Is my wall plug watt meter at all accurate at measuring system power draw?

3t3P

3t3P

3t3P

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I've read around that it's not entirelly accurate using a plug-pass-through £20 power meter to measure system draw.

What margin of error are we talking here and why is it not accurate?

Just need to know cos got a second GPU coming tomorrow and want to be careful with watts drawn on current PSU...
 
I take it your adding a second gtx 470? If so the psu in your sig may struggle with an overclocked setup, running sli 470's here and according to my wall plug the total system draw is 630w when gaming, cards clocked at 750/1500/1674 on stock volts, cpu at 4.2ghz. Even taking into account how accurate/inaccurate theese monitor things are, youd be asking a lot of the psu to run two of theese cards.
 
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Why wouldn't it be accurate?

What you have to remember is that the reading at the socket isn't ther same as the output from the PSU.

Let's assume your PSU is 85% efficient.

If you were drawing 600W from the socket then that would mean your PSU was outputting 510W (600*0.85).

Alternatively for your PSU to be outputting all 620W it would need to draw ~730W from the socket (620/0.85).

FYI running a PSU close to it's full output for long periods is not recommended. For SLI'd GTX 470's you really need something beefier.
 
ah well, so much for SLI being a 'cheap', £200 upgrade (+ eventual waterblock) lol

yes I could see no reason why they would not be accurate for total power drawn but can understand why it is NOT telling your the PSU output.

Will an AX850W be ok for me? I like that PSU:)
 
3t3P said:
ah well, so much for SLI being a 'cheap', £200 upgrade (+ eventual waterblock) lol

yes I could see no reason why they would not be accurate for total power drawn but can understand why it is NOT telling your the PSU output.

Will an AX850W be ok for me? I like that PSU:)
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How can a power meter know what's being output at the the end?

It can only measure the power being drawn through it.

A much cheaper option than the Corsair AX 850W:

OCZ Z-Series 850W '80 Plus Gold' Modular Power Supply
 
I've seen a DOA PSU from OCZ, i'm gonna get the AX850. The corsair PSU's are lurrrvely, can't wait to open a new one!
 
Should be accurate to a few watts, at least telling you how much power you use. You have to factor in your PSU efficiency to determine how much power is used out of the PSU at the other end. PSU efficiency would be between 80% to 90% of the measured wattage at the plug, with a good power supply.

Expect around 200 watts extra for another 470, ballpark figure.
 
is it true the second card in SLI uses less than the theoretical maximum?

so really my rig is using 400watts load?! (450x0.9)
 
3t3P said:
is it true the second card in SLI uses less than the theoretical maximum?

so really my rig is using 400watts load?! (450x0.9)
Click to expand...

1) Under optimum conditions SLI/Crossfire will run both cards at 100% load. It doesn't always work that way though so for that, and other reasons, 2 cards rarely give double the performance of 1.

2) If it's drawing 450W from the mains then the PSU is outputting ~400W as you say. Try running Prime95 and the Heaven benchmark at the same time and you'll likely see power consumption go up.
 
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