PSU in low-power PC build

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pegleg1402

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Aug 12, 2014
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I am trying to build a low power PC in an older case that has an older PSU. My question is, will the PSU consume its maximum wattage even if I install a low power CPU, motherboard, and hard disks? Do I need a lower power PSU to see any power savings?
 
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geofelt
A psu consumes only the power demanded of it, regardless of it's max capability.
One possible issue with older psu's is that they were not designed to deliver power @+12v where modern cpu's and graphics cards need it.

The power you need is largely determined by your graphics card. Most modern parts with integrated graphics will run on 100-150w.

If your old psu is of good quality, use it. If it is a cheap unit that commonly comes with a pre built pc, then you are probably better off replacing it regardless.
Exactly what brand/model/wattage do you have?
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InvalidError

InvalidError

Titan
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May 18, 2007
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PSUs only use whatever power they need to maintain output voltages and the amount of power that requires is whatever the loads actually use plus whatever the PSU's losses are.

If you have an old, low-quality no-name brand PSU, its efficiency will likely be in the 50-70% range, meaning the PSU itself wastes nearly as much power as the equipment connected to it uses. In that case, if you are genuinely concerned about efficiency, getting a better PSU would be a good idea... depending on how questionable your existing old PSU is, a new PSU might be a good idea just for safety.

If your CPU uses 40W, your GPU uses 40W and the rest of your components use 20W, your PSU needs to provide 100W. If you have an old no-name PSU with 60% efficiency at 100W, the PSU will draw 167W from the wall. If you upgrade to a 80+ Bronze PSU which will be 82-85% efficient, it will draw about 120W from the wall and you save 45W.
 
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geofelt

geofelt

Titan
Oct 9, 2006
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A psu consumes only the power demanded of it, regardless of it's max capability.
One possible issue with older psu's is that they were not designed to deliver power @+12v where modern cpu's and graphics cards need it.

The power you need is largely determined by your graphics card. Most modern parts with integrated graphics will run on 100-150w.

If your old psu is of good quality, use it. If it is a cheap unit that commonly comes with a pre built pc, then you are probably better off replacing it regardless.
Exactly what brand/model/wattage do you have?
 
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