Power question -- something doesn't add up

[bm5]

Hi There.

Please help a n00b figure out this power stuff. I'm building a new system for lan gaming. So it's a mATX system -- case comes with a 350W PS (antec NSK-1380). the powersupply is 80+ rated. It's a non-standard shape, so no replacing the powersupply.

I'm worried about power since 350W seems way low to what the card manufacturers recommend (usually at least 450W or more). To help understand power draw I did a little experiment hooking up the system to a kill-a-watt meter.

To help do the experiemnt I threw in a 9600GT card (figured it would draw a bit less power than a 8800GT). System specs are:

E8400 (overclocked to 3.6)
Asus P5E-VM HDMI
EVGA 9600GT
Raptor 150
DVD

I thought with the above components I should be at or close to the limit of power draw from that puny powersupply. The results are puzzling.

To test max power draw I ran both Orthos (small FFT) and ATI tool (scan for artifacts). The kill-a-watt meter shows a max of 180W draw. Since the PS is 80+ certified, let's take an efficiency rating of 80% (could be slightly higher) X 350W = 280W max output.

So if I have 280W max output and I'm only drawing 180W, seems to me that I have plenty of headroom to go with a better card, perhaps even the 8800GTS due to it's efficient heatsink design (vent outside the case which is a definate plus considering the small case).

The other thing I thought about is max amp draw. The PS has two 12V lines each rated at 18A (so 36A ). If I use one of those dual 4 pin molex to 6 pin PCI-E adaptors I should be in good shape.

what's wrong with my analysis? surely there must be a reason why the manufacturers are saying you need 450+ W power supplies?

appreciate your input. thx

 

[Seb1]

...
To test max power draw I ran both Orthos (small FFT) and ATI tool (scan for artifacts). The kill-a-watt meter shows a max of 180W draw. Since the PS is 80+ certified, let's take an efficiency rating of 80% (could be slightly higher) X 350W = 280W max output.
...
what's wrong with my analysis?

I'm quoting " PSU 101: Electrical Basics" from JonGerow (JonnyGURU):

g. Efficiency:

The ratio between the useful output of an energy conversion device and the input is called efficiency. With computers, the term is often used to describe the ratio of AC power inputted into the power supply unit and the DC power coming out. The difference is exhausted as heat from within the power supply.

So, the PSU can deliver 330W for the computer components and can draw 330/0.80= 412.5W from the wall outlet.
Your PSU it's fine and usually the power requirements are overrated because of several reasons. (that I'm not going to discuss )

 

[Mega666]

280W draw / 350W (rated for power supply) = you're using 80% of the power supply's rated output.

Max total draw from the wall = (Power supply max) Watts / efficiency
350W / .8 = 420W draw from the wall max.

P = V * I
350W = 120V * I
I = 350W / 120V = 2.92 amps from the wall socket.

What is you're concern about the amps?

 

[HemiRick]

Othos alone is not testing you max power draw as the video card is NOT working you need to run a game or 3dmark.....and test the power draw then.

 

[Dangman]

@ HemiRick

The OP is also running ATI Tool at the same time.

To help understand power draw I did a little experiment hooking up the system to a kill-a-watt meter.

FYI, the Kill-A-Watts are not that accurate. Hit up this post for a bit more info on the Kill-A-Watt's inaccuracy:
http://hardforum.com/showpost.php?p=1032584323&postcount=68

The other thing I thought about is max amp draw. The PS has two 12V lines each rated at 18A (so 36A ). If I use one of those dual 4 pin molex to 6 pin PCI-E adaptors I should be in good shape.

No. That is not how you come up with the total amperage for the +12V rail. You determine the amperage on the +12V rails by first finding out what's the total combined, max load, combined or max wattage aside for the +12V rails/section alone. Then divide that total by 12 and you get how much amps the PSU has on the +12V rail.

That's the correct way to find out how much amps a power supply has. Don't add up the amps on the +12V rail to figure out the amperage. Doesn't work that way. If the total combined or max wattage can't be found on the power suppply, check the manufacturer's page for that PSU for that info. If the manufacturer doesn't provide that information, it's generally a good sign for you to drop that power supply from consideration.

So for the AR-350 PSU that comes with the Antec NSK1380, if you underneath PSU table, you can see "+12V combined max. output: 300W" Divide 300W by 12 and you get the max amperage on the +12V: 25A on the +12V rail.

surely there must be a reason why the manufacturers are saying you need 450+ W power supplies?

Main reason why is due to all the shitty PSUs out there. Not all 450W PSUs are the same. Some have as much as 33A on the +12V rail while some only have 15A on the +12V rail. Some can't even provide the amount of power listed on their label.

So to cover their asses, video card manufacturers recommend higher wattage PSUs just to make sure that there's a good chance that one will get a PSU with enough amps on the +12V rail to power their rigs. At least that would be my guess. If you check eVGA's website, they actually do list how amperage a system with one of their video cards need. Even then the amperage listing is a bit over-specced just in case. Your 25A on the +12V rail is enough for a system with a 9800GTX! (only requries 24A)

You need to read this PSU FAQ:
http://jonnyguru.com/forums/showthread.php?t=1036

 

[bm5]

thanks everyone, all of the responses were very helpfull. I think I'll wait for the new cards to come out to put into my main system, and then move the current 8800GTS I have into the mATX system -- now that I know I have enough power

Thanks again, cheers!