Oh, the lowly PC power supply. Of all a PC's components, it gets the least respect. Why? Because it's not very sexy. PC do-it-yourselfers may covet their neighbors' faster processor or high-end graphics card, but they rarely compliment them on their power supply's ample +12 volt rail. Maybe they should.

How many times have I heard somebody describe their new rig this way? "Dude, I got an Intel 3.2-GHz P4, 1GB of Kingston HyperX RAM, an ATI Radeon 9800 XT graphics board, two 10,000-rpm hard drives, and an 8X DVD-R drive."

Okay, although I live in California, I don't know that many people who say "dude." Still, when I ask them about their power supply, they often respond along these lines: "I totally got a good deal--I got my case and the power supply for $29.95."

Dude, that's not a deal; it doesn't even make sense. You just spent a small fortune on the best PC parts you can buy, and now you're powering them with some no-name power supply--specs unseen--that came with your cheapo case? Let me put it another way: Would you drop a Ford Pinto engine into your Lamborghini? Assuming your answer is no, then why would you do that to your beloved new PC?

Power Supply Blues

I speak from experience. A few years back I started experiencing frequent hangs and occasional reboots on my then-two-year-old whitebox PC. Like any red-blooded American I blamed Windows ME for my problems (darned Microsoft OS). It wasn't until I was digging around inside the case that I noticed the power supply was a 200-watt piece-o-junk.

Cue the light bulb.

Basically, the PC was underpowered when the vendor shipped it. Then, over the course of several years, I had added more components: an additional hard drive here, a second optical drive there. The poor little bugger never had a chance.

So I went out to buy a replacement power supply--and that's when I realized just how confusing the process of finding the right one can be. Sure, you could just buy the unit with the highest wattage listed on the box. If you buy from high-quality vendors like Antec, Enermax, or Zalman (to name a few) you'll probably be just fine. But I'm a geek, and I want to get exactly the right power supply. Of course, to do that you need to dig a little deeper.

Watts the Problem?

The best way to pick the right power supply is to compile a detailed list of the components you're putting into your new PC. Then you add up the various power requirements, including the maximum number of watts each can use, and how much juice each component draws from the power supply's three major rails (or outputs): +3.3, +5, and +12.

For example, an Advanced Micro Devices motherboard might require a maximum of 23 watts while drawing 3 amps on the +3.3 rail, 2 amps on the +5 rail, and 0.3 on the +12 rail.

It's nearly impossible to find these power specifications for many components. This is very annoying to Type-A system builders like myself. I can understand why retailers don't put the specs in their ads--no need to bore everyone--but at least the vendors could list them somewhere on their Web site.

Barring a movement by component vendors to placate my power-supply specification needs, I offer up a helpful but aging PDF document from AMD. There's a helpful worksheet on page 12, and some basic specifications on page 13 can help you get a ballpark figure. (Note: Figures for power-hungry items like your processor and graphics cards will likely be higher than those listed in the document.)

Unfortunately, neither AMD nor Intel has more specific, up-to-date information these days, offering instead basic suggestions like AMD's Athlon 64 Power Supply Guidelines and Intel's guide to selecting system components.

When totaling your numbers, round up. Given that it's rare for all of a system's components to draw their maximum power requirement simultaneously, your final number should skew a little high. But as with your car engine, you don't want your power supply revving at full speed all the time.

When you're done, you should have a big round number for total wattage as well as total amps for each rail. That second set of numbers is key, as a power supply that meets your overall wattage needs may not match up when you look at its maximum output for the +3.3, +5-, and +12 volt rails.

Happily, most power supply vendors list detailed specifications. For an example of how some manufacturers display their power-supply specifications, check out the details for Antec's True430. It's in these details that the higher-quality units begin to stand out from the rest.

Pick a Winner

You'd think that from here it would be an easy matter of matching up specs. But there are many other factors to consider, tops among them the fact that even the best power supplies don't output the wattage listed on the box. According to one professional system builder I consulted, even the best units from the best companies will likely fall short of their advertised spec by 15 percent or more. Depending on your level of skepticism, figure most power supplies will offer anywhere from 65 to 85 percent of their listed power.

Once you've done that final bit of math (that's it, I promise), consider the following short list of additional details to ponder before buying.

• Warranty: No-name brands may not offer one at all. You'll likely pay more for products from name-brand companies, but you'll generally get better quality from those willing to back their products such as Enermax (one year) and Antec (three years). Higher-end units are also more likely to provide more stable voltages on their different rails.
• One fan or two? A two-fan power-supply (one drawing in air from the bottom; one pumping air out the back) aids with air flow in the case, but some argue that the warm air puts extra strain on the unit. Some power supplies let you adjust the fan speed to reduce noise when maximum cooling isn't required; others use ultra-quiet fans or even heat sinks to further eliminate noise.
• Cables and connections: If you're running a Serial ATA hard drive, look for a newer power supply with jacks to power it. And watch cable lengths, particularly if you're using a three-foot tall tower. That third hard drive isn't much fun without any juice.

Finally, go read some reviews: PC World hasn't traditionally offered much in the way of power supply write-ups, so I'm forced to point you elsewhere. I recommend reading customer comments about specific products at builder-friendly sites such as NewEgg.com. The contributors are generally savvy, and careful reading can point you in the right direction.

What's your take on power supplies? (And don't write to tell me that you're all for them.) Ever gone on a quest for the best, or suffered at the hands of a particularly shoddy model? Have some additional tips to offer? Drop me a line--I'd love to hear more of your stories. And thanks to all who wrote in to talk about their own upgrade fiascos.