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don_svetlio

Graphics card TDP and Power Consumption Explained

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Posted · Original PosterOP

So, seeing as there seems to be a bit of misconception about what TDP and Power Draw are exactly and how they correlate so I decided to try and explain it as best I can.
For testing purposes and in order to avoid brand bias - I will be using the GTX 760 and GTX 960 as my main sources for the numbers and measurements. Here goes nothing.

 


TDP:

The thermal design power (TDP), sometimes called thermal design point, is the maximum amount of heat  generated by the Central/Graphics Processing Unit that the cooler attached to it has to dissipate under normal operation. TDP does not account for power viruses (furmark, kombustor, prime95) or any other sort of torture test and can easily exceed a product's rated TDP if one of those is applied.

Since safety margins and the definition of what constitutes a real application vary among brands, TDP values between different manufacturers cannot be accurately compared. For example, while a graphics card with a TDP of 200 W will almost certainly use more power at full load than a GPU with a 190 W TDP from the same manufacturer, it may or may not use more power than a GPU from a different manufacturer that has a 190 W TDP.

The dynamic power consumed by a switching circuit is approximately proportional to the square of the voltage:

P=CV2f

where C is capacitance, f is frequency, and V is voltage.

Now, boring math aside. Let me get more into the part of how exactly heat is generated and what affects the TDP of each graphics card, to some extent.

As we all know - each GPU core has an absurd amount of transistors we hear AMD or Nvidia brag about.
Nvidia:"Titan X has 8 billion transistors!"
AMD:"Oh yeah? Fury X has 8.9 billion transistors!"
                   ...and this goes on...

So what's the significance of said transistors for TDP? Why am I bringing it up? Well - as we all know - by lowering the manufacturing process we usually have an increase in transistor count to go with it. Going to a smaller die size usually means reduced power as it allows the voltage across the transistors to be reduced. That also increases speed as the channel is now shorter so you can have faster switching transistors or more complex architectures running at the same speed. And as we all know - the lower the voltage applied to transistors - the less heat they generate. So say - if GPU A (which is 40nm) has 1B transistors @ 1.5V it would output more heat than GPU B (which is 28nm) with 1B transistors running @ 1.25V. Now. Although process shrinkage and transistor count cancel each other out to some extent - because GPUs have been stuck on the same 28 nm process for the last 3-4 years - we have only seen an increase in transistor count and thus an increase in temps as more transistors = more heat generation = higher TDPs. So, why is the 960 suddenly so much cooler than the 760? Well. It has less transistors. The 760 (3.5B transistors) has a TDP of 170W. The 960 (2.94B transistors) has a TDP of 120W. Yet, the 960 outperforms the 760 consistently and by a decent bit. Well - that's architecture for you. Optimization goes a long way. Realistically, the TDP in Maxwell wasn't achieved magically but by reducing the amount of heat-generating components of the GPU core. Now - it's nowhere near as simple as transistors being the only important thing - there are other factors as well.

Now that we've established what exactly produces the heat and how process shrinkage affects it - let's move onto the most frustrating part - power draw.

 


Power Consumption

Now - remember the TDPs of the cards we're looking at? 120W for the 960 and 170W for the 760. Well - according to bit-tech and anandtech - those TDPs have little to do with how much power each card actually pulls.

Bit-tech used Unigine valley to determine GPU power-draw as they deem it to be mostly GPU-centered. Using a watt meter - they determine the 960 system using about 240W of power and the 760 system about 300W.

Anandtech use "Battlefield 3" as it's an actual game and got a reading of the 760 system using 335W of power from the wall. I couldn't find them doing a similar test for the 960 but taking the offset of 30W from the other test - It'd be safe to assume a 960 test bench would use about 275W of power for the whole system.

Factoring out about 100W in both cases of the game benchmark for the CPU, HDD and RAM and we have these estimates. The 960 uses about 175W of power and the 760 uses about 235W of power.
Going back to the TDP of the card - 120W(draws 175W) for the 960 and 170W(draws 235W) for the 760. Suddenly - we see how TDP does not accurately reflect how much power GPU A or GPU B would draw. In most cases - power draw is "guessed" based on TDP and sometime that's somewhat accurate
but usually it's totally wrong.

Now - I'm sure everyone and their grandma can throw an infinite amount of benchmarks showing GPU A, B or C drawing less power than my speakers but you need to remember that in order for these mid or high end products to actually deliver their rated performance, they also need the power. The thing is - Nvidia have managed to reduce the amount of heat given off by individual transistors better than AMD has and thus resulting in lower TDP - that causes people to immediately assume that you need a 1000W PSU to run a 380 and that you can run the 960 off a few potatoes and some wires. In almost all cases - that's totally wrong.

In order to accurately assess how much power a component is drawing, you need highly specialized testing equipment - such equipment is used by sites like Bit-tech or Anandtech in their testing and those test are almost always more accurate then a wild guess based on how much heat a certain component puts out. Example - the 7950 Boost has a TDP of 200W, yet draws exactly the same amount as a 760 - a card with a 170W TDP. Another example would be the 380 - it has a TDP of 190W - 70W more than the 960. In Legitreviews test, they used a P3 Kill-A-Watt power meter to determine that the 380 uses about 180W of power - almost identical power draw to the 960 but the TDP is vastly different. The same argument could be made about the 970 and 390 and so on.

All in all - I simply want to advise everyone to look for reputable sources with the appropriate equipment when researching power and thermals - do not simple decide that the card with the lower TDP will automatically draw less power. Always review the model :)
Small example
NVIDIA-GTX-960-System-Power-Consumption.

Sources:
http://www.legitreviews.com/sapphire-nitro-r9-390-8gb-nitro-r9-380-4gb-video-card-review_166123/11
http://www.bit-tech.net/hardware/graphics/2015/01/22/nvidia-geforce-gtx-960-review-feat-asus/10
https://www.techpowerup.com/reviews/Gigabyte/GTX_960_OC/25.html
http://www.anandtech.com/show/7103/nvidia-geforce-gtx-760-review/16

Tags to @Aniallation @STRMfrmXMN @EllieThePurpleFuzzy @quan289 @themaniac

Feedback, suggestions and corrections are appreciated. Thanks for your time :)


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Posted · Original PosterOP

Linus made an as fast as possible for this shit, but thanks.

That one didn't really explain anything - not for me at least.


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Guides are really fun to read on mobile.


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Posted · Original PosterOP

Lol "asses" - that little write up of yours is great, I'd always wondered what TDP actually referred to

Whooops - fixed :D


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@LinusTech @Slick @nicklmg or mods this needs to be pinned


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Posted · Original PosterOP

@LinusTech @Slick @nicklmg or mods this needs to be pinned

Hopefully it does :D

I also have to add a list of the relative power consumption for all current cards. Feel like that would be a neat addition.


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Thank you for this article it was very helpful.


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Posted · Original PosterOP

Thank you for this article it was very helpful.

Happy it helped you ^_^


Archangel (Desktop) CPU: i5 4590 GPU:Asus R9 280  3GB RAM:HyperX Beast 2x4GBPSU:SeaSonic S12G 750W Mobo:GA-H97m-HD3 Case:CM Silencio 650 Storage:1 TB WD Red
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Hopefully it does :D

I also have to add a list of the relative power consumption for all current cards. Feel like that would be a neat addition.

yes that is needed


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I read that they had the 390x pulling 200 MORE watts from the wall than the 980 was, what does that mean?

 

Soooo how many potatoes do I need to run a R9 390?

about the same amount of transistors it has, because 1 potato equals one transistor


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                         The Austrailian king of LTT said that I'm awesome and a funny guy. the greatest psu list known to man DDR3 ram guide

                                                                                                               i got 477 posts in my first 30 days on LinusTechTips.com

 

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pin it! pin it now! just pin it! pin it!


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pin it! pin it now! just pin it! pin it!

agreed @LinusTech @Slick @nicklmg @Mods_o_joy


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Posted · Original PosterOP

And another note is how these two different chips have such a huge difference in TDP and Power Consumption yet their temperature margins are so close...

http://www.techpowerup.com/reviews/Powercolor/R9_390_PCS_Plus/34.html

http://www.techpowerup.com/reviews/EVGA/GTX_970_SC_ACX_Cooler/29.html

The power consumption is a lot closer than TDP but I do attribute that to the beefier cooler on the 390 usually. But still - it goes back to the point that every manufacturer rates TDP they way they feel it should be, not based on a unified source or reference.


Archangel (Desktop) CPU: i5 4590 GPU:Asus R9 280  3GB RAM:HyperX Beast 2x4GBPSU:SeaSonic S12G 750W Mobo:GA-H97m-HD3 Case:CM Silencio 650 Storage:1 TB WD Red
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Windows 10 is now MSX! - http://linustechtips.com/main/topic/440190-can-we-start-calling-windows-10/page-6

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@don_svetilo

While some of your statements are true my experiences with nivida GPUs are different. Sadly I have no AMD card to test.

If you don't OC, the long term power draw must not exceed the rated TDP. Due to GPU boost the card is pinned to 80-100% TDP all the time under load even if the pipeline is only about 50% filled.

As far as I know it's the same with AMD cards. So TDP actually equals to power draw under load due to the GPU is throtteling itself to maintain the TDP rating.

Sure when the load is not that high and the GPU don't have to burn all powerallowd it's different and I can't say how the difference on the power bill is. But I don't care about the power bill, I care about the maximal long term power draw under full load. This will define the size of the cooler I need.

To the mods: please don't lock for necro.


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