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Why You Should Get Lower Wattage Psus

I admit it, it looks better to have a 750W+ PSU. It's also comfortable to know you have some headroom in case you'll ever upgrade. But every component that comes out consumes less than the previous it replaces, so do we need the headroom, and what is the downside of getting a high-power PSU?

Here's my experience that triggered this 1st blog post... as the story goes in the last month I thought of building a low power home server.

So I got this:
  • Case + PSU Asus Pundit AE3 - µATX desktop case + 275W PSU
  • Atom bog stock mobo w/ D510 dual core HT enabled CPU
  • Seagate Momentus .4 OS drive
  • 2GB DDR2 in 1 stick
...and added some Windows Home Server OS sauce. It is a headless server, so WHS was the obvious choice.

What about "Power Consumption"? Math warning! It took 41W at the wall socket irrespective of the load and here's why:
  • all the components listed above can't possibly consume more than 20W
  • the difference to 41W must be PSU inefficiency
  • 41W represents 14% PSU rated power; 20W is under 10%
  • even a 80+Gold rated PSU (which mine isn't) is horrendous @10% load.

Yesterday 2x 2TB Samsung EG arrived and the server now consumes 45W at the socket. Xbitlabs measured these HDD's load power consumption @6W - and both on my rig added only 4W. What happened is:
  • component's consumption probably rised by 12W thus pushing the total consumption to some 32W by my guesstimate
  • 32W is ~12% PSU rated power; the PSU is more efficient at this load than at the previous 8%
  • efficiency rose from ~50% to 70%
...and no, it's not unusual. Below 20%*PSUpwr efficiency rises from 50% to 80%+
I got my case+PSU for cheap, but in the long run I may have to change the PSU with a lower wattage model. The idea is to land the power consumption in the maximum efficiency range, which is between 20 and 80% of the rated power.

It's maybe hard to grasp that below 20% PSU efficiency can be so low, so I wrote this post to share my thoughts. Here is my guide to buying a PSU:
  • compute idle power (iW); I'll peg iW @25% PSU rated power so that efficiency is good @idle usage
  • multiply iW by 4 to get the necessary PSU rated wattage (25%*4=100%)
For example: midrange rigs based on sk 1156 or AM3 with Nvidia 4xx or AMD 5xxx/6xxx graphics cards have very low idle power won't ever consume more than 60-70W @idle. Pair them with 600W PSUs and you'll notice that at the plug they actually pull 100W+
These midrange PCs don't need more than 400W PSUs. Having a lower power PSU is cheaper to purchase and run and it is quieter because losses are low.


Comments (1)

Nice read. I totally agree with you!
I hear people buying 650+ b/c they plan on adding a second GPU to SLI / CF. That's just stupid because SLI / CF drivers are crap, single faster GPUs generally perform better, and you could just sell your old card and buy a faster single GPU.
So IMO everyone should be buying a quality 450w PSU at most.
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