Debunking Power Supply Mythsby Christoph Katzer on September 22, 2008 3:00 AM EST
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Anyone building a computer system should eventually pose the question: How much power does the system actually require? This is an important consideration, since it's impossible to choose an appropriate power supply without actually knowing the demands of your system. Unfortunately, many users take the easy way out: just grab a 500W power supply and call it good. If you really want to be safe, you can even grab on 800W PSU... or if you plan to run multiple graphics cards perhaps you really need a 1000W unit, right?
If people really took the time to examine system power requirements, we would see a tremendous increase in sales of 300W to 400W PSUs. The truth is that the vast majority of systems would run optimally with such a "small" power supply. Even if you're running SLI/CrossFire, you don't actually need a 750W power supply. (Of course, we recommend purchasing a good quality power supply, as there are certainly "750W" PSUs out there that can't reliably deliver anywhere near that much power.) To help dispel some myths relating to power requirements, we've put together a couple of charts.
|GPU Power Consumption*|
|NVIDIA GeForce 9600 GT||49W||107W|
|NVIDIA GeForce 8800 GT||64W||115W|
|NVIDIA GeForce 9800 GTX||79W||116W|
|NVIDIA GeForce 9800 GX2||90W||179W|
|NVIDIA GeForce 8800 Ultra||100W||186W|
|ATI Radeon HD 3650||17W||32W|
|ATI Radeon HD 3850||53W||82W|
|ATI Radeon HD 3870||62W||92W|
|ATI Radeon HD 2900 XT||67W||104W|
|ATI Radeon HD 3870X2||55W||130W|
* Actual power consumption for the graphics cards only. Results taken at idle on the Windows desktop and under full load running the Fur benchmark.
|CPU Power Consumption**|
|Manufacturer||Idle (EIST or CnQ Enabled)||Idle||Load|
|Intel Core 2 Duo E4500||14W||17W||36W|
|Intel Core 2 Duo E8500||18W||22W||43W|
|Intel Core 2 Quad Q9550||19W||23W||60W|
|Intel Core 2 Extreme QX6850||29W||32W||103W|
|Intel Core 2 Extreme QX9770||26W||56W||86W|
|AMD Athlon 64 X2 5000+||33W||47W||89W|
|AMD Athlon 64 X2 6000+||25W||74W||160W|
|AMD Phenom X3 8750||50W||67W||86W|
|AMD Phenom X4 9600 BE||29W||36W||101W|
|AMD Phenom X4 9850 BE||38W||53W||126W|
** Actual power consumption for just the processor. Results taken at idle on the Windows desktop with either EIST/C&Q enabled or disabled, and full load generated using BOINC.
|Chipset/Motherboard Power Consumption***|
|Platform and Chipset||Load|
|Intel P35 (775)||37W|
|Intel P965 (775)||39W|
|Intel X38 (775)||52W|
|Intel X48 (775)||40W|
|NVIDIA 680i (775)||46W|
|NVIDIA 790i (775)||51W|
|NVIDIA 750i (775)||59W|
|NVIDIA 780i (775)||69W|
|NVIDIA 8200 (775)||29W|
|AMD 690G (AM2)||34W|
|AMD X3200 (AM2)||35W|
|AMD 770 (AM2)||40W|
|NVIDIA 570 (AM2)||40W|
|AMD 790FX (AM2)||42W|
|AMD 790X (AM2)||43W|
*** Actual power consumption for the motherboard and chipset. Idle and load power do not differ by any significant amount.
Top-end graphics cards are clearly one of the most demanding components when it comes to power requirements in today's systems. Only heavily overclocked CPUs even come close to the same wattages. Note that the above chart only includes last generation cards; NVIDIA's latest GTX 280 requires even more power.
Looking at the processor side of the equation, Intel's Core 2 Duo/Quad/Extreme CPUs in general have very low power requirements. AMD's latest Phenom processors aren't far behind, however, especially in light of the fact that they include the memory controller rather than delegating the task to the chipset. We should also mention that part of the reason for the extreme power requirements on the X2 6000+ come from the use of an older 90nm process.
Naturally, motherboards also require a fair amount of power. Current motherboards average around 47W for socket 775 and 39W for socket AM2/AM2+, but features and other factors can heavily influence that number. Outside of their IGP solution, NVIDIA's chipsets tend to use more power than the competition; AMD chipsets on the other hand typically require less power. Again, numerous other aspects of any particular motherboard will impact the actual power requirements, including BIOS tuning options.
Hard drives and optical drives account for another 10 to 20W each. However, remember that hard drives are a relatively constant 10 to 15W of power draw (average is around 12W) since the platters are always spinning (i.e. idle), and movement of the drive heads during read/write operations (i.e. load) only increases power draw slightly. Optical drives on the other hand stop spinning when idle, requiring only about 5W, while during read or write operations they need around 18W.
RAM power requirements measured a constant 2W per DIMM, regardless of capacity (though clearly not including FB-DIMMs). That figure is estimated, unfortunately, as we could not measure DIMM power requirements directly; we measured power draw with two DIMMs and then again with four DIMMs to arrive at the reported figures. It's also not possible to easily separate memory power requirements from the motherboard and chipset, as they share many of the same power connections from the PSU.