Calculate required wattage and headroom

Colonel_Panic

Limp Gawd
Joined
Oct 5, 2009
Messages
328
Recently while doing various upgrades to my workstation and a friend's triple SLI GTX580 rig, I've had to learn a lot about power supplies. I had a few questions, though, that I think other people might have and want to see if we can pull some answers together.

I have an OCZ Series Z 850W PSU in my workstation, powering 2x GTX470 and an i7 950 (normally overclocked). According to my wall power meter, I was drawing a peak of 711W playing some games (BFBC2 seems to push it the hardest) and occasionally the system shuts down. No BSOD, just black screen. Is this indicative of insufficient power? The same thing happened to my friend's system after we installed the 3 GTX580 in SLI (he went from a 1kW to 1.2kW unit). I brought my CPU back to stock speeds, and there hasn't been a reboot yet (wall meter says ~600W now).

For a bit of math, this 711W represents ~83% of the max sustained power load of my PSU. Using a high percentage of available power output will likely reduce the life of the supply, but would this usage be "too much" on a daily basis?

Finally, what method do you folks use to estimate required power usage? I tried a number of online calculators, and they all fluctuated when I entered my info (One had 650W, Newegg suggested 860W).

TL;DR
  • What typically happens when a computer draws too much power from the PSU?
  • What percentage of the rated power output of a PSU is safe to use on a regular basis?
  • How can PSU requirements be safely estimated?
 

Profumo

Gawd
Joined
Jul 10, 2011
Messages
681
Online PSU calculators are complete and utter shit.. if you want some proper data check this out..

No offence, but you fail to understand some basic stuff.. first of all, PSUs are rated at output not input, and as so your 711W peak load does not translate in to an actual system draw of 711W because your PSU is not 100% efficient(no PSU is), your actual system draw is about 630W at worst which is well inside the power envelope of this PSU.
Also, that Using a high percentage of available power output will likely reduce the life of the supply is only applicable for low quality units using low quality caps, which is not the case here.
There are two probable scenarios here with the first one being, you're not achieving a stable overclock so you need to increase the voltage more to achieve stable operation at the desired clocks.. and the second one(far less likely) your PSU has a regulation problem and it's not providing proper voltages to the cpu/mobo/gpu and the system is forced to shutdown to protect itself, very slim chance of this being the case though.. either way, is not an issue of undersized PSU, on the contrary, i've seen GTX 470 SLI setups run on 650W units without any issue whatsoever..
 

Colonel_Panic

Limp Gawd
Joined
Oct 5, 2009
Messages
328
Thanks, Profumo, that's all information I was hoping to get out of this thread. I'm going to take a look at my overclock again to find its weak point, maybe stress test it longer to verify stability.
 

BostonHXC

Limp Gawd
Joined
Oct 3, 2008
Messages
283
Sorry about that I misread your post and after posting realized that you had already looked at online calculators.
 

pchow05

n00b
Joined
Aug 6, 2011
Messages
0
your gpu/cpu will start to underperform. gpu will start artifacting

most online calculators are quite accurate to safely estimate how much watts you need.
 
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