October 25, 2009

We take a break from our overclocking series this week to take a look at an important component of every computer build; the power supply unit, or PSU. While at first glance this component may seem pretty straightforward, there are actually quite a few nuances that make selecting the best power supply a little tricky. It is not enough to simply pick the one with the most power that you can afford; rather, there are some key specs that you should watch for and select based upon.

As with most components, one of the most important considerations is brand reputation. While it is possible to get a bad power supply from any manufacturer (it sometimes just happens), selecting a PSU made by a quality, well-known company with a good warranty and reputation will greatly decrease the chances of receiving a DOA unit, due to the resources they devote to quality control during the manufacturing process. Corsair by far consistently gets the best ratings and reviews, and makes some exceptional power supplies. Some other leading manufacturers include PC Power & Cooling, Antec, and OCZ. However, this doesn't mean you have to pay a ton for a quality power supply. With the abundance of sales and mail in rebates available, it is not too difficult to find an excellent PSU for a good price.

Once you have established a budget and have narrowed it down to a few good brands to look at, it is important to evaluate your needs and select the best PSU you can afford based on that. First it is necessary to decide how much power your system actually needs. An excellent PSU Calculator can be found here, or you can do an online search for a similar calculator and get a second opinion. Newegg's calculator can be found here. Select the components you have or will use for your build, and then consider giving yourself a little headroom for future upgrades. For modern computer systems, 400W - 500W will be more than adequate, unless you are planning on running multiple video cards in SLI or Crossfire configurations for gaming. If so, add 150W - 200W per video card.

There are only a few other key considerations to look at. If you are planning on running multiple video cards as mentioned above, you also need to make sure your power supply has enough cables to support it. While it is possible to use adapters in some situations to give you the extra connectors you need, it is better by far and will result in more stable power distribution to simply select a PSU that has the required number of connectors to begin with. Most video cards use the PCI-E cable (check your documentation to be sure), so you will want to ensure the PSU you select has enough of these cables to accommodate the number of video cards you plan to run. Additionally, some power supplies are SLI/Crossfire "Ready" or "Certified" and it may give you more peace of mind to select one of these if you are planning on running such a configuration.

Another important point is to ensure that there are enough amps on the main +12 Volt rail to support your system. The explanation of what that means and why is pretty technical, but what it means to you is that when you are looking at the specs for your shiny new PSU, you want to make sure you have one with a high amp rating on the +12 Volt rail. This will usually be listed in the detailed specifications of the item description. While you'll get a number of different opinions on this, in general 20 amps is a good number to shoot for, and I would be hesitant to put anything less than 18 in anything I build. Where this comes into play is with the popular multi-rail supplies which advertise 4 or more +12V rails. While this sounds great at first, when you dig deeper you usually find out that each rail is only rated at 14 amps, which can cause instability in any system and would definitely not be sufficient for your high-tech video card. Find one with a single +12V rail rated at 20 amps or more (30 to 40 amps are not hard to find). The image the right is from a Newegg product page and shows what you are looking for.

Finally, there is the debate on modular versus fixed power supplies. Modular PSU's allow you to only connect the cables you need to use, which results in a much cleaner installation and build. Fixed cable supplies don't give you this flexibility, but have less resistance and result in larger amounts of cleaner, more stable power being supplied to your system. I use fixed cable systems in all of my builds, but the choice is up to you.

If you are worried about energy efficiency it is possible to buy more efficient power supplies, including ones that are certified "80 PLUS" or more, which means they are always at least 80% efficient. Make sure the power supply you buy also has enough power cables for the number of hard drives and optical devices you plan to connect, and finally, make sure the dimensions will fit inside your chosen computer case. : )

(Images courtesy of Newegg.com.)