Power load and efficiency XFX Pro 450W

[Otakon7]

Hello,

I've been trying to wrap my head around how the whole PSU thing works but to no avail (doesn't help I have a migraine so even that sticky isn't too helpful atm), not to mention quite a few of the examples present across the board use maximum theoretical values which further confuse me.

So, I have questions and I will be using the SPU mentioned in the title as an example.

#1 80% Efficiency
From my understanding, the efficiency shows the offset on how much more power will be needed to be pulled from the wall, with 100% being the exact power consumption of a system.

So in example, if a system uses 300 watts, since the efficiency is 80%, it means it will actually require 20% more power to provide those 300, making the total power used 360W.

Is this correct?


#2 Minimum load
I keep hearing it's bad to have a PSU that is too big and your system spends too little power (less then 10% in example) which would result in your system failing.

But from what I could gather, I won't fail, just that it will be less efficient, which would make a 100W system waste just as much power as a 150W would in example.

Is this correct?


#3 450W is not 450W
When I hear people say you need 500-600W for a system that uses up 300-350W peak it makes me think all PSU deliver 20% less power then what they show.

What it really seems to be about is the "myth" about 50% PSU usage being the sweet spot in efficiency so people say you need more then needed to get that sweet spot.

I can see this lowering heat and giving you a lower power bill, but I do not think it means your 450W PSU is useless is you use up in example 380W power (efficiency included), even if you take aging into consideration, you should have watts to spare.

Is this correct?


#4 GPU Power consumption
Similar to #3, I can see people run 180W cards on the XFX 450W, coupled with AMD cpus, but when you ask if you can run something more energy efficient, like gtx 960, you get people saying once again, no, you need 500-600W PSU to be able to run it.

This is mostly tied to people taking the recommended PSU wattage on the GPU manufacturer site at face value I believe and once again, the 50% efficiency thing I believe, but there is no reason why a quality 450W PSU wouldn't be able to run a system that runs 300W peak under stress testing (provided you have the necessary connectors to even run the card).

Is there truth to this?


#5 Big PSU, small system
This is more of a personal question; Would be alright running a Skylake non-k cpu with no GPU on a 550W PSU? This is tied to #2, I don't think the system will spend THAT little energy but doesn't hurt to ask.

Thank you for putting up with my questions, I will reply to your comments in a timely manner.

 

[Math Geek]

1. 300 watts used is actually 375w from the wall. you have to divide the used amount by .8 to get the wattage from the wall. keep in mind this assumes that the 300w load is within the 80% efficiency band. specs can show what %'s of usage fall under what efficiency.

2. very low usage won't kill the psu. this may have been true many years ago but a quality unit can easily handle a low load. it will lose some efficiency although some units can pull the 80% at pretty low loads. again have to look at specs to see the efficiency curve

3. what is recommended actual has nothing to do with the efficiency rating but rather the fact that is not ideal to run a psu near full load all the time. it will fail rather quickly. using 80-85% of the theoretical max of the psu is considered to be "safe" usage. therefore if you will pull 300w, then a unit capable of between 350-375w would keep you in "safe" usage of the psu.

a bigger concern is also the fact that many people buy what are basically junk psu's. they may say the can give 500w, but reality is they may barely push 300w where it counts. many recommendations take this into account and suggest higher power than needed to account for the crappy psu's they also sell. a quality unit can actually push the power it says it does therefore a less powerful one is actually better than a higher wattage crap unit.

4. a lot of what people suggest is based on a lack of understanding of the math involved. so they overestimate but present it as a MUST HAVE!! total wattage is not the important thing here, it is the 12v amps the psu has. the cpu and gpu pull from the 12v rail and are the heaviest draw. therefore, it is important to know what the 12v rail is capable of and ensure it is enough to feed the parts and keep it in safe operating range.

for instance a 150w gpu needs 150w/12v= 12.5 amps off the 12v rail. add in a typical 90w cpu and you'll need (90w + 150w)/12v = 20 amps. to stay in the safe range you'll want between 23.5 and 25 12v amps from the psu for these 2 parts. add in the motherboard and "other" pieces and you'll be adding about 100w more on the 12v rail. (this varies but i found that 100w is a good guess that keeps the numbers pretty good for a generic guess). now we are at about 340w total which is 28.3 amps. so in the 80-85% range of usage, this is 33.3 - 35.4 12v amps.

these numbers are way more meaningful than looking at the total wattage of the psu. a crap psu may claim 750w but often it only gives 300 or so on the 12v rail making it pretty useless to a system that actual needs 700w or so.

5. to have a bit more power than needed is not a bad thing unless it is grossly overkill. only pulling 200w from a 1200w psu is not the greatest idea. but pulling 300w from a 500w one is not really a waste as the numbers show it to be almost ideal. the extra wattage also keeps it a bit future proof for upgrades dwon the line. 550-600w is enough for just about any single gpu system exept for the absolute high end. therefore, it is a good way to go with a build to ensure it meets future needs. you may only need 300w now but a gpu upgrade can easily move this into 400w or more territory which the psu can handle since it is a bit bigger than needed now.


hopefully this helps you understand the numbers some. total wattage is not really the key but rather the 12v rail is what you are really looking at. if you're not sure what is a good psu or not, tom's has a nice list ranking many units that can help steer you to a good purchase. http://www.tomshardware.com/forum/id-2547993/psu-tier-list.html

the xfx units are high quality and if it is enough power, then it would be a good choice.

 

[Otakon7]

Ah, now I finally understand how amps work. Thanks a lot.

So to sum it up, Watts are used to determine the usage ratio of your system, with about 80-85% usage being the safe limit to make the most out of your PSU, but with 50-80% being optimal. Amps are used to determine if your PSU can actually sustain your system (I guess an analogy would be giving a steam engine coal, even if you have enough coal, if you can't provide enough at a time, the engine will shut down) with also about 15-20% headroom.

(Kinda talking to myself here, no need to respond, but feel free to correct me if I'm wrong)

Thanks once again.