Power Supply - Basic Question

Tempest · 15-02-2010, 09:11

Something I've never really been 100% sure about, so perhaps someone can explain.

Does a higher power power supply in a PC (in itself) use any more power than a low power model (assuming its being used to power the same amount of stuff?)

So, have a PC, and take it's 450w power supply out, and replace it with a 1000w power supply.

Obviously the 1000w model is going to be able to power the PC a lot easier (way below it's abilities)

But, would you actually use more electricity with the bigger supply in place?

Uncle Nick · 15-02-2010, 09:36

In general, yes.

PC power supplies have an efficiency curve, and typically they get less efficient (by a few % anyway) as you drop down towards <25% usage.

So if your PC only draws 150W (quite plausible if it's not got enormous graphics cards in it, or it's just sat on the desktop) then that would be 33% capacity on the 450W PSU but only 15% capacity on the 1KW PSU.

If the 1KW PSU is less efficient at 15% than the 450W is at 33% (again, quite plausible as the headline efficiency rates are calculated between 20-85% and other loads don't "matter"!) then you will be using more electricity to provide the 150W to the PC.

If you've got some time on your hands, read the PSU reviews over at www.jonnyguru.com , these guys know quite a bit about PSU's and the comments attached will fill in lots of detail about what's desirable in a PSU these days.

movaado · 15-02-2010, 10:19

Chances are the more expensive supplies will be better designed and more efficient at the same power rating as well.

Tempest · 15-02-2010, 12:04

Thanks for the advice and the link.

I guess, in reality I'm only thinking of over-speccing by a few 100w

Perhaps the machine should have a 600ish supply in it to be safe, but I see a 1000w model for sale and think well, I may as well get the 1000w one and be set for life then, rather in another 3 years (perhaps) need a bigger one.

akira9000 · 15-02-2010, 12:22

My PSUs have been getting smaller over the past 2 years as the cpus and gpus consume less and less power. All my machines could get by on 300W these days easy. I stick to 450W quality brands for a just in case scenario but still have tons of headroom

Tempest · 15-02-2010, 12:33

Quote:

Originally Posted by akira9000

My PSUs have been getting smaller over the past 2 years as the cpus and gpus consume less and less power. All my machines could get by on 300W these days easy. I stick to 450W quality brands for a just in case scenario but still have tons of headroom

I thought GPU's still were going up and up and up.

Will await to see the new cards from Nvidea in the next couple of months.

akira9000 · 15-02-2010, 12:41

Quote:

Originally Posted by Tempest

I thought GPU's still were going up and up and up.

Will await to see the new cards from Nvidea in the next couple of months.

Im always pretty mid range with GPUs, I know some high end need 150W+
but even so, my current setup is

i5-750....73W max
9500GT...50W max
2 x 1Tb hdds..10W max

you get the idea

Uncle Nick · 15-02-2010, 12:48

There's an upper limit in the PCI-E spec for how much power a card can draw - something like 300W IIRC. I think that ATI crippled their current top end (5790? the one that's actually two cards strapped together...) to keep it inside the spec and be allowed to sell it, but they say "*nudge nudge, wink wink* it can go faster than this you know" and let the dedicated user push it past the power requirement limit.

I've got an 8600 card in mine, and the whole PC (monitor included) uses less than 250W at the wall (unknown PSU efficiency).

15-02-2010, 09:11	#1
Tempest Calm outside Storm Inside Join Date: Jun 2000 Location: Horley (Gatwick) Posts: 15,728 Thanks: 8 Thanked 14 Times in 12 Posts	Power Supply - Basic Question Something I've never really been 100% sure about, so perhaps someone can explain. Does a higher power power supply in a PC (in itself) use any more power than a low power model (assuming its being used to power the same amount of stuff?) So, have a PC, and take it's 450w power supply out, and replace it with a 1000w power supply. Obviously the 1000w model is going to be able to power the PC a lot easier (way below it's abilities) But, would you actually use more electricity with the bigger supply in place? Last edited by Tempest; 15-02-2010 at 09:11.

15-02-2010, 12:22	#5
akira9000 Team Rod Join Date: Oct 2000 Location: Lincs. uk Posts: 1,520 Thanks: 6 Thanked 17 Times in 13 Posts	My PSUs have been getting smaller over the past 2 years as the cpus and gpus consume less and less power. All my machines could get by on 300W these days easy. I stick to 450W quality brands for a just in case scenario but still have tons of headroom __________________ Mac rich - Time poor

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15-02-2010, 09:36	#2
Uncle Nick Bear with me, I'm slow Join Date: Mar 2001 Posts: 3,704 Thanks: 8 Thanked 8 Times in 8 Posts	In general, yes. PC power supplies have an efficiency curve, and typically they get less efficient (by a few % anyway) as you drop down towards <25% usage. So if your PC only draws 150W (quite plausible if it's not got enormous graphics cards in it, or it's just sat on the desktop) then that would be 33% capacity on the 450W PSU but only 15% capacity on the 1KW PSU. If the 1KW PSU is less efficient at 15% than the 450W is at 33% (again, quite plausible as the headline efficiency rates are calculated between 20-85% and other loads don't "matter"!) then you will be using more electricity to provide the 150W to the PC. If you've got some time on your hands, read the PSU reviews over at www.jonnyguru.com , these guys know quite a bit about PSU's and the comments attached will fill in lots of detail about what's desirable in a PSU these days.

15-02-2010, 10:19	#3
movaado XBL: movaado Join Date: Mar 2001 Location: Chatham, Kent, UK Posts: 3,952 Thanks: 21 Thanked 12 Times in 11 Posts	Chances are the more expensive supplies will be better designed and more efficient at the same power rating as well.

15-02-2010, 12:04	#4
Tempest Calm outside Storm Inside Join Date: Jun 2000 Location: Horley (Gatwick) Posts: 15,728 Thanks: 8 Thanked 14 Times in 12 Posts	Thanks for the advice and the link. I guess, in reality I'm only thinking of over-speccing by a few 100w Perhaps the machine should have a 600ish supply in it to be safe, but I see a 1000w model for sale and think well, I may as well get the 1000w one and be set for life then, rather in another 3 years (perhaps) need a bigger one.

15-02-2010, 12:48	#8
Uncle Nick Bear with me, I'm slow Join Date: Mar 2001 Posts: 3,704 Thanks: 8 Thanked 8 Times in 8 Posts	There's an upper limit in the PCI-E spec for how much power a card can draw - something like 300W IIRC. I think that ATI crippled their current top end (5790? the one that's actually two cards strapped together...) to keep it inside the spec and be allowed to sell it, but they say "nudge nudge, wink wink it can go faster than this you know" and let the dedicated user push it past the power requirement limit. I've got an 8600 card in mine, and the whole PC (monitor included) uses less than 250W at the wall (unknown PSU efficiency).