Is my PSU enough for 2 x GTX 780 DirectCU II OC cards in SLI?

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skplion

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Hello, I have a fairly new system (about 2 months old) and am intending on doing SLI. I am wondering if my PSU will be enough to power the system or if i should invest in something more powerful.

My system:
CPU: Intel Core i7 - 4770K 3.5 GHz Quad-Core Processor (OCed at 3.9 GHz)
CPU Cooler: SCYTHE SCSMZ-2100 SAMURAI ZZ REV.B CPU COOLER
Motherboard: ASUS MAXIMUS VI EXTREME LGA1150 Motherboard
Memory: G.SKILL F3-1600C7Q-32GTX 32GB (4X8GB) DDR3 PC3-12800 1600MHZ TRIDENTX
Storage: KINGSTON SKC300S37A/120G SSDNOW KC300 120GB 2.5'' SSD SATA3
Storage: SEAGATE BARRACUDA 7200.14 ST3000DM001 3TB SATA3
Video Card: ASUS GTX780-DC2OC-3GD5 3GB DDR5 PCI-E
Video Card: ASUS GTX780-DC2OC-3GD5 3GB DDR5 PCI-E (Not yet in SLI)
Case: THERMALTAKE VN10001W2N LEVEL 10GT
Monitor: SAMSUNG S24C300HS 23.6'' LED MONITOR FULL HD
Keyboard: LOGITECH G19 GAMING KEYBOARD
Mouse: LOGITECH G700 RECHARGEABLE GAMING MOUSE
Power Supply: OCZ OCZ750FTY-UN FATAL1TY SERIES POWER SUPPLY 750W

I just got my hands on the second GTX 780 (not installed yet), and from what i gather around the internet my PSU should be enough for SLI but it seems like a close call. So i would appreciate if someone could give me an informed answer before i start with any experimentations.

Many thanks in advance!
 
Solution
photonboy
YES, it is.
I do agree on possibly getting a better Power Supply though for two reasons:
a) Overclocking graphics may put you over, and
b) NOISE would be higher from the power supply. Most are quietest below 50% load then start ramping up the fan. You'd be roughly 80% load.

More info:
- Both cards use roughly 440Watts max http://www.guru3d.com/articles_pages/geforce_gtx_780_sli_review,4.html
(wall power, not card power which is 80% of that if 80% efficiency of PSU)

- add roughly 140 Watts for rest of system (not sure of exact amount) so this is 570Watts

- Your power supply can draw 720Watts wall power at 80% load (make sure you are comparing wall power for proper results)...
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Dark Lord of Tech

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skplion

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Thanks for the fast answer SR-71 Blackbird! So i take it you suggest changing my PSU. There is no chance that my current one will hold up?

Also, i didn't mention that at the moment my single gpu is overclocked (via GPUTweak) @ 1050 Mhz core clock and about 7100 MHz memory clock (don't remember exactly and i'm not at home right now). I am willing to return it to stock clocks after i do the SLI. Is two of those cards in SLI at stock clocks a noticeable improvement over a single one at those clocks?

Again, thanks for the feedback!
 
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photonboy

photonboy

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YES, it is.
I do agree on possibly getting a better Power Supply though for two reasons:
a) Overclocking graphics may put you over, and
b) NOISE would be higher from the power supply. Most are quietest below 50% load then start ramping up the fan. You'd be roughly 80% load.

More info:
- Both cards use roughly 440Watts max http://www.guru3d.com/articles_pages/geforce_gtx_780_sli_review,4.html
(wall power, not card power which is 80% of that if 80% efficiency of PSU)

- add roughly 140 Watts for rest of system (not sure of exact amount) so this is 570Watts

- Your power supply can draw 720Watts wall power at 80% load (make sure you are comparing wall power for proper results) http://www.hardwaresecrets.com/article/OCZ-Fatal1ty-750-W-Power-Supply-Review/1019/7

So you need at most about 630Watts from the wall, and can draw 720Watts at 80% load.

OVERCLOCK INFO:
It never scales linearly, thus a 20% overclock might use 30% more power. I don't know your exact specs or max overclock but if you multiply 440W x1.3 that comes to 616Watts (wall power) which would either put you over or be too close for comfort.
 
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Solution
photonboy

photonboy

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http://www.pcpartpicker.com/part/evga-power-supply-120g21000xr (EVGA 1000W, full modular)

I haven't researched this carefully, but it's got good customer feedback and is $150 after MIR.

Other:
You might wish to research G-Sync and maybe get the upcoming Asus ROG Swift which is a 27", 2560x1440 120Hz gaming monitor. https://www.asus.com/us/News/xXtX0FNhXQWPrry7

IMO you'd have a far better gaming experience with this monitor and a single GTX780 compared to GTX780 SLI and a non-GSync, lower res monitor.

Very few people understand G-Sync yet: http://www.hardwarecanucks.com/forum/hardware-canucks-reviews/64586-week-nvidias-g-sync-monitor.html

You get a really smooth gaming experience at lower resolutions than with a normal monitor so don't require as much graphics hardware (thus don't really need the 2nd card). It's also far easier to tweak games as there's no VSYNC cap you need to worry about. Thus, just tweak to about 50FPS for really demanding games or just put game quality to MAXIMUM if it can go higher.

LIGHTBOOST can be confusing. Basically you should only use it if you can maintain above 100FPS. There is an FAQ for this feature.

"If there is one issue with G-SYNC it is that you have to see it to believe what it can accomplish. Describing the changes it made to how I approached gaming is impossible. Right now, you may think that your current 60Hz or 120Hz panel provides a great experience. I know I did. My tune changed from the moment I started playing Battlefield 4 multiplayer with G-SYNC enabled. "

*Resolution TIP:
Whether using G-Sync or not, if you have a 2560x1440 monitor then my general rule of thumb is only game at this resolution if you can maintain your target refresh with MAX settings such as 60FPS (if using VSync or Adaptive VSync). Otherwise, 1920x1080 is probably better. Many games like Crysis 3 aren't much better at 2560x1440 and the graphical power would be better put towards different settings instead of simply raising the resolution (which mainly only affects the HUD/Text. In some cases it MAY help anti-aliasing but often 1920x1080 with a higher AA can do the same thing at a higher frame rate).

2560x1440 is really awesome however for games like Starcraft 2 and CIV5. Mainly top-down, "GOD" games. Deus Ex HE can run at 2560x1440 easily with a GTX780 and the slight STUTTER that I get even at other resolutions at 60FPS would likely disappear with G-Sync.
 
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skplion

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Thanks for the answer photonboy!

So you are saying that the PSU might be enough but it would be nearing its limits. If i'm not mistaken, hardware components don't always draw maximum wattage from the PSU. So the two cards should draw 440 Watts under full load. Otherwise, the power consumption should be lower and the PSU should hold up. Is that correct?

Also, about the efficiency, if i understand it correctly 80+ bronze means that near 100% load it should have about 82% efficiency (81.6% if i understand correctly the article you posted). So if it draws approximately 750W from the wall, it outputs about 82% * 750 = 615W. The two cards draw 440W (according to your link, elsewhere i saw about 500W), the CPU has 84W TDP, and that leaves 91W for the rest of the system (31 if we say that the cards draw 500W). All the above calculations without considering the overclock. Are the above calculations correct?

The above leads me to believe that the current PSU should be enough for the system to work under minimal to medium load, but if i stress it the PSU will fail. If it is so, i am thinking of installing the second card tomorrow to test how it behaves as it is, and buy a new PSU later on. I would buy a new PSU anyway, i was just thinking if my current one is enough to buy a new one later on. Coincidentally, the one SR-71 Blackbird recommended was the one i would most definitely buy.
 
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photonboy

photonboy

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You are mostly right, however you've forgotten about the power used by the following components:
- Motherboard
- 32GB DDR3 memory
- Drives

My calculations still conclude the Power Supply should be sufficient if not overclocking the graphics cards. I'm not sure how loud the PSU fan would get though at about 80% load.

I still think the high-res G-Sync monitor would be the better way to go. G-Sync aside, several games look FAR better at 2560x1440 instead of 1920x1080 and the extra 3" does make a big difference as well.

The ultimate gaming experience has several factors, but the biggest one is SMOOTH gameplay, with minimal stutter and lag. That's what G-Sync fixes, and again can also be smoother at a lower frame rate.

More links for G-Sync:
Linus (Tech tips): https://www.youtube.com/watch?v=3PJjhBUSuHk

NVidia (click "Product Demo Video"): http://www.geforce.com/hardware/technology/g-sync
 
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skplion

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Thank you both for the insight SR-71 Blackbird and photonboy. The answers were spot on. I tried installing the second card with the current PSU, and the system ran without any problems but the PSU fan did indeed get very noisey. So noisey in fact, that i wouldn't give it much lifetime if it kept working under these conditions. From fear of frying anything, i didn't try to stress it as it would most likely fail but instead i uninstalled the second card untill i can get the PSU that SR-71 Blackbird mentioned or the 1000W one.

For anyone who reads this thread wondering if their PSU is enough for GTX 780 SLI, even though mine worked with a 750w i would also suggest getting at least a 850w PSU.
 
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