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PSU sizing and efficiency

E

ericsan

Weaksauce
Joined
Feb 11, 2002
Messages
103
Looking at the ratings curves for efficiency vs load, I wonder if a 850 would produce less heat than a 650, and draw less power?

Suppose my rig draws a steady 400 watts doing some activity. A 400w Gold PSU would be at nearly 100% and lower efficiency than a 800w Gold PSU running at 50% load in the same rig.

Now yes, if I have a 400w load, I would pick a 650 Gold PSU probably for that rig. But even still, a 650 would put me at 75% load during that activity.

So would a PSU, double (100%) times the load, draw less power from the outlet and put out less heat than a PSU that is 50% of the load? (give the same Gold rating)

I know this is a very specific example, I'm just looking at the math against the curves I see about 80+ specs.
 
Let's look at the Load Testing page of the recent HardOCP review of the Seasonic Snow Silent 750W. There, on a PSU that doesn't suck, we see that efficiency varies by about 2%. While the heat does go up a bit from 47C at 25% to 52C at 100%, you must understand that HardOCP runs these tests in a 45C hotbox so you would probably not see so much variation in a case with some cooling.

Since that might be the best PSU on the market today, here's a recently reviewed 550W Gold rated PSU's load testing and while it doesn't do quite so well as the Silent Snow, it too only varies about 2% in efficiency and has pretty much the same temperatures.

When you commented about comparing 400W PSUs to 800W PSUs, that's a problematic comparison because good quality PSUs start at around 550W these days and most of the stuff below 500W is junk that you shouldn't be using at all, with the exception of a few very expensive SFF models.

So, to your question, I would say the answer is no, there would be no significant difference in power draw from the outlet or heat output if you got a larger PSU, unless there was something terribly wrong with the smaller PSU.
 
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