Learning about PSUs

[xFlankerx]

So my eyes are just being opened to the fact that you don't need 700W PSUs to power most high-end systems. The thing that basically triggered this enlightenment was this article on DailyTech about the 8800GTX, and the total system power consumption shown in a small chart near the bottom. As you can see, the Kill-A-Watt Power Meter shows that the PSU is pulling 321W from the wall socket. At 85% efficiency, it would mean that the system was actually being fed 273W.

So I started to wonder, what kind of a power supply would actually be needed to power this 8800GTX system? My original belief was that you would need a (273W/.7) = 390W PSU @ 70% efficiency to supply that. But then someone came along and told me that the power supply actually supplies just as much power as it's rated to, and it pulls extra from the wall socket, which is then lost as heat. A scenario came up;

Assume that we are testing a system that has 340W being supplied to the system. This person told me that the power supply that would be needed for this system, would be one that is called a 340W Power Supply. The actual draw from the wall would be 485W at 70% efficiency, and 400W at 85% efficiency. But in both these cases, it would still need to be rated a 340W PSU.

So if the 8800 system draws 321W at the wall, at 85% efficiency it's only sending 272.85W to the components. Person says that only a 273W PSU would be needed to power this system.

So, accounting for inconsistency and other bad things that happen to PSUs, is it true that you could run a 8800GTX system on a quality 350W PSU with lots of Amps on the +12v rail(s)?

 

[Kurz]

Problem with that theory is that PSUs hate being used at MAX load all the time. Personally I would get a ~550W to use on the 8800GTX, to have some headroom for decaying power output as the PSU gets older. Its a good rule of thumb to try getting about 50%-100% more potential watts over the actual Watt use.

Amps is an important factor as well.
Waits for a more knowledgable Poster.

 

[billb]

So my eyes are just being opened to the fact that you don't need 700W PSUs to power most high-end systems.

Lets do an automobile analogy:
Driver A and driver B both drive the same trip every day on the same highway at the same speed. (i.e. same MOBO, CPU, Video, and the same games/programs)
Driver A has a 100hp motor.
Driver B has a 300hp motor.
Driver A does the trip at 100% throttle.
Driver B does the trip at 33% throttle
Which motor will last the longest?

Using this (http://www.extreme.outervision.com/) to calculate that you need , say, 645 watts and then buying a 650 watt PSU is sheer folly.

As for a reccomendation, many have found the PCPower&Colling PSUs to be excellent.
Others will say 1kw (or 850 watts for that matter) is excessive, but those with the high power PSUs don't post about boot issues while those with the $124, 650 watt PSUs wonder why they have problems!

 

[funnyperson1]

I do think that many people here go way overboard on the watts, but billb has a nice analogy, running a system with an 8800GTX and other high end components is like redlining your car all the way to work everyday. Yes a high quality PSU of that wattage can sustain the demand, but over time it might not be able to take it. Also the choice of high quality PSUs in the 300+ watt range isn't too hot, off the top of my head, the only real winners there are the Seasonics and Enhances.

I do like a safety buffer, but I think it should be more like 50-150 watts. For example, for the stated 8800GTX machine, I think a 450 or 500W psu would be more than adequate, as long as you get a good one. However some people would be quick to tell you to get a 1kW PCP&C psu for that same system.

On another note, PCP&C rebrands great PSUs, but again, they rebranded them. Why pay an extra $100+ for a PCP&C psu when you can get the same Etasis build from Silverstone?

edit: actually I was mistaken, PCP&C are made by Seasonic and Win-Tact among others.

 

[xFlankerx]

Problem with that theory is that PSUs hate being used at MAX load all the time. Personally I would get a ~550W to use on the 8800GTX, to have some headroom for decaying power output as the PSU gets older. Its a good rule of thumb to try getting about 50%-100% more potential watts over the actual Watt use.

Amps is an important factor as well.
Waits for a more knowledgable Poster.

Yeah, but it won't be running 100%. As you can see, the 273W being fed to the system is at load, so the maximum power draw at startup shouldn't really be that much higher. And even that load won't be 100% of the time, how often do you have to have an 8800GTX running a game at full blast? So that power is actually around 78% of the maximum load of a 350W PSU.

But going even further than that, wouldn't the Forton 400W PSU with a total of 34A on the +12v rails be enough? As far as I know, it doesn't get much better than Forton (FSP) for quality PSUs. That would have the PSU running at 68% at that kind of load. Or in other words, an overhead of almost 50%.

550W PSU on a system that actually consumes 273W? 100% overhead? I'm pretty sure that no system is ever going to consume twice what it consumes at that kind of load, not even at startup.

Using this (http://www.extreme.outervision.com/) to calculate that you need , say, 645 watts and then buying a 650 watt PSU is sheer folly.

From the way you phrased that, it would seem that the calculator tells you how much power your system is going to consume, not the Power Supply rating that you should get. But in a system with these specs;

* Intel Core 2 Extreme QX6700
* NVIDIA nForce 650i SLI based motherboard
* 2x1GB PC2-6400
* NVIDIA GeForce 8800GTX
* Western Digital Raptor 150

...it only consumed a total of 273W at load. I highly doubt any system is actually going to consume ~650W.

****************

PLEASE correct me if I'm wrong, but I'm only going by facts that I see. I won't pretend to know everything about PSUs, I'm a relative noob, but I DO think that people grossly over-exaggerate the Power Supply that they need for their system, simply because of the large numbers that PSU companies are pumping out.

 

[Kurz]

550W to 500W is a safe number however if you want to spend less on the powersupply then go right ahead. I am just suggesting a higher powersupply since I tend to leave my computer on to fold, so having a PSU that is hovering around 50% actual draw is actually pretty good in my eyes. Since folding is pretty stressful on power and the CPU its good to have a wide safety zone.

Unlike some people I cant stand not having my computer working if the powersupply dies.

Its not like you are wasting energy by buying a higher Wattage PSU.
As long as you get an effeicient one.