Amps or Watts. Which is more important?

[devjt2]

I'm looking at putting a 9800 GTX+ in my system and it is calling for at a minimum a 450w PSU with 24 Amps on the 12V rail.

I've got a 450 watt power supply. Check. It is a Corsair VX series 450w. I'm a little uneasy as this is the bare minimum that the card is calling for.

However, this PSU has 33 amps on the 12V rail. So there's some headroom there.

Think it'll be okay?

The rest of my system:
i7 930
Asus P6T
12 GB DDR3
2x WD 640GB HDD
Generic DVDRW
3 Case fans
No overclocking

 

[MrK6]

The 450W rating is really a safety net to cover cheaper/generic PSUs which generally don't have as robust 12V rails as does that Corsair unit. You'll be perfectly fine using that PSU for your system.

 

[MagickMan]

Corsair 450 is like an "average" 550-600W. You're fine.

 

[Stefan Payne]

Well, Watt = Voltage * Amperage.

So in the end, the power on the +12V rails is the important value...

 

[yh125d]

9800GTX+ doesn't really require as much power as the requirements say. You're fine

 

[Modelworks]

Also when reading capacity on a power supply it is best to take 10% off for a safety margin. If it says 12VDC @33A , figure 33x10% 3.3A . So about 30A capacity.
Usually the max rating isn't continuous.

 

[RaiderJ]

Also when reading capacity on a power supply it is best to take 10% off for a safety margin. If it says 12VDC @33A , figure 33x10% 3.3A . So about 30A capacity.
Usually the max rating isn't continuous.

I think in the case of Corsair PSU's, the rating is for a continuous output (which is why some say they are underrated)

 

[Meghan54]

Also when reading capacity on a power supply it is best to take 10% off for a safety margin. If it says 12VDC @33A , figure 33x10% 3.3A . So about 30A capacity.
Usually the max rating isn't continuous.

I'd tend to disagree. A competent power supply from a reputable brand should have no problem running at 100% of capacity. Not that I'm advocating doing such because doing just that will shorten the power supply's life, give higher heat production, louder fan noise, and probably be at its worst level of ripple/noise generation and worst voltage regulation.

But, it should do that for a loooong time. In fact, good power supplies are in reality underrated as to their ultimate output. This is demonstrated through testing at Hardware Secrets over and over.

Take the Corsair HX620. It managed to put out a tad over 750W and still remained within specs. Or the Antec True Power 750 that managed to put out just over 900W, and again managing to stay within ATX electrical specs. And the list is quite long....

As for peak output vs. continuous output, the fact of the matter is that the reputable brands rate their power supplies as continuous, not peak. It's the bottom feeder brands, like HEC or Huntkey or Deer or the like, that attempt to fool the consumer with peak ratings, such as the HEC 1080W power supply which is in reality an 800W continuous and 1080W peak.

But brands like Seasonic, Antec, Corsair, Enermax, Silverstone, and other reputable brands do not rate at peak, but continuous.

 

[SparkyJJO]

Good units - like corsairs - list their outputs as their continuous outputs. They can actually peak higher than that.

Generic PSUs typically label their max outputs, their continuous being much lower than the label.

That's why the gfx card makers have the requirement of "450W" or whatever to put a good safety margin on those cheaper power supplies. They are taking into account the max vs continuous amount, and then adding a safety margin on top of that just to be sure that the $30 special won't totally blow up under the load. At least not right away.

 

[Modelworks]

I'd tend to disagree. A competent power supply from a reputable brand should have no problem running at 100% of capacity. Not that I'm advocating doing such because doing just that will shorten the power supply's life, give higher heat production, louder fan noise, and probably be at its worst level of ripple/noise generation and worst voltage regulation.

I have yet to see any power supply in computers or anything else designed for continuous output at maximum rating, it is sort of a conflict of terms. The reason is that when you design a power supply you have to allow for peaks in usage as no circuit consumes exactly the same amount of power from the time it is switch on to the time it is turned off. If your device uses 20A then you have to add what the max peak is to that. If you were to use a supply that says 20A @ 12VDC max and you switch on a 20A load it will fail .The reason is inrush current.

Good video here:
http://www.youtube.com/watch?v=c5FKurAdh-Q


Most things like video cards in a pc now use DC-DC converters. They take the 12VDC @ 20A and convert that to 3.3V @ 72A . It is a lot of power to convert and depending on how the circuit handles the ramping up of the current draw it will exceed the continuous usage rate . Even if the circuit is already connected the second you were to play a game and the card now needs more power the DC-DC converter is going to instantly demand more current, current that it will take time for the inductors to pass and level out. Until that happens there will be excess current usage.

The problem really is in the labeling. Unless the manufacturer states 20A continuous you can't be sure what the continuous rating is and have to assume that it is less than the labeling.

Corsair is probably one of the few that does say it provides continuous but there will still be peaks and they don't list what the limits for those are.
I would like to see someone really test some of the higher end PSU. Let them run for 6 hours at the rated power to check the failure rate. Also throw in some normal things that happen to a psu , like suddenly increasing and decreasing the load several times an hour.

 

[TemjinGold]

Not really a conflict per se. What these fellas are saying is that the good brands don't put their peak value on the label. They rate their PSUs at the highest point that it can safely put out continuously. A Corsair HX520 is rated for continuous because 520W isn't actually the maximum it is capable of putting out. It is the maximum that Corsair feels it can safely put out continuously.

In other words, look at it as the HX520 being a 650w PSU that Corsair is guaranteeing can run 24/7 at 520w. Most good brand PSUs do that.