do i need 850watts for my system or 1000watts?

750w is plenty. Anything more is pointless.

The Gigabyte GTX 980 G1 Gaming 4 GB has a maximum power consumption of 342 Watts when running Furmark Stability Testing.

http://www.techpowerup.com/reviews/Gigabyte/GeForce_GTX_980_G1_Gaming/25.html

Lotsa ways to look at this:

1. According to nVidia

http://www.tomshardware.com/answers/id-2311121/power-supply-requirements-nvidia-gpus.html#14243229

98 C = Maximum GPU Tempurature (in C)
165 W = Graphics Card Power (W)
500 W = Minimum System Power Requirement (W)
2x 6-pins = Supplementary Power Connector

Click to expand...

SLI Power Requirement per nvidia = 500 + 1 x (165) = 665 watts for 2 cards

2. Using this:

http://extreme.outervision.com/PSUEngine

I get a recommended size of 559 watts which includes the CPU overclock but not the GFX cards OC which it assigned ONLY 136 watts per card ....

Guru3D tested this "at the wall" and found that the card draws 198 watts before overclocking :

http://www.guru3d.com/articles_pages/msi_geforce_gtx_980_gaming_oc_review,7.html

So cased upon "at the wall" testing, we are 62 watts short per card. Adding a 14.6 % OC per

http://www.techpowerup.com/reviews/MSI/GTX_980_Gaming/28.html% OC,

Puts us up at a 227 watt max "at the wall" .... with a 90% efficient PSU, that's 204 watts PSU output ... 68 watts less than the calculator.... so that needs fixing

559 + 68 + 68 = 695 watts but that is without any capacitor aging which could hit 20% after the 1st year .... with a quality PSU, I use 15%.... which brings us up to 800 watts.


3. Guru3D testing

Here is Guru3D's power supply recommendation:

GeForce GTX 970 or 980 - On your average system the card requires you to have a 500 Watt power supply unit.
GeForce GTX 970 or 980 in 2-way SLI - On your average system the cards require you to have an 800 Watt power supply unit as minimum.

If you are going to overclock your GPU or processor, then we do recommend you purchase something with some more stamina.

Click to expand...

Note that Guru3D uses a LGA 2011 CPU in their tests which adds 100 watts....so they say 700 watts w/ haswell ... more if overclocking

4. From my own "at the wall" testing .....

MoBo (40 watts)
CPU (130 watts)
RAM (3-5 watts per stick)
HD (10 watts each)
SSD (2-3 watts)
Fans (2 watts each)
Water Pump ( 8 - 24 watts)
Cards (165 watts + 20 watt peaks + 12% OC = 205)
USB Devices (2-3 watts each) x 5 (Mouse, KB, Headset, Phone\+ 1 Optional)


5. Actual testing in a real build.

My water cooled system with twin 780s pulls 785 watts from the wall running Furmark. CPU load = 40 watts during the test. Again with 90% efficiency, that's a PSU output of about 713 watts. The 980 draws about 30 watts less at the wall (27 PSU output) so subtract 54 watts ... so if I swapped for 970s, I shud be 659 watts ... so a 750 would cut it, that is before capacitor aging.....If I use 15% that puts me about 757 watts w/ capacitor aging.... but again, with a 40 watt load on my CPU. So based upon my own power meter readings and the 30 watt per card difference.... I like the 850. But I have a 26% OC on my cards under water and a 4770k at 4.6 Ghz (1.325 volts). If you don't go so high, you will need less.

So, I generally use 750 watters for twin 970's and 850 watters for twin 980's. Three reasons:

1. Overclocking stability depends on stable voltages and if you look at the test results, the larger voltage variations occur as you get close to the rated voltage.

2. Capacitor aging.... even the best caps age over time....in that 3rd year, your power output is significantly less; can be 20% or more after 1st year if puter is on 24/7

3. The closer you get to the rated voltage, the louder the PSU fan is.

750w is enough given the power efficiency of the GTX 980, but like these guys said it's probably better to get a little bit extra. You can definitely get by, but still I would suggest that you go for 850w to give yourself some room for upgrades in the future. It is so much better to spend a couple of extra bucks in the start than have to worry about getting a new PSU when you're thinking of running 980 3-way SLI or whatever. Trust me, it's wise to spend extra in the early stages of your build, so you don't have to upgrade again in the future. I am speaking from personal experience.