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Author Topic: Measuring Rig Power Consumption  (Read 1952 times)
MegaBux
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September 29, 2011, 04:28:55 AM
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I'm trying to add another card to my rig and when doing so, the system becomes unstable and eventually crashes.  Consequently, I suspect that I may be drawing too much power from my power supply.  I have a Corsair AX1200 80+ Gold power supply; it has one 100.4A rail.  Under full load, this power supply is rated to be about 88% efficient.

When I measure power consumption at the wall, I read about 1050W.  What does this number represent?  Is it the power draw of the power supply, after which 924W are provided to the system?  Or is it the actual consumption of the system after taking into consideration its efficiency (1050 / 88% = 1193W)?
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September 29, 2011, 04:39:32 AM
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1200W is the max Power that comes out of ur PSU, and 88% eff means: if ur System is running at 1200W it would consume 1200/88% = 1363W. If it consumes 1050W then ur system runs at 924W (PSU -> MOBO...).
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September 29, 2011, 10:29:02 PM
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When I measure power consumption at the wall, I read about 1050W.  What does this number represent?  Is it the power draw of the power supply, after which 924W are provided to the system?
Yes this is correct.

Quote
Or is it the actual consumption of the system after taking into consideration its efficiency (1050 / 88% = 1193W)?

No this is incorrect.  

IF your powersupply is 88% efficient then 1050W AC at the wall means the power supply pulls in 1050W.  It converts 88% of that (924W) into useful DC electricity and converts the remaining 126W into heat.

Remember though efficiency varies depending on load.  Your powersupply is well matched.  Generally you want load to be 60% to 80% of peak because that is where powersupply is the most efficient.

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September 30, 2011, 07:19:46 PM
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1200W is the max Power that comes out of ur PSU, and 88% eff means: if ur System is running at 1200W it would consume 1200/88% = 1363W. If it consumes 1050W then ur system runs at 924W (PSU -> MOBO...).

Is this true of all power supplies?  When I check the specs for an Enermax 1350W supply, it shows a "peak power" rating, which is generally higher than the wattage rating of the supply (but within the efficiency).  I couldn't find the same specs for my Corsair AX1200.

Basically, I'm looking to see what the peak power is for the AX1200, which should be at least 1500W since it is 80+ efficient.
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Gerald Davis


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September 30, 2011, 07:51:04 PM
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1200W is the max Power that comes out of ur PSU, and 88% eff means: if ur System is running at 1200W it would consume 1200/88% = 1363W. If it consumes 1050W then ur system runs at 924W (PSU -> MOBO...).

Is this true of all power supplies?  When I check the specs for an Enermax 1350W supply, it shows a "peak power" rating, which is generally higher than the wattage rating of the supply (but within the efficiency).  I couldn't find the same specs for my Corsair AX1200.

Basically, I'm looking to see what the peak power is for the AX1200, which should be at least 1500W since it is 80+ efficient.

No you are misunderstanding the specs.

Power rating is in DC load.  Period. 1200W means it can handle 1200W of DC load (whatever that may be in AC due to inefficiency).   The Peak power rating is also measuring DC load.  Neither rating is in AC load.    The difference between power rating & peak power rating is the power rating indicates what the powersupply can handle continuously and the peak power rating is what spikes the powersupply can handle briefly.

Assumming you trust manufacturer specs exactly (which you shouldn't) a 1200W power supply w/ 1500W peak power rating can handle a continuous 1200W load 24/7/365 (assuming you trust the manufacturer specs which you shouldn't) and @ 88% efficiency it would draw (1200/0.88) 1363W at the wall (AC).  The same power supply can also  briefly handle spikes up to 1500W and @ 88% efficiency it would draw (1500/0.88) 1704W at the wall (AC) when DC load spikes to 1500.

Remember power supply is best loaded at 60% to 80% of continuous rating (not peak rating) because that is where efficiency is the highest.


Cliff notes:
Peak power rating refers to the power limit the supply can handle for brief periods of time.  
Peak power rating is pretty much worthless spec.
Power supplies are rated by their SUSTAINED DC load rating.
Due to inefficiency power supplies will ALWAYS draw more AC then then DC load they are supplying.
AC load = ( DC load / efficiency)
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September 30, 2011, 09:01:01 PM
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Thank you very much for that detailed explanation.  It has definitely cleared up a lot of my confusion!
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September 30, 2011, 09:20:41 PM
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Very good explaination of PSU's Smiley

My rule of thumb....200-300 watts more than I need,a nice comfort zone.

I have a Kingwin 800 watt with XFX 6970's in crossfire,when OC'ed my system uses 720 watts Shocked

With no OC,610 watts.

I'll be upgrading to 1000 watt ASAP Wink

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