# Thread: PSU based on % of Recommendation

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## PSU based on % of Recommendation

I apologize if this has been stated somewhere, but I haven't found anything giving a simple way to knowing roughly what size PSU to get, based on:
1. An established estimate of the system's energy requirements;
2. An accepted performance rating of the PSU one desires to get; and,
3. A estimated % one wishes one's PSU to operate at.

Simply: I'm trying to get a good idea of what size PSU I need, based on power consumption estimates and the information given on PSU performance.

My system's estimated Energy usage is: 550W.
I want a gold PSU.
I would prefer it to not be 100% stressed for any long period of time. I am aiming at 70-80% of a full load.

Does this mean I should be looking at an 850W PSU?
I am thinking it should be:
Power*(1/% efficiency)*(1/% of estimated draw on PSU)

yielding:
550W*(1/.87)*(1/.7) = 896W

900+ Watts seems overkill, and I suspect the online PSU calculators typically over-estimate. Thus, the 550W is an upper-bound.
I used 87% because it represents a lower-bound for efficiency of Gold rated PSUs.
I used 70% as a rough idea of where I wanted my system to be loaded at.

I am confused, because some sites recommended using a 650W PSU; but, after one considers the efficiency being around 87% when the draw would come close to 100% of the rated PSU, puts the power draw at 632W.

So is a 850W PSU what is best here?
Is my math brain leading me astray? lol. That's entirely possible.

Quoddity

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I would add that I have used several online PSU calculators and they all roughly give 550W.
I have a X570 with Ryzen 5 3600x.
Gigabyte 5700 XT OC
cooler master hyper RGB air cooler
2x 8GB Ram 3200
one 500 GB NVMe card
6 Sata HDs
2 200mm fans
3 140mm fans
1 120mm fan
all fans rgb
My thoughts are on either an RMx850 or one of the EVGA gold series PSUs.
I'd be happy going with a 750W. I'm just trying to figure out roughly what I should be considering in terms of overall power without going really overboard.

I plan to lightly over-clock the GPU. I also plan to get another NVMe card and more RAM.

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Originally Posted by Quoddity
I used 87% because it represents a lower-bound for efficiency of Gold rated PSUs.
I used 70% as a rough idea of where I wanted my system to be loaded at.
I think you got the wrong idea when looking at PSU Wattage labels. They are labeled for OUTPUT POWER, with efficiency already factored in.

Also: why do you want the PSU to only be loaded at 70%? There's nothing wrong with using a PSU at its max rated power.

As to your 3600X and 5700XT build, I suspect it would draw 350W or less, maximum overclock. 5700XT - 250W, 3600X - maybe 70W, everything else - 30W. Keep in mind that not everything will be on 100% use at the same time.

You got a couple good choices for your build. The Corsair CX550 at \$75, Antec EAG for \$83, and RMx 550 at \$110 if you desire something more high end...

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Originally Posted by hello_there_123
I think you got the wrong idea when looking at PSU Wattage labels. They are labeled for OUTPUT POWER, with efficiency already factored in.
Thanks Hello_there_123 for confirming my suspicion that I don't need to go to 850W PSU.

I do suspect these online calculators may over-estimate system power consumption.
It certainly is true that all the components of my system will not be active at the same time. eg., HDs.

But I don't believe I have the wrong idea of PSU Wattage labels.

I used the estimated wattage derived from several online system wattage calculators. (I'm not assuming they are accurate - it's simply where I got my starting information.)
From that estimated wattage, I assumed that the number given is the amount the system draws, not what the PSU is needing to output to meet those draw demands.
This is why I added the efficiency to the PSU requirements; since, if my system needs to draw, for example, 100watts and the PSU at that point is running at 80% efficiency, my PSU would need to provide 100*(1/.8) watts = 125 Watts.
So setting aside the % load the PSU would be under, if my system needs 550W and is running at 87% efficiency at that point, the PSU would need to provide 632W.

But maybe I am incorrect.
Are these online system wattage calculators, such as the one through cooler master, assuming PSU efficiency?

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Originally Posted by Quoddity
I do suspect these online calculators may over-estimate system power consumption.
It certainly is true that all the components of my system will not be active at the same time. eg., HDs.
Your suspicions are correct. The worst ones, like Outervision, usually overestimate by a factor of 2 or more to push their affiliate sales cash. Probably the best calculator would be BQ's one, but even then I would subtract 50W from its estimation.

Originally Posted by Quoddity
But I don't believe I have the wrong idea of PSU Wattage labels.

I used the estimated wattage derived from several online system wattage calculators. (I'm not assuming they are accurate - it's simply where I got my starting information.)
From that estimated wattage, I assumed that the number given is the amount the system draws, not what the PSU is needing to output to meet those draw demands.
This is why I added the efficiency to the PSU requirements; since, if my system needs to draw, for example, 100watts and the PSU at that point is running at 80% efficiency, my PSU would need to provide 100*(1/.8) watts = 125 Watts.
So setting aside the % load the PSU would be under, if my system needs 550W and is running at 87% efficiency at that point, the PSU would need to provide 632W.
As I said before, the PSU is rated for OUTPUT POWER. You are indeed correct, a 550W PSU running at 87% efficiency would need to draw 632W from the wall. However, you don't see the PSU being labeled as a 632W PSU. It's labeled as a 550W PSU.

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Originally Posted by hello_there_123
As I said before, the PSU is rated for OUTPUT POWER. You are indeed correct, a 550W PSU running at 87% efficiency would need to draw 632W from the wall. However, you don't see the PSU being labeled as a 632W PSU. It's labeled as a 550W PSU.
Ok then. I did not see that coming! You're right, I did not understand how PSUs are labeled. Thanks.

I suppose since you seem to be active, perhaps you or someone else can tell me why EVGA PSUs are more expensive (at least in my area).
I've always used Corsair and am comparing the RMx with EVGA's G series. From what I can tell, I want to stay away from the most recent G5 variety, but nevertheless, EVGA's PSUs are \$20 or more than Corsair's offerings. Is this marketing? local availability adjusting the price? or is EVGA better at Ripple, etc...?

Whether you answer or not, thanks for the input

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Originally Posted by Quoddity
I suppose since you seem to be active, perhaps you or someone else can tell me why EVGA PSUs are more expensive (at least in my area).
I've always used Corsair and am comparing the RMx with EVGA's G series. From what I can tell, I want to stay away from the most recent G5 variety, but nevertheless, EVGA's PSUs are \$20 or more than Corsair's offerings. Is this marketing? local availability adjusting the price? or is EVGA better at Ripple, etc...?
EVGA PSUs being overpriced isn't specific to Canada, it's most countries, leading me to believe that EVGA is riding off their reputation. However it is my personal belief and I'm not saying that it's 100% the correct answer. There's probably some underlying factors as well, like EVGA not having a dedicated PSU divison.

And to answer your last question: no, the RMx is better than all of the G-series.

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