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 Post subject: You may not need as much wattage as you think.
PostPosted: Tue Mar 20, 2018 8:09 am 
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Website wattage calculators err on the side of caution, to a somewhat extreme degree sometimes. Also, some enthusiasts make recommendations on the basis that the GPU (and GPU) would be overclocked to the highest possible maximum, when most people don't do that.

A build without a video card most likely does not need more than 250w. Even a system with a video card usually doesn't need or benefit from more than 500 watts. A system I built for a friend runs perfectly fine with a GTX 1080 Ti and a 550w power supply. 600+ watts is exaggerated; perhaps for SLI or cryptocurrency mining builds, but neither option is practical for most people.

Some people recommend larger capacities for future-proofing, but due to processors becoming more power efficient over time, it is likely that your more powerful upgrades in the future will be consuming less power - not more.


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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Wed Mar 21, 2018 2:30 pm 
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This is entirely true. Buying a power meter is not an expensive way of checking actual AC consumption to make an educated choice on PSU size.

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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Thu Mar 22, 2018 8:31 pm 
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Are power meters cheap enough to justify buying them just to test a power supply. In that case, if a builder is uncertain, maybe they should just spend $10-20 more on a power supply with 100 extra watts. Most power supplies today have at least 350-400 watts. If you don't use a video card, this is more than plenty (I doubt a test is even required). Maybe this is only a concern for those looking to overclock their processors to the max.


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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Fri Mar 23, 2018 6:57 am 
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You can get a Kill-a-Watt power meter for under $20. It's a useful tool for seeing what your PC is doing at idle and various loads. It's also a great tool to have around the house to see what appliances are consuming.

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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Tue Aug 07, 2018 2:45 pm 
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This is my proposed rig I am planning:

Ryzen 7 2700 X
MSI GeForce GTX 1060 DirectX 12
16 GB RAM
4 fans (including cooler fan)
optical drive
possible 2 PCIe NVMe SSDs
AsRock X470 mobo

I put the components through a PSU calculator and it recommended a 650W PSU. However, I had already recently bought a 550W PSU before doing the online calculator. I just calculated it in my head. Now I'm beginning to wonder if I should have bought a 650W instead.

Will 550W be okay for this system?


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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Tue Aug 07, 2018 4:14 pm 
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whispercat wrote:
Will 550W be okay for this system?
Yes it should be fine, even an overclocked i7 8700K @5hz + GTX1080 barely breaks 350W on prime95/furmark, on gaming it usually hangs around 250W, your setup wont draw as much, so 550 is more than enough.

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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Tue Aug 07, 2018 9:30 pm 
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From reviews I've read so far, the 2700 X maxes out at around 220W
The MSI GTX 1060 maxes out at 135W
RAM about 5W
PCIe NVMe SSDs about 10W
Fans would be 10W
MoBo ?

total ~ 380W - 400W

I read somewhere that you should also leave about 20% wattage for headroom. So that would be 550W - 20% (=110W) = 440W.

So it may be using about 3/4 capacity power when my system is fully maxed out, which would be rare. I guess it should be okay. I suppose compared to a 650W PSU, it would run a bit warmer with the fan coming on more often.

Thanks


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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Tue Aug 07, 2018 10:56 pm 
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whispercat wrote:
I suppose compared to a 650W PSU, it would run a bit warmer with the fan coming on more often.


Not necessarily. The efficiency dropoff at max load is small so the extra heat output (even if you were running at 500W+) would be only a Watt or two. With higher rated PSU's they will often up the fan rating, or perhaps change the fan curve as naturally a 650W PSU will put out more heat at max load than a 400W PSU. The assumption that a higher rated PSU will be quieter is often incorrect. Sometimes it is correct, but often it isn't.

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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Wed Aug 08, 2018 6:16 am 
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whispercat wrote:
From reviews I've read so far, the 2700 X maxes out at around 220W

It's a 105W TDP processor. So, either the reviews are talking about total system power or they've got an insane overclock/overvolt going on.

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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Wed Aug 08, 2018 8:52 am 
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CA_Steve wrote:
whispercat wrote:
From reviews I've read so far, the 2700 X maxes out at around 220W

It's a 105W TDP processor. So, either the reviews are talking about total system power or they've got an insane overclock/overvolt going on.


Bit-tech has these graphs labeled in reverse, but you're right, it's total system power. My mistake. https://www.bit-tech.net/reviews/tech/c ... -review/7/

So where are the other 115 watts coming from then?


Last edited by whispercat on Wed Aug 08, 2018 9:03 am, edited 1 time in total.

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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Wed Aug 08, 2018 9:01 am 
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edh wrote:
whispercat wrote:
I suppose compared to a 650W PSU, it would run a bit warmer with the fan coming on more often.


Not necessarily. The efficiency dropoff at max load is small so the extra heat output (even if you were running at 500W+) would be only a Watt or two. With higher rated PSU's they will often up the fan rating, or perhaps change the fan curve as naturally a 650W PSU will put out more heat at max load than a 400W PSU. The assumption that a higher rated PSU will be quieter is often incorrect. Sometimes it is correct, but often it isn't.


Interesting. I just assumed that a 550W PSU supplying say 500 watts would be under a heavier load and thus putting out more heat, than a 650W or a 1000W under the same 500 watt load. Like when you run a 1500W blender at low speed, it doesn't get as hot as at high speed. Is this incorrect?


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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Wed Aug 08, 2018 9:46 am 
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CA_Steve wrote:
whispercat wrote:
From reviews I've read so far, the 2700 X maxes out at around 220W

It's a 105W TDP processor. So, either the reviews are talking about total system power or they've got an insane overclock/overvolt going on.


I believe TDP is only for base clocks. The Ryzen X models have built in overclocking features (I forget the exact details, but its features like XFR2, PBO, etc), which I imagine will take you well above TDP. Pretty sure it can boost even higher if it detects a safe CPU temp (i.e. if you have a great watercooling loop, it'll autoboost even more). Again, this is all built-in, no manual OC needed. The 220W figure does sound a bit high, though, if I had to guess I'd think it would top out at 150-180W, but I don't really know.

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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Wed Aug 08, 2018 9:54 am 
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whispercat wrote:
edh wrote:
whispercat wrote:
I suppose compared to a 650W PSU, it would run a bit warmer with the fan coming on more often.


Not necessarily. The efficiency dropoff at max load is small so the extra heat output (even if you were running at 500W+) would be only a Watt or two. With higher rated PSU's they will often up the fan rating, or perhaps change the fan curve as naturally a 650W PSU will put out more heat at max load than a 400W PSU. The assumption that a higher rated PSU will be quieter is often incorrect. Sometimes it is correct, but often it isn't.


Interesting. I just assumed that a 550W PSU supplying say 500 watts would be under a heavier load and thus putting out more heat, than a 650W or a 1000W under the same 500 watt load. Like when you run a 1500W blender at low speed, it doesn't get as hot as at high speed. Is this incorrect?


He's wrong, you're right. PSUs are at peak efficiency around 50% of rating... it'll be way more than a watt or two to have a 500W PSU delivering 500W vs a 1000W PSU delivering the same 500W, more like at least 30W.

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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Wed Aug 08, 2018 11:45 am 
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flyingsherpa wrote:
He's wrong, you're right.

I would beg to differ. The efficiency drop off is extremely small with modern PSU's. Please read and familiarise yourself with efficiency ratings for the different 80 Plus standards before drawing conclusions:
https://en.wikipedia.org/wiki/80_Plus

Efficiency dropoff from 50% to 100% is of the order of 2-4%. You can assume much of this dropoff comes above 90% so as I said, the difference will be a watt or two between a low power and high power PSU at high power loads. At low powers (like those that you will actually use, not those invented for marketing purposed) the smaller PSU will win out on efficiency.

Yes, the difference between a 500W PSU running at 500W and a 1000W running at the same would be 30W based upon a 3% difference in efficiency between 50% and 100% but you won't be running at 500W all of the time! At lower powers the 1000W PSU will be far more wasteful and even at 400W, which you might actually be at, the 500W PSU at 80% power will be more efficient that a 100W PSU at 40%. It also does not inherently mean that a higher wattage PSU will be quieter. Please reread and reconsider what I have previously posted.

The blender analogy doesn't really relate that well. A higher power motor will have more metal to it, more turns on the motor windings, more rotating area cutting through the air to provide air cooling. Therefore a bigger motor will inherently by design be able to dissipate a higher level of heat than a smaller motor.

With PSU's there are differences between components on lower power and higher power components but you're not magically going to make the extra heat go away. Sometimes the difference between power levels on PSU's is reflected in different temperature ratings of components, eg. 105C rated capacitors instead of 85C, sometimes internal heatsinks might differ also but there is also going to be a lot of binning involved with the same components having different ratings. It doesn't inherently mean that a higher rated PSU will run cooler so therefore you can't assume that the fan speed will be lower - they might have rated the fan higher anyway to deal with higher heat loads.

It's a moot point though as most people are using a small fraction of their PSU wattage during typical use - running Furmark doesn't count as normal usage.

You could of course give up the pretensions of massive power PSU's and buy a completely fanless PSU, then it will always be silent.

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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Wed Aug 08, 2018 1:54 pm 
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Thanks. I see what you mean. However, people don't buy a PSU for normal usage, they buy a PSU to accommodate their estimated heaviest workload needs, so that they have enough headroom for those few times when they do use a lot more power.

I could see how using a much higher capacity PSU might draw more energy when idling or doing normal tasks. This could be a consideration when judging what power level of PSU to get.


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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Thu Aug 09, 2018 1:32 am 
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whispercat wrote:
Thanks. I see what you mean. However, people don't buy a PSU for normal usage, they buy a PSU to accommodate their estimated heaviest workload needs, so that they have enough headroom for those few times when they do use a lot more power.

I could see how using a much higher capacity PSU might draw more energy when idling or doing normal tasks. This could be a consideration when judging what power level of PSU to get.

Which is why people are buying PSU's far bigger than they need for no practical reason. I guess it's their money to waste if they wish but then they shouldn't whinge and moan about how expensive food, housing and other vital things are.

I'm all for capitalism but it is down to the individual to spend their money as they wish and sadly most people are swayed far too easily by marketing.

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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Fri Aug 10, 2018 10:49 am 
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If people are buying a PSU not for their normal usage but for their peak usage, then they are not "buying PSU's far bigger than they need for no practical reason". They still need a PSU that will deliver power for their peak usage.


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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Fri Aug 10, 2018 12:14 pm 
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For me there are 3 relevant things to consider when buying a PSU, in terms of usage.

1) Your idle wattage
2) Your average load wattage
3) Your peak wattage

1 n 2 are the most important, 3 is just a safety, usually good branded power supplies can draw much higher than what they are rated for shorter times, aka peaks. Titanium PSU help a lot into making 1 much more viable with a bigger PSU, the 80+ standard ask for 90% efficiency at 10% draw (platinum and below dont ask till 20%, favoring choosing smaller psu), so for me i try to aim my usual load usage to where its the most efficient, usually around 40-60% of the PSU, and try to have my idle around 10% at the least.

Now if i were to go gold/platinum, although it depends on the PSU, the setup im planning is an i7 8700K + GTX1080, already tested it and it draws 50W idle / 250W gaming / 350W stress, going by my gaming load, 250W would drive me toward a 500W PSU, but 20% would be 100W, so im going to be idling below 20%, so ill probably settle on a 450W, below that its harder to get quality PSU either way. Again depends on the PSU, but certainly Titanium makes it much easier to stay in the efficient zone.

Now where it becomes hard to go with a smaller PSU is on higher end CPUs, while they are still very efficient at idle on stock clocks, they can consume a ton of you overvolt, a TR/i9 can go from 50W idle / 250W load to 600W+ if you overvolt/overclock, add a couple of GPUs and you are getting close to 800W maybe more.


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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Fri Aug 10, 2018 2:10 pm 
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Are those online PSU wattage calculators reliable?

They keep telling me my wattage use would be around 450W. I recently bought a 550W (Seasonic Prime Ultra Gold) but I'm not sure how much headroom to give my planned system. It will be a Ryzen 2700 X with a GTX 1060 (possible upgrade to a 1080 later). I will be doing video editing, a little 3-D viewing using Navisworks,, and apart from that, just office work. No gaming or VR. I also won't be overclocking.

But with 8 cores on the 2700 X I wonder if a 650W would hit a sweeter spot between idle and peak on average?

I just don't know how to calculate it. These are the Seasonic specs: 80 PLUS GOLD - 87% efficient at 20% load, 90% efficient at 50% load, and 87% efficient at 100% load


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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Sat Aug 11, 2018 1:59 pm 
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whispercat wrote:
Are those online PSU wattage calculators reliable?

In general they aren't because at multiple stages additional wattage is added, either as a safety factor or by disinformation. Component manufacturers list TDP which is purely theoretical and over the top and also PSU recommendations which are nonsense. Review sites list AC draw, not DC so efficiency losses are included and the misinformed assume this is the same. Online calculators will add a bit to each component, just in case, PSU manufacturers have a vested interest to sell you more than you need. 'Enthusiasts' will often read these numbers, and because they are enthusiasts are commonly happy to spend more money as a result, then some guy on a forum will end up suggesting you should sell your house and all of your belongings to be able to buy a 1500W PSU.

A power meter is not expensive and is a long term investment to manage household electricity usage, for example making a business case for replacement of domestic appliances.

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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Thu Aug 23, 2018 4:51 am 
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whispercat wrote:
whispercat wrote:
From reviews I've read so far, the 2700 X maxes out at around 220W
... it's total system power.
... So where are the other 115 watts coming from then?
Total system power imply that some power is drawn/lost to other components than the CPU. The VRMs do have some loss, and graphics, RAM, system disk, audio, USB, network and what have you also use power.

On top of that the regulators (at default settings) do allow the new Ryzens to draw well above TDP if the motherboard can provide reliable power it and the cooling is sufficient.
With a high end motherboard and efficient cooling I wouldn't be surprised if the 2700X would stay put closer to 150W while running heavy loads.


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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Thu Aug 23, 2018 8:19 am 
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Quote:
... So where are the other 115 watts coming from then?


Approximately:
  • 22 W lost in the PSU at 90% efficiency (Gold)
  • maybe 40 W for the motherboard (variable, 20-40W isn't uncommon, this includes conversion losses for powering the CPU. Higher-end mobos tend to use more power, partly due to having more features.)
  • 10 W for the GPU at idle
  • 5 W for two sticks of RAM (a couple of watts per stick)
  • 5 W or so for the SSD

This would leave 138 W for the CPU, which sounds like a reasonable ballpark figure - I'd expect something in the 110-150 W range. Tom's Hardware actually measured the 2700X at 105 W under Prime 95, but they were using a water chiller for cooling, and their 'AVX offset auto' setting may behave differently than whatever settings bit-tech used.

The TDP is something like "power at a typical full load", with some manufacturer leeway for which loads they consider 'typical'. Prime95 is far from typical, it can easily draw more than the TDP.


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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Thu Aug 23, 2018 11:27 am 
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Thanks faustus, edh, Olle P, and Abula.

So it sounds like most current systems on average, would use around 80-110 watts just idling. Then when the CPU and/or GPU are loaded, this will add whatever those components are rated at (or have been reviewed at). I'm assuming the CPU and GPU would almost never be maxed out at the same time, correct? Maybe there are some 3-D games or VR software that can do that.

At least I understand power consumption a lot more now. In my case, all my components maxed out at the same time would still only come to about 400 Watts. So a 550W PSU (80 Plus Gold) should be good.


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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Thu Aug 23, 2018 12:47 pm 
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Yeah, you're good. In general, there's one more thing to check: Power supplies may have multiple 12 V 'rails', say one for the mobo/CPU, one for a GPU, and one for molex/SATA power. Here's an arbitrary example from a 550W unit (source). Although it can deliver 550W in total, it's limited to 22 A * 12 V = 264 W per 12 V rail, no more than 444 W in total across all 12 V rails, and no more than 135 W for the 3.3 V and 5 V combined.

So you need not just enough power in total, you need enough for each of the sub-rails and sub-limitations as well. This particular 550 W unit would be a bit small for your estimated 400 W system, since the combined 12 V power is just 444 W. It would probably work, but the headroom is uncomfortably small. Likewise, a 300 W GPU (say a 250 W 1080 Ti with an overclock) might not work even if the total system power was less than 440 W, because it exceeds the capacity of a single 12 V rail.

But your unit (Seasonic Prime Ultra 550 Gold, right?) has a single 12 V rail that supports 540 W according to the specs. That's plenty.


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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Thu Aug 23, 2018 6:51 pm 
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So when looking at specs, do I add up all the 12 V rails?

For my Seasonic 550W that would be: 540W + 3.6 W = 543.6 W. Is this correct? How do I know which 12 V rail is for the CPU, GPU, etc.?


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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Fri Aug 24, 2018 2:47 am 
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Quote:
So when looking at specs, do I add up all the 12 V rails?

No. If there are multiple 12 V rails their sum is generally larger than the total power it can handle, you can't load all the rails to the max at the same time. The total 12V power is a separate specification. E.g. the example I linked (the image seems to be blocked, see Technical specs - Additional information instead) sums to more than 800 W over three 12 V rails, but it also says "+12V1-3 combined maximum output: 444W". So for this particular PSU, it's at most 444 W at 12 V.

Quote:
For my Seasonic 550W that would be: 540W + 3.6 W = 543.6 W. Is this correct?

Your Seasonic has only one +12 V rail. The 3.6 W is -12 V, a negative voltage. I'm not entirely sure what -12V is actually used for, but CPU and GPU power is from the +12V.

Quote:
How do I know which 12 V rail is for the CPU, GPU, etc.?

In a single rail design like yours there is only one option. In a multi-rail design it's not always obvious what goes where, but it should be marked somewhere. E.g. this modular unit has labels like "12V1/HDD" where the cables plug in. Or it could be listed in the documentation.


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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Wed Aug 29, 2018 1:20 am 
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whispercat wrote:
So it sounds like most current systems on average, would use around 80-110 watts just idling. ...
Idle system use is less than half of that, typically 25-30W, unless you overclock manually.


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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Wed Aug 29, 2018 8:40 am 
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edh wrote:
flyingsherpa wrote:
He's wrong, you're right.

I would beg to differ. The efficiency drop off is extremely small with modern PSU's. Please read and familiarise yourself with efficiency ratings for the different 80 Plus standards before drawing conclusions:
https://en.wikipedia.org/wiki/80_Plus

Efficiency dropoff from 50% to 100% is of the order of 2-4%. You can assume much of this dropoff comes above 90% so as I said, the difference will be a watt or two between a low power and high power PSU at high power loads. At low powers (like those that you will actually use, not those invented for marketing purposed) the smaller PSU will win out on efficiency.

Yes, the difference between a 500W PSU running at 500W and a 1000W running at the same would be 30W based upon a 3% difference in efficiency between 50% and 100% but you won't be running at 500W all of the time! At lower powers the 1000W PSU will be far more wasteful and even at 400W, which you might actually be at, the 500W PSU at 80% power will be more efficient that a 100W PSU at 40%. It also does not inherently mean that a higher wattage PSU will be quieter. Please reread and reconsider what I have previously posted.

The blender analogy doesn't really relate that well. A higher power motor will have more metal to it, more turns on the motor windings, more rotating area cutting through the air to provide air cooling. Therefore a bigger motor will inherently by design be able to dissipate a higher level of heat than a smaller motor.

With PSU's there are differences between components on lower power and higher power components but you're not magically going to make the extra heat go away. Sometimes the difference between power levels on PSU's is reflected in different temperature ratings of components, eg. 105C rated capacitors instead of 85C, sometimes internal heatsinks might differ also but there is also going to be a lot of binning involved with the same components having different ratings. It doesn't inherently mean that a higher rated PSU will run cooler so therefore you can't assume that the fan speed will be lower - they might have rated the fan higher anyway to deal with higher heat loads.

It's a moot point though as most people are using a small fraction of their PSU wattage during typical use - running Furmark doesn't count as normal usage.

You could of course give up the pretensions of massive power PSU's and buy a completely fanless PSU, then it will always be silent.


Nothing I said was wrong. You're the one saying it'd only be a watt or two, then later agreed with me that it'd be at least 30 W. Take your own advice and learn about this stuff before giving people false info.

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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Wed Aug 29, 2018 8:46 am 
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Location: UK
flyingsherpa wrote:
Nothing I said was wrong. You're the one saying it'd only be a watt or two, then later agreed with me that it'd be at least 30 W. Take your own advice and learn about this stuff before giving people false info.

Nothing I said was wrong either... if you're not making the incorrect assumption that Furmark and Handbrake constitute a typical usage scenario.

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 Post subject: Re: You may not need as much wattage as you think.
PostPosted: Mon Sep 03, 2018 12:13 am 
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Joined: Tue Jul 22, 2008 6:05 pm
Posts: 309
Location: Calgary, AB, Canada
Olle P wrote:
whispercat wrote:
So it sounds like most current systems on average, would use around 80-110 watts just idling. ...
Idle system use is less than half of that, typically 25-30W, unless you overclock manually.


Well, this test on the 2700X showed 68W at idle at stock.

https://www.bit-tech.net/reviews/tech/c ... -review/7/


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