PSU Wattage has to be exagerated..

[asdasd123123]

Ok, So I went out and bought one of those wattage calculating devices that THG uses regularly, because I seriously doubt my electric bill to be accurate.

Well, the point is this, I hooked it up to the outlet that leads to my Amplifier, my two monitors the computer and a few peripherals.
I hit 280watts idle, and barely 320watt total when starting a game.

I've got a 400watt PSU, but this seriously makes me doubt if I ever need that much...

If I run only the computer, it draws 150w..

Thing is, I don't exactly have a weak rig running.

What I have is
Opteron 2,4ghz
2x1gb ram
2x36gb Raptor
1x320gb WD
Nvidia 7800GT
Creative X-Fi Elite Pro
A DVD, card reader, mic, a red CCFL etc

So... Erhm, 400watt for a new rig? 500-1Kw? I doubt it..

 

[waylander]

It's not just about the wattage, the amps the 12v rail is probably even more important. If you could concentrate all the wattage required to when you need it to the components that required it then you're probably right that no one would need more than 400-500w... too bad it doesn't work that way.

 

[pengwin]

as waylander said. the PSU needs good amount of amps on its 12v rail.


what PSU do you have?

 

[michaelahess]

Amperage off the ps isn't the same as amperage out of the wall, your average home circuit is 15 or 20 amps, considering most people have a tv, stereo, computer and lights on one circuit makes it obvious that a higher amperage ps of the same wattage will be higher quality. I have 6 computers, 1 with x1800xt and one with x800, a total of 20 hd's in those machines, all running off 300-350 watt ps's except the x1800 which is a 500watter, a 21" crt, 19" crt, and 2 17" lcd's, and I don't blow the circuit. Now when I put a small window ac unit in, it killed it but that takes around 10 amps by itself.

I'll be receiving an amp/watt tester from my employer today to test actual wattage draw, it's some industrial strength thing we use for our fiber node tests.

Basically wattage is the important thing on the wall side of the ps, simple formula to determine watt/amp usage from the wall is watt's divided by volts=amps.

So if your meter shows 230watts you'd do:

230/110=2.1amps

However most meters should also show the amps. Again this is NOT the same as amps on the dc side of the ps so don't get them confused!

 

[asdasd123123]

I have a Chieftec 400watt PFC with an 120mm fan, it blows practically cold air out the back.
It can sustain 18A on the 12v supply.

But, point is, no matter how you do the math, if the card needs 100watts, you will need 8 ampere to the card itself. The 7800GT needs 60w btw, so 22A like Nvidia recommends seems a little overkill..

 

[waylander]

I think they are trying to take into account numerous hard drives, optical drives, peripherals, fans... etc.

The other big draw is the CPU of course which can pull an avg of 80w which is 7amps, add that to your card of 5amps and you're at 12 of 18amps. That leaves 6 amps for the rest of the hdds, fans, optical drives... etc....

If you OC that cpu and gpu then you're closer to 16 - 17amps... that 22 amps doesn't look so stupid now does it?

 

[pengwin]

I think they are trying to take into account numerous hard drives, optical drives, peripherals, fans... etc.

The other big draw is the CPU of course which can pull an avg of 80w which is 7amps, add that to your card of 5amps and you're at 12 of 18amps. That leaves 6 amps for the rest of the hdds, fans, optical drives... etc....

If you OC that cpu and gpu then you're closer to 16 - 17amps... that 22 amps doesn't look so stupid now does it?

well put.

 

[jaguarskx]

But, point is, no matter how you do the math, if the card needs 100watts, you will need 8 ampere to the card itself. The 7800GT needs 60w btw, so 22A like Nvidia recommends seems a little overkill..

As Waylander stated, the 22A recommendation is for the entire system. Not just the GPU. If you know of a way to power up all the other components on thin air, then please let us all know. Can't beat free energy.

22A may seem high, then again it may not. ATI and nVidia tries to cover there a$$es by recommending PSUs that are kinda powerful simply due to the fact that they don't know what other components you have in your system.

You may have an Athlon X2 4800+ at stock speed which sucks up 96w (8amps) of juice. Or you may have a Pentium D 930 which sucks up around 130w (10.8amps) of juice.

Other power hungry components are the hard drives small ones like 80GB HDDs may only consume 18w (1.5amps) of power, but the Western Digital 74GB 10,000RPM Raptor sucks in as much as 40w (3.33amps) of power. People also tend to have mutiple drives as well.

Also, when measuring power from the AC outlet take into consideration that Power Supplies are not 100% efficient. That means there will be wasted electricity that will be given off as heat. The typical PSU is about 70% efficient; the most efficient PSU are rated at 85%, but they also tend to be slightly more expensive.

Assuming a hypethetical PC requires 200w to power the entire system. A 70% efficient PSU will draw 285.7w (200w / 70%) from the AC outlet. A 85% efficient PSU will draw 235.3w (200w / 85%) from the AC outlet. The extra 85.7w or 35.3w are wasted as heat, fear not because the utility company will charge you for that "heat".

The efficiency of a PSU is actually a bit more complicated than a simple single number. A PSU tends to be at it's worse efficiency at low loads. Say a 70% efficient 500w PSU has the following characteristics:

55% efficiency @ 1% - 25% load
60% efficiency @ 26% - 40% load
65% efficiency @ 41% - 60% load
70% efficiency @ 61% - 90% load
65% efficiency @ 91% - 100% load

What this means is that 70% is the peak efficiency. As you can see this imaginary 500w PSU is at it's most efficient when you put a 61% - 90% load on it. That translates to a load of 305w to 450w. Below 305w the PSU is less efficient. Above 450w and again your PSU is less efficient. Beyond 500w... start saving money for replacement parts.

Getting back to the 200w load this hypethetical PC puts on a 500w PSU, this means that there is only a 40% load (200 / 500). Based on the above specs, the PSU will only be 60% efficient. The imaginary PC will be actually drawing 333.33w (200w / 60%) from the AC outlet. Of which 133.33w are being wasted as heat.