Choosing the right PSU for your PC

So, just how big a power supply unit does a PC really need? In this post I'll show you how you can work out how big a PSU your system needs and some of the considerations that you need to keep in mind when buying a PSU.
Written by Adrian Kingsley-Hughes, Senior Contributing Editor

So, just how big a power supply unit does a PC really need?  In this post I'll show you how you can work out how big a PSU your system needs and some of the considerations that you need to keep in mind when buying a PSU.

It seems to me that one of the most misunderstood aspects of building or upgrading a PC is the PSU.  I've read more rubbish written about the PSU than almost any other hardware component in the PC (notice that I said hardware there, when it comes to the amount of rubbish written about software, all bets are off!).

Obligatory warning and disclaimer: Messing about with PSUs is potentially hazardous - they can deliver lethal levels of charge.  If you are in any way unsure about what you are doing or how to use your equipment, consult a qualified electrician.

Any PC has a specific amount of power that it needs to draw to work.  What you need to have in mind when choosing a PSU is the average running load that your PC consumes when running.  Peak load can come into play sometimes and this can be significantly higher than the average running load, but as a rule high loads are more likely at start up than any other time.

So, how do you work out your average load?  Well, thankfully the days of having to pull up specs on the individual components are over.  On the web there are a number of really good and up-to-date power supply calculators that you can use.  You just enter the spec of your PC and the calculator will work things out for you.  One of the best online power supply calculators that I've come across is made available by Antec.  By using the simple form you can quickly and easily find out how many watts your PC will need and use this information to choose a PSU that delivers this.

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What the power supply calculator on Antec's site is very good at is making you think not only about the standard components that are inside your PC (CPU, RAM, hard drives and so on) but also non-standard items such as water pumps for a cooling system and cold cathode lights.  Also, it allows you to factor in that you hook up to your PC via USB and Firewire.  The calculator generates some very good data that you can work with and even allows for you to figure in surge compensation and capacitor aging (as the electrolytic capacitors inside the PSU age they lose some of their initial capacity). 

For example, I entered in the information for the following system:

  • Intel Core 2 Duo E6700
  • Four sticks of DDR2 RAM
  • Twin NVIDIA GeForce 8800 GTX SLI
  • Two SATA hard drives
  • One DVD-RW/DVD+RW drive
  • One SoundBlaster sound card
  • Four USB devices drawing power from the system
  • Three 80mm fans

The recommended PSU wattage for the system was 487W.  Factoring in 15% surge compensation and 20% for electrolytic aging and this gave a total of 658W.  For a system with the above specification I would have recommended a 650W PSU, and it seems that I would have been right.

This calculator is also handy for finding out how far you can upgrade a PC that you bought without having to fit a beefier PSU.  I've found that as a rule manufacturers save money by fitting PSUs that are able to cope with the demands that the system places on it but not much more.  This means that if you want to take your Dell or HP PC beyond the spec much, you'll have to bin the PSU.

I know a lot of people who build PCs based on a small PSU.  The idea is that it is cheaper and the PC doesn't waste power.  While it's true that it might be cheaper (a good quality 400W PSU is cheaper than a 600W PSU of equivalent quality), the idea that a bigger PSU is less efficient is false.  The PC will only draw the power it needs and no more.  In fact, having a larger than required PSU can be an advantage since it doesn't need to work as hard to deliver power at the lower end of the scale and it can even allow your PC to ride a very brief power dip without a hiccup.  While I wouldn't recommend putting a 1KW PSU in a PC that only needed 500W, remember that unless you buy a high quality 1KW PSU, it's only really likely to give you, at best, 75-80% of the power they claim anyway (tests show that the failure rate of PSUs at full load is pretty high).  And if you factor in that cheaper PSUs lose efficiency much more rapidly than a quality one as they heat up, it becomes obvious that buying cheap can mean buying twice.

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So how do you spot a good quality PSU?  There are a number of ways.  First off, buy a decent brand name.  No-name PSUs are generally inferior and if you plan on making your PC work for its keep (pushing it hard, leaving it on 24/7) then it's false economy. Another good way to tell a good PSU from a cheap one is weight.  Cheaper PSUs will be lighter and feel flimsier than quality PSUs.  Also, try to get a PSU that has a 12cm (4.8") fan which doesn't need to rotate as fast to keep the PC cool, making it a lot quieter.

Also, remember to check the label on the PSU.  Most good PSUs will list the ratings for the +3.3V, +5V and +12V rails.  You can see an example of this kind of label here on the Newegg site.  These ratings will be optimistic, but provide a starting point.  Make sure that the power rails (especially the 12V rail) gives you ample power.  Also, read the label carefully.  For example, two 12V rails rated at 22A doesn't mean that you've got 44A to play with.  You have to take the total Watts on the +12V rail and divide that by 12 (so on the label I linked to above the 12V rails have a total rating of 384W, so 384W/12V = 32A).

Also, don't be tempted to test the rails using a multimeter - most multimeters can't handle above 10A.  If you want to test the power output either rely on a software monitor which can read the values directly off the board such as Motherboard Monitor (anyone know what's happened to the official Motherboard Monitor site?) or read the values from the BIOS "PC Health" screen, if available (however, be aware that these values aren't always that accurate - I've seen BIOS updates which change them significantly). 

If you know what you are doing with resistors and don't mind doing a little math you can use a multimeter, but be careful not to blow it up!  Unless you know 100% what you are doing, don't try this at home!  It's far safer to test the voltages at the rails - if this fluctuates, you can be sure that current does too.

Also, while talking about PSUs I should probably mention that having a good quality UPS that conditions the power going into your PC is a good idea if you want a stable, reliable system.  Most PCs are happy with input voltages which vary by ±10%, but beyond that things get hairy.  A good quality UPS gives you the piece of mind that under and overvoltages aren't going to trash your system.

Thoughts?  How do you choose your PSU?  Do you buy cheap or do you spend a little extra?

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