi-rail system that's lost a rail will most likely fail/crash anyways.

Now, as far as "enough power" I tend to go by AMPERAGE not wattage. All high end video cards have a recommended (amperage) to them, it may be hard to find in the specs but they do. This is more important of a number to go by because you can have two 500 watt power supplies and one have 22amps and one have 30amps. If the card you have requires a 25amp minimum, you're more likely to crash at extended periods of max gaming on the 22amp PSU. And, it's tricky because sometimes a manufacturer will say "minimum 500w" but one has less amperage (less current strength) than the other and may not support the max load, especially in SLI/Xfire. Side note, most "GOOD" power supplies, will hold steady a little past there posted values and have some room to push it, GOOD ONES. Crappy ones are probably over rated and preform less than posted values or not at a constant rate.

A handy (but general) formula for figuring out how much amps are in a unit is Total Number of Watts/12 (Volts for the rail) = Amps... so a 875Watt PSU would have 72 amps... NOW, for a GEFORCE 780 I believe the minimum recommended amount of Amps for that card is 40... now, the second card won't run at exactly the same TDP as the first card but at 72 amps you are REALLY close to the limit. To be safe, I'd give yourself some more head room, if it's not too late, and go with an 80 or 85 amp unit with a single +12v rail and if you want to really be safe go with Silver or Gold rated because they more efficient with more constant values and without as much minor current loss as lower rated PSUs.

If you REALLY want to learn a WHOLE LOT about power supplies go here and read this > http://www.hardwaresecrets.com/article/Anatomy-of-Switc...
it's my favorite site with the best info IMO about power supplies and reviews. EXCELLENT SITE.

Hope it helps.

At least that's my jist of it.

The 40amp is for the whole system so the second card needs a lot less or each card peaks at 328watts http://www.techpowerup.com/reviews/ASUS/GTX_780_Ti_Dire... which equals 27.33 amps wich makes 2 cards consume only 54.66amps under stress test. Here is actually total system power consumption measured with 2 x 780Ti in SLI maxing at 654watts so OP is fine http://anandtech.com/show/7492/the-geforce-gtx-780-ti-r...


I am confused... It does not appear to 40amps for total system according to this statement on the specs for an EVGA GTX 780 on new egg. Here's the quote >

"600 watt or greater power supply with a minimum of 42 amps on the +12 volt rail.
One 8-pin PCI Express power connector or two available 6-pin PCI Express power connectors and one 6-pin PCI Express power connector or two available hard disk power connector."

Here's the link > http://www.newegg.com/Product/Product.aspx?Item=N82E168...

You'll notice it says "42amps on the +12V" also...the entire system is not fed by +12V rail. If I remember correctly the +3v or +5V feeds the cpu and some other components but it's been a while since I researched it can't remember and in a hurry so can't look atm. However, it would appear that Newegg add is stating the card needs a minimum of 42amps so I am not clear about what you mean by 40amps for total system ? Did you mean that with a single card running the entire system will Draw 40amps ? Well if so, doesn't that still mean you would need 40amps (or in the GTX 780 link above 42amps) to run the card since the card can't run without the system ? lol. I do agree with you that the second card in SLI does not require as much amps as the first card. In fact, I did say "now, the second card won't run at exactly the same TDP " in my post.

I think I get what you mean though and I do see by this article http://www.guru3d.com/articles_pages/gtx_780_ti_sli_gef...

Both GTX 780 cards running in SLI caused a TDP for the CARDS THEMSELVES at 508 watts and the Total system draw was 617Watts. I know test vary from review to review but the 508watts is even less amperage required than the 54.6 amps you mentioned below. But good to know and thank you for expanding on my quantitative values.

My main point to the readers is that going by Wattage alone is a more general approach to picking a proper PSU and if you want to dig deeper or you're going to do SLI, taking a closer look at amperage might be worth the consideration; especially, if you're dumping gobs of $$$ on a high end SLI/Xfire system and have multiple PSU candidates to consider, wouldn't you want the most amperage for the $$$ ? I know I do, that's why when I spend money on a PSU I also look at which one is rated better on amperage and I love a single +12V rail personally.

But your info is good to know and thank you for clarify what the manufacture's sales statements and expanding on my general (and apparently incorrect) quantitative values.
Sorry my reply sloppy in a hurry.


600watt with 42 amp on the +12 volt actually means for the total system! If you think of it this way the card gets 75 watts through the slot, 75 watts through a 6 pin connector and 150 watts through a 8 pin connector it maxes out at 300watt.


Ok gotcha, so theoretically, based on the guru3D report I showed for 2 GTX 780's in SLI showing a total power draw of 617W = 51.41 Amps my Seasonic X650 gold rated at 54amps (but capable of past 60amps in real world test) should also be able to handle 2 GTX 780's in SLi ?
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a b ) Power supply
January 3, 2014 6:58:10 PM

lostmish said:
Ok gotcha, so theoretically, based on the guru3D report I showed for 2 GTX 780's in SLI showing a total power draw of 617W = 51.41 Amps my Seasonic X650 gold rated at 54amps (but capable of past 60amps in real world test) should also be able to handle 2 GTX 780's in SLi ?


Using guru3d is a very bad idea when trying to figure out power requirements.

You want a test that isolates only the video card you want and then a separate test that isolates only the CPU + base system that you want. Guru3d does not do this.

Why?
* You need to add up YOUR CPU and YOUR GPU separately.
* What if you have a different CPU or other components from theirs? This makes their numbers utterly useless if you are trying to figure out if YOUR system has enough power.
* They may not stress out the CPU to the max (which you need to do for a true max power test)... so how will you know if your system can ever handle max load when both CPU and GPU are maxed out.
* They are testing using AC power, using a different power supply that may have a different efficiency rating from you. (Testing in DC is more accurate.)



Here's what I suggest instead:
* You want to figure out the maximum power draw your system can pull in the worst case scenario and then get a power supply that can handle that worse case scenario.
* Basically, add together benchmarks of the CPU+base system you want together with benchmarks of the video card you want, then add some extra for hard drives, some extra padding so you don't run at max and the result is a pretty safe worst case scenario number.

For CPU: Look for a benchmark that ONLY focuses on the CPU (ignore idle load or 3d tests)
The following are total AC power for CPU + base system (motherboard & RAM):
http://anandtech.com/bench/CPU/52
http://www.tomshardware.com/charts/cpu-charts-2013/-38-...
http://hexus.net/tech/reviews/cpu/38421-intel-core-i5-3...
http://www.xbitlabs.com/articles/cpu/display/core-i7-37...
http://hexus.net/tech/reviews/cpu/38421-intel-core-i5-3...
Personally, I think it would be much better to have DC numbers. Because these are AC numbers, you need to convert them to DC by accounting for power supply efficiency.
Example:
http://www.xbitlabs.com/articles/cpu/display/core-i7-37...
They list the i5 3570K as taking up 109 watts (AC)
If I look at their "Testbed Configuration" I see the system is using a "Tagan TG880-U33II" power supply, which gets like 82% efficiency.
So... 109W (AC) * .82 efficiency = 89W DC power

Anyways, this gives you a good base system for the CPU... now you gotta add the GPU.


For video cards, look for tests that check DC power, NOT total system power. Total system power is useless if you are trying to add CPU and GPU together. You need an accurate video card measure.

Here are examples of good video card tests that show the DC power only for the video card:
http://www.techpowerup.com/reviews/ASUS/GTX_780_Ti_Dire...
http://www.tomshardware.com/reviews/geforce-gtx-760-rev...

Examples of bad video card tests:
anandtech.com
hexus.net
(they use total system power, which is not helpful)
Why it total system power testing bad for video cards?
... reasons mentioned earlier...

So... take the numbers for your CPU (and convert that number to DC using power supply efficiency), then add the numbers for the GPU and you have the total max power draw in DC for your CPU base system & GPU. Add some padding for things like hard drive, and the possibility that benchmarks may not be entirely accurate, and you have a good number to start with.

Example:
Let's say I want to find out:
i5 3570K + GTX 760
CPU: 109W(AC) @ 82 efficiency = 89W(DC) http://www.xbitlabs.com/articles/cpu/display/core-i7-37...
GPU: 168W(DC) http://www.techpowerup.com/reviews/EVGA/GTX_760_SC_ACX_...
89 +168 = ~257W (DC)
or 257/12v = 22amps
So: 257W on the +12v rail or 22amps on the +12v rail
Of course, you want to add some padding to that number to account for:
* hard drives
* in case the benchmark did not stress it to the absolute max
* so you don't run the power supply at max load

This is how I might calculate power requirements by using benchmarks.

Other notes:
* Pay very close attention to which numbers the reviews are telling you. Is it DC or AC? Is it total system power draw or only that one component?
* Make sure you get the max load number and the benchmark that maxes out ALL cores (if it's a CPU); not the idle or average load numbers.
* wattage in DC or AC are two different things. The power supply converts power to DC to be used by the components, but in the process it loses power. So it might draw 100w watts from the wall but only supply 80 watts of DC power for an 80% efficient power supply.
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a b ) Power supply
January 3, 2014 7:17:48 PM

So, taking my own advice, here's what I get for your i7 3960x and two GTX 780's. :) 

195w(AC) @ 85% efficiency > 170W(DC) http://hexus.net/tech/reviews/cpu/32591-intel-core-i7-3...
213w(AC) @ 85% efficiency > 181W(DC) http://www.xbitlabs.com/articles/cpu/display/core-i7-39...
So... let's say 180W(DC) for the CPU + base system.

548VA (DC) (274*2) http://www.techpowerup.com/reviews/EVGA/GTX_780_Ti_SC_A...
574 http://www.tomshardware.com/reviews/geforce-gtx-760-vs-...
Can't get an accurate # on the video card. I only found a single card on techpowerup (so multiplied by two).
The tomshardware numbers seem off, as though they are using AC power, which would make them skewed quite a bit. Anyways, with lack of any other info, let's go with the highest number.

754W(DC) = 180W + 574W

Now, let's assume the worst case, that this is all using the +12V rail (most of it is using +12v).

For your i7 3960x and two GTX 780...

You need more than this to be absolutely safe:
754VA total on the +12v rail or:
63amps of +12v power

So you will need to check the actual rail info on your power supply and not the wattage. This is at the bare minimum (for the worst case scenario). You need to add some more to that number for hard drives, other parts, and the fact that the benchmarks may not have stressed out the CPU and GPU as far as they could.

After all this, you also need to look at how many +12v rails you have, because you need to split them up correctly.

What is the exact model power supply or computer you have?




As a side note: there is an alternative way to calculate the power requirements. The benchmarks are probably closest to reality, but you can also check by adding up the power plugs connected to your components.
The GTX 780 has 1 6-pin PCI-E connector and 1 8-pin PCI-E connector.
This means it could draw the following:
75VA from the PCI-E slot
75VA from the 6-pin
150VA from the 8-pin
Which means a max of 600VA for two cards. Then you add the CPU + base system. Anyways, this is not necessarily reality... more like a max based just on the cables connected. This way of calculating things *might* matter if you wanted to overclock the video cards.

Read about the cables here: http://www.playtool.com/pages/psuconnectors/connectors....
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