The Inner Workings

Think about the function a power supply performs. It converts power coming from a wall socket–120V, 60Hz, alternating current–into energy that your computer can use. Various components utilize different voltage levels, and so the power supply must generate those, as well, delivering ample current across each voltage to drive the connected devices. It's a tough job that's more complicated than a wattage rating would suggest. Modern processors, for example, get their power from a +12V rail (the term used to describe each independent voltage output). Many disk drive motors also use the +12V rail, while AGP graphics cards use +3.3V, and other peripherals operate on +5V power.

Understand, then, that when someone cites the power output of their supply as a single number, they are using the most readily understandable point of comparison, but one that is not very telling. You see, power output can be expressed in a number of different ways. There's peak output, continuous output, and individual output ratings for each rail in addition to a combined maximum for the +3.3 and +5V rails. If you're wondering why you should care, consider that buying a power supply solely on its marketed wattage doesn't tell you anything about its capacity to handle the demands of a high-end Pentium 4 on the +12V rail, along with disk drives and PCI Express graphics cards that also piggyback on the same pipeline.

Naturally, going cheap on such an important component is a sure way to immediately introduce instability, especially on workstations. That doesn't mean you need to encourage 600W power supplies either, though. Using some simple math, a bit of research, and discerning judgment, you can demystify power supplies for you customers and make more educated recommendations.

Lead By Example

Grab the power supply nearest you and turn it over in your hands. Good power supplies are naturally somewhat heavy by virtue of their large capacitors and ample heatsinks. Most come in ventilated steel enclosures, but manufacturers increasingly use lights and windows to add enthusiast appeal, especially on higher-end models. What we're looking for, though, is a sticker on the chassis itself, usually displaying a table that breaks down each unit's vital statistics.

Leader of the Pack Armed with two PCI Express cables and six SATA connectors, PC Power and Cooling’s Turbo-Cool 510 SLI is the Lexus of power supplies.

For each rail, you'll see a current rating, reflected in amperage. Using basic physics calculations, such as P(power) = V(voltage) * I(current), it's easy to figure out how much power can really be delivered. With 34 amps on the +12V rail, you're looking at more than 400 watts of power to drive processors, graphics cards, and disk drives. Let's say the +5V rail boasts 40 amps. There's another 200W of available power. Finally, you have 30 amps on the +3.3V rail, yielding 100W more. While it'd be reasonable to rate such a power supply at 700W or more, there is likely a limitation on the combined +3.3V and +5V output that falls below the sum of both rails independently, scaling that number back to some degree. Consequentially, PC Power and Cooling, Inc., who actually manufactures a unit with those specs, rates its power supply for 650W of peak output.

Just as a car can't operate indefinitely at redline, however, power supplies only sustain peak output levels for 60 or 70 seconds at a time before overheating, according to Eric Nelson, product development manager for OCZ Technology. And that's why PC Power and Cooling sells the Turbo-Cool 510 SLI as a 510W unit, which more accurately reflects its real-world performance. Emphasis on +12V robustness is a fairly recent phenomenon triggered by increasing processor consumption and a new generation of PCI Express graphics cards. Because the Turbo-Cool 510 SLI is designed to drive hardware compliant with NVIDIA's SLI technology, it has to feed up to two GeForce 6800 Ultra graphics cards in addition to the standard-fare components. "If you think it's bad now," says John Nguyen, product manager at Tyan, "just wait until dual-core processors emerge later this year with even loftier requirements." That's right, folks, this issue will amplify in the next six months.

Avoiding Pitfalls

"By an overwhelming majority, resellers prefer buying cases with built-in power supplies," says Scott Richards, worldwide vice president of sales and marketing at Antec. "They're both cheaper to import and easier to implement because they take the guesswork out of purchasing." But while the processor, motherboard, and graphics worlds are rife with recognizable names, there's often little perceived value associated with offering customers top-quality cases and power supplies. It becomes tempting to skimp on one of the most important components, which, unlike the beefy PC Power and Cooling example, are sometimes rated in dubious and misleading ways. Rather than citing continuous power, for instance, culprits will market peak numbers instead. They'll often add up the combined output of each voltage, disregarding the ceilings imposed by yoked +3.3V and +5V or dual +12V rails.

Even when voltages are reflected accurately, generic power supplies notoriously shortchange that critical +12V rail. To illustrate, I randomly chose an inexpensive yet obscurely unique looking case from a prominent online retailer selling for about $40 with a bundled 480W power supply. Though presumably capacious, the power supply was rated for 16A on the +12V rail, a maximum of about 190W. Now build a system consisting of an Athlon 64 FX-55 (104W), two GeForce 6800 GT graphics cards in SLI mode (50W each, conservatively), and a pair of Serial ATA hard drives in a RAID 0 array (about 12W each). Admittedly, your customer may never apply 100 percent load to the whole system, but even at 80 percent, they'd bang right up against the power supply's peak output and that's not a good place to be. Sound far fetched? Consider that NVIDIA suggests between 420W and 480W for a GeForce 6800 GT SLI machine. Yeah, ouch.

Mainstream Reliability Antec's TruePower delivers reliable power affordably and it’s optimized for low noise output.

It's even tough to compare two units with the exact same voltage output ratings. If company A tests their products in a realistic operating environment of, say, 40 degrees Celsius, and company B generates the same rating at 25 degrees, an optimistically low temperature, you can bet that the second power supply will shed efficiency to the tune of 30 percent or so. That's really bad news when you're already pushing a power envelope and one reason Doug Dodsen of PC Power and Cooling says he has his products tested at 50 degrees. Customers know that they're getting a power supply that will continue running, even under more extreme conditions.

Stepping Up To The Plate

So what do you get when you decide to shun those lesser power supplies and tap something a little juicier? First of all, there's the piece of mind that goes along with ample headroom. Strong rails deliver clean, stable power to sensitive components, ensuring optimal reliability. And when a power supply is built to last, its manufacturer is more likely to back it with a generous warranty. OCZ Technology guarantees its PowerStream models for five years, for example. If a product fails in the first three years, the company's PowerSwap program provides for a replacement. After that, OCZ will repair the unit. PC Power and Cooling goes all-out with a flat five-year guarantee on its hardware, while Antec offers three years of warranty coverage on power supplies. Antec is also unique in that it offers direct reseller support through email and a priority phone number. Special reseller rebates are posted to antec.com on a monthly basis, as well.

Beyond the service and support that accompanies premium-grade power supplies, you'll also find features generally geared toward the tweaking crowd. Adjustable rails, for example, make it possible to individually manipulate output voltages. Three LED lights on OCZ's PowerStream supplies indicate if outputs are within the five percent variance range specified by Intel, overpowered, or underpowered, while adjustment dials facilitate fine-tuning to compensate for extraneous loading.

Tweaker's Delight OCZ's PowerStream line offers adjustable voltage outputs for optimal power delivery to your high-end customers.

Another popular trend right now is modularity. Rather than ship with an array of connectors snaking from the power supply itself, many manufacturers are selling the cores and including a box of accompanying cables. Ultra Products' X-Connect is one of the most popular products in that category. And indeed, OCZ also has its own modular model, as well. "Modular power supplies work well for system builders because they minimize loose connections within a case," says OCZ's Eric Nelson. At the same time, though, they introduce one more point of failure should one of the add-on cables come unplugged. More of a novelty than anything right now, you should at least keep an eye on the expanding modular power supply market.

Finally, the most prominent power supply manufacturers take special care to follow industry trends. Are you using more Serial ATA hard drives this year compared to last? The best power products will make concessions for plenty of native Serial ATA connectivity, minimizing the number of adapters you're forced to add. Seeing a surge in PCI Express graphics sales? Rather than monopolizing two large, four-pin power plugs on another adapter, grab a power supply that natively offers the six-pin PCI Express auxiliary power interface required by most high-end video cards. And, of course, if you plan to offer early adopters bleeding edge hardware centering on SLI graphics, double check that you have two of those PCI Express plugs and enough juice on the +12V rail to maintain stability.

In Retrospect

"When we started advertising our power supplies a few years back, many people didn't know the difference between them and UPS backup systems," says Scott Richards of Antec. It's truly amazing how little emphasis is placed on the quality of such an important component, since power supplies are key in defining overall stability.

The story is changing now, in part because Intel and AMD processors consume an increasing amount of power, despite shrinking manufacturing techniques. New graphics technologies are to blame (or thank, depending on your point of view), as well. And the ever-pervasive quest for more performance has prompted the enthusiast community to seek out flashier units with greater output potential.

At the end of the day, power supplies aren't glamorous or sexy, and you certainly won't sell systems because you opt for higher-quality units. However, you will minimize support calls from power-related stability issues and maximize the flexibility of your customers' machines.