Power Supply Fundamentals

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While SPCR's reviews test the PSU to full output (even >600W!) in order to verify the manufacturer's claims, real desktop PCs simply do not require anywhere near this level of power. The most pertinent range of DC output power level is between about 50W and 250W, because it is the power range where most desktop systems will be working. To illustrate this point, consider the power consumption of the systems described below. They were tested with the same equipment used to test power supplies.

Total Power Draw
AC Input
DC Output
A: Low P4

Intel Pentium 4-2.8C
AOpen MX4SGI-4DL2 motherboard
2 x 512 mb OCZ PC3700 DDRAM
Seagate 7200.7 120G HDD
Seagate Barracuda IV 40G HDD
Matrox P650 VGA (dual head mode)
Seasonic Super Tornado 350W PSU
Asus QuieTrack CDRW
6-in-1 card reader / floppy drive
3 low speed fans

Folding @ Home




B: High A64

Athlon A64-3800+ (130nm core)
Soltek SL-K8TPro-939 motherboard
4 x 512 mb OCZ PC4000 DDRAM
ATI 9800-256 Pro VGA
Hitachi 7K400 HDD (400gb)
Samsung P160 HDD
Silverstone ST30NF PSU (fanless)
M-Audio Firewire 410 external firewire-driven sound card
low speed 80mm fan

Folding @ Home




C: Mid P4

Intel Pentium 4-3.2 (Northwood)
Intel D875PBZLK motherboard
2 x 256MB HyperX DDR400 PC3200
ATI Radeon 9800XT 256MB DDR
16x Sony DVD-RW
Zalman 400W PSU
Samsung HDDs
Creative SB Audigy-2 ZS Platinum
2 x 120mm fans and 1 80mm fan

Folding @ Home




NEW! E: A64 Dual Core

AMD A64-4800+ X2 EE (2.4 GHz, 65W TDP) socket 939
DFI RS482 Infinity motherboard
2 x 1024MB Corsair DDR2-6400 RAM
ATI Radeon X1800GTO PCIe graphics
Seagate 7200.7 80GB SATA hard drive
Maxtor Diamondmax 10
Seasonic SS-400HT ATX PSU
Zalman 9500 HSF
w/ Nexus 92 fan
1 x 120mm fan

Folding @ Home




D: High P4

Intel Pentium 670 (Prescott, 3.8GHz)
Intel D915PBL motherboard
2 x 512MB Corsair DDR2 RAM
AOpen Aeolus 6800GT PCIe VGA
2 x 250 GB Western Digital Caviar SE HDD
Seasonic S12-430W PSU
Creative SB Audigy-2 ZS Platinum
3 x 120mm fans

Folding @ Home




NEW! F: ATI X1950XTX / Pentium D950

Intel Pentium D950, overclocked / overvolted 10% (Presler, 3.74GHz)
Asus P5LD2-VM motherboard
4 x 1024MB Corsair DDR2-6400 RAM
ATI Radeon X1950XTX-512 PCIe graphics
Hitachi Deskstar 7K80 80GB hard drive
WD Raptor WD1500ADFD 150GB
10krpm HDD
Seasonic SS-350ET ATX PSU
1 x 120mm fan

Folding @ Home




  • All the systems have two hard drives.
  • The AC input power was measured directly; it is what the PSU draws from the AC line. The DC output power was calculated based on the efficiency of the PSU used in each system. It is the DC power delivered to the components in the system by the PSU. We have measured the efficiency of all the actual PSUs used in the above systems. The results have been posted in previous PSU reviews. The exception is system D, where a clamp meter was used to measure the DC power delivered.
  • It IS possible to obtain about 10% higher peak power draw with the same systems by engaging all the drives in the system to write and read to each other simultaneously. The actual work a PC can do under such multitasking conditions is minimal, and this kind of usage can be considered a gross abuse of the system. However, ensuring some overload margin is not unwise.
  • Folding @ Home is a pretty good realistic maximum power draw number for a system used with many software applications. The power draw during folding also closely approximates turn-on maximum power draw in these systems.
  • The highest power consumption was achieved while running PCMark, a system benchmark which brings the VGA card into play. The recorded wattage is the highest peak seen during this benchmark. Sustained maximum was about 5~10% lower.
  • These numbers are accurate and based on repeated empirical testing, but for argument's sake, you could say they're as much as 10% too low. The max power draw of any system we've discussed here would still be less than 300W DC.
POSTSCRIPT — April 4, 2007
  • The ATI X1950XTX / overclocked Pentium D950 system information was added to round out the data. There were snickers that the nVidia 6800GT graphics card of system D was completely outmoded. The 1950XTX still remains about the most power hungry graphics card on the market today. The Pentium D950 actually has a higher TDP (130W) than most current CPUs; we made it run hotter by overclocking and overvolting 10%. This system has a higher DC power demand than just about any single vidcard, single CPU system you can build today. Yet, the total DC power demand is just 256W. (Note: It runs perfectly stable with a new very high efficiency 350W power supply from Seasonic.)
  • A high performance A64X2-4800+ dual-core system with a mid-range graphics card and dual drives was also added. Again, the power demand is very modest, falling well under 200W at the AC outlet.
  • In the context of SPCR, dual-graphic card SLI / Crossfire systems are excessive. For the SPCR audience, the heat, power consumption and noise issues around dual graphics card systems makes them difficult to embrace.


This was a topic of discussion in one of the sessions at the Spring 2005 IDF in San Francisco: the concept of choosing a PSU whose efficiency curve is well-matched to the system power load. Such matching can yield incremental improvements in average power consumption and ensure minimum waste heat generation. Correct PSU sizing is very carefully practiced by tier one computer makers concerned with maximum cost effectiveness.

SPCR's own PSU testing has shown that power efficiency in PSUs varies with load, and the load at which the best efficiency is reached varies from model to model. In a given line of PSU models, the maximum and average efficiency tends to be very similar; where the peak occurs depends on power rating. Most PSUs reach peak efficiency between 50~75% loading, tail off a bit at maximum power and drop at least 10% at minimum load.

PSU efficiency data from SPCR review database.
The vertical scale has been truncated for clarity; please see text below for full discussion.

A system that draws ~250W maximum and idles at <100W would be a good match for the 300W PSU shown above.The efficiency power curve of the 600W PSU is better suited for a system than idles at >150W and peaks at >300W. It would be a substantially worse match for the system of the previous example, as the PSU would be operating at a mediocre <75% efficiency in idle, and only just reaching 80% at peak.

From a PSU heat waste point of view, the differences are significant:

At 200W load,

  • the 300W model would generate 44W of heat (18% of 244W AC input);
  • the 600W model would generate 50W of heat (20% of 250W AC input).

At 90W load,

  • the 300W model would generate 23W of heat (20.5% of 113W AC input).
  • the 600W model would generate 32W of heat (26% of 122W AC input).

Using the 600W PSU with this system is an example of incorrect, costly PSU sizing. It is practised most frequently by gaming enthsiasts who are encouraged to believe that greater power capacity is always better. Whether 480W, 550W or >600W PSUs are suitable for system that cannot possibly draw even 250W is a type of question asked almost daily in the SPCR Forums.

The counterpoint to "correct PSU sizing" comes from very high efficiency power supplies with very flat power efficiency curves. The models in the Seasonic S12 series, for example, differ so little in efficiency at the same 65~300W power levels that there's no real power consumption cost in choosing a high power model even if it is going to be used at lower power levels. With a system that only needs a maximum of 200W, the power consumption using a S12-500 or a S12-330 is essentially the same. Aside from the initial cost difference, there's no operational cost due to lower efficiency at lower power levels. With such power supplies, it makes sense to buy higher power capacity than currently needed in anticipation of future component upgrades that will demand more power.

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