PowerSupplyGuide - SH/SC Wiki

How to pick a power supply

April 2007 warning: This guide hasn't been updated in over a year and is now pretty badly out of date. Finding isolated power consumption numbers for individual components is pretty difficult lately. There's also not much of a point in doing it anymore, because pretty much all newer, quality PSUs have more than enough headroom on their +12V rail(s).

If you really have trouble finding a power supply, look at a video card power consumption benchmark on a good hardware site. Find your graphics card, make sure the specs of the review system aren't lower than those of your own system, and take note of the load power number. That should give you a general idea of how much power your own machine will draw. Give yourself 100-200W of headroom for good measure, and you should have a rough target for your power supply.

Generally speaking, a quality 450-500W PSU should have more than enough headroom for most modern systems with one graphics card. Make sure the unit has good reviews on Newegg and elsewhere, and try to find something with an 80% power efficiency rating if possible. PSUs that are cooled with a single 120mm fan are also preferable, because they tend to be quieter than other models. Brand-wise, I'd avoid Antec, since I've heard an unusual amount of complaints about them lately. Seasonic, Corsair, OCZ, and Fortron should be better choices.

Picking parts for a new computer and unsure of which PSU to get? Just upgrading and wondering if your current PSU will handle your new parts? Read this.

Step I - Determine +12v rail output for the PSU

Power for a PSU is split between three rails (+12v, +5v, and +3.3v), and the wattage rating is an aggregate of their output. Unfortunately, many PSU manufacturers inflate their wattage ratings by increasing tolerance for the least used rails (+5v and +3.3v), all the while neglecting the +12v rail which is the most used. You should therefore ignore wattage ratings, and base yourself solely on the +12v rail to judge a PSU's actual power output.

+12v rail output is given as a max value in amperes (A), either on the PSU label or in the PSU's product description ("+12v: 18A", for instance). Note that newer ATX12V 2.0 spec PSUs have two +12v rails, which split power load between the CPU+motherboard (+12v1 rail) and everything else (+12v2 rail). This helps both current stability and power delivery, so you should try to get a PSU with dual +12v rails if possible.

Step II - Ensure accuracy

If you have a $20 PSU that claims 28A on the +12v rail, it's very unlikely that it'll actually be able to put out that much. Use your own judgement there.

Step III - Determine +12v power output for your components

Since power consumption values for processors and video cards are impossible to obtain normally, I've compiled some numbers from these three articles:

(The 7800GTX draws about as much as the 6800 Ultra)

You can also count fixed values for the following components:

7200RPM Hard drive - 2A
DVD/CD drive - 1A
Cooling and motherboard - ~1A

Step IV - Do the math

This is self-explanatory, but here's an example system: Athlon64 3200+, GeForce 6800GT, 3 hard drives, a DVD burner and a DVD-ROM.

From the charts above, we can count 4.2A for the CPU as well as 2.9A for the GeForce. Adding in the fixed values for hard drives and CD/DVD drives gets us the following:

3 Hard Drives = 3 * 2 = 6A
DVD burner = 1A
DVD-ROM = 1A

Lastly, we add 1A for the motherboard and cooling.

That adds up to 16.1A of total +12v current draw, or if we split power for a PSU with dual +12v rails, 5.2A for the CPU+motherboard (+12v1) and 10.9A for the rest (+12v2).

Step V - Give yourself 10% headroom

PSU ratings aren't always accurate, and you don't want to be running your PSU too close to maximum spec anyway.

If the PSU has a single +12v rail
+12v = 16.1 * 1.1 = 17.7A

If the PSU has dual +12v rails
+12v1 = 5.2 * 1.1 = 5.72A
+12v2 = 10.9 * 1.1 = 12A

So your system will need 17.7A on the +12v rail for a single +12v rail PSU, or 5.72A on the +12v1 rail and 12A on the +12v2 rail for a dual +12v rail PSU.

Step VI - Get a quality PSU

If your existing PSU isn't enough, or you're getting a new one anyway, spend at least $40 and get a quality one. You will get more stable voltages, less ripple, higher efficiency, more reliable power output, and longer product life. Good PSU brands include:

- Enermax
- Antec
- Seasonic
- PC Power & Cooling
- Tagan
- Fortron

Typically anything above $40 should be at least decent. Do double-check +12v rail output, though, and don't buy a $100 500W PSU "just to be safe" if you figured you only need 15A on the +12v rail.

Appendix I - unlisted hardware

For video cards, models in the sub-5200FX and sub-9500 Radeon series have no significant power draw on the +12v rail, so you needn't worry about them.

For unlisted A64s and P4s, you can match up values from listed ones (e.g. a 3500+ is between the 3200+ and 4000+, so power draw will likely be around 5.5A).

For AthlonXPs, you can get a rough idea of power consumption by going to this page, grabbing the max TDP in Watts for a given chip and dividing it by 12. If you're unsure, you can probably count 6A tops for any AthlonXP.

Appendix II - 20-pin vs. 24-pin

The latest ATX standard specifies a 24-pin power connector from the PSU to the motherboard, as opposed to 20-pin which has been the standard for the past few years. New motherboards will therefore have 24-pin power connectors, whereas most people's PSUs will only have 20-pin connectors.

20-pin power supplies work just fine in 24-pin motherboards. The 20-pin connector will only go in one way. If you somehow end up having problems, 20-pin to 24-pin adapters are available fairly cheap anyway. You therefore don't need a brand new PSU if your old one is 20-pin and your new motherboard is 24-pin.

Appendix III - supplemental links

http://dansdata.com/psus.htm
http://www.silentpcreview.com/section4.html

Changes

2005-10-20: Added Celeron D/Sempron power consumption values from here.
2005-10-07: Added X2 and P-D power consumption values from here.
2005-07-21: Added long overdue section on 24-pin vs 20-pin compatibility.
2005-05-23: Major changes - rewrote steps I, III and IV for greater clarity, cleaned up a few other things, added Dan's Data PSU article to links.
2005-05-17: Added instructions for PSUs with dual +12v rails.
2005-04-04: Added Venice power consumption figures from here.
2004-11-29: Added Winchester power consumption figures from here.