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Eight power supplies compared

There really is a difference

I'VE EXTOLLED the virtues of quality power supplies so many times that saying they're the most often overlooked PC component feels like a cliche. But it's true. In my experience, poor quality generic-brand power supplies are often the root of PC stability problems. Also, as new processors and graphics cards demand more power than ever before, quality power supplies are becoming more important.

But what makes a good power supply, anyway? Most assume that power supplies carrying higher wattage ratings are superior, but as the collapse of the so-called "megahertz myth" illustrates, higher numbers aren't always better. First and foremost, a power supply should deliver clean, consistent power to system components. Power supply efficiency is also important; an efficient power supply can save you money on every electricity bill, especially if you have your system running 24/7. Environmental variables like temperatures and noise levels matter, too.

Armed with a couple of test systems, temperature probes, noise level and power consumption meters, an oscilloscope, and an all-important bathroom scale, we can test all those metrics, plus a few others, ourselves. There's no need to rely on manufacturer spec sheets, marketing claims, or wattage ratings. Are cheap, generic power supplies really that much worse than high-end models that cost twice as much, or more? And among those high-end power supplies, is there really much difference from manufacturer to manufacturer? Let's find out.

Comparing specs
Here's how the power supply units (PSUs) we'll be looking at compare in terms of specifications. By the way, the SH-ATX465P4 is a no-name unit I picked up at a local shop for $40 for the sake of comparison.

PSU Wattage PFC Fans Warranty Price
Antec NeoPower 480W Active 1 120mm 3 years $110
Enermax EG485P-SFMA24P 485W Passive 2 80mm, 1 blower 3 years $129
OCZ PowerStream 520W Passive 2 80mm 5 years $130
SH SH-ATX465P4 465W NA 2 80mm NA $40
Silverstone SST-ST30NF 300W Active None 3 years $155
Ultra X-Connect 500W None 2 80mm 1 year $99
Vantec Ion 2 350W None 1 120mm 3 years $45
Zalman ZM400B-APS 400W Active 1 80mm 1 year $87

I should mention that we also invited RaidMax, Thermaltake, and PC Power & Cooling to participate in this comparison, but they either declined or didn't respond to our requests.

We're looking at a full range of power supply wattages—from 300W all the way up to 520W. Conventional wisdom would probably suggest that more wattage is better, but as we'll soon see, the PSUs with lower wattage ratings hold their own.

In addition to a range of wattages, we also have several power factor correction (PFC) implementations. Power factor (PF) is the ratio of true power to apparent power where true power refers to power that's actually being consumed and apparent power is calculated by multiplying the voltage by the current being drawn. If voltage and current are out-of-phase, apparent power will be greater than real power. This is a common condition for circuits that contain capacitors and inductors—power supplies being a perfect example—and it results in a PF less than 1. PFC aims to bring voltage and current back into phase, raising the PF towards a goal of 1, where real power equals apparent power.

In their quest for a more ideal power factor, the Antec, Silverstone, and Zalman power supplies employ dedicated circuitry to correct out of phase voltage and current. This is referred to as active PFC. The Enermax, and OCZ power supplies use passive components like capacitive filters to correct power factor, which is appropriately referred to as passive PFC. In general, active PFC results in a higher power factor than passive PFC, which in turn results in a higher power factor than no PFC at all. But does having a higher power factor really do you any good?

Yes and no.

If you're a large business running banks of machines, the power company may charge you for the apparent power draw you present to the grid rather than your real power consumption. In this case, a lower power factor can cost you money over time. If you're running an uninterruptible power supply (UPS), a lower power factor can also result in shorter run times off battery. However, if you're a regular home user with no UPS and no power factor-related charges on your electricity bill, PFC isn't going to do anything for you. It will, however, make your local power company happy; higher client power factors allow for more efficient distribution of power across the grid.

Speaking of efficiency, overall power supply efficiency is also something to consider. In this case, efficiency refers to the percentage of real power drawn by the power supply that's actually delivered to system components. More efficient power supplies will draw less real power to run a system, saving even home users on electricity costs. More efficient power supplies should also run cooler since the percentage of real power drawn that isn't delivered to system components is radiated as heat. Not all power supply manufacturers publish efficiency specs, but we'll be able to compare the relative efficiency of the power supplies we're looking at by measuring real power consumption at the outlet.

Our stack of power supplies offers plenty of variety on other fronts, like cooling. At one end of the spectrum, there's a passively cooled unit from Silverstone. At the other, we have an Enermax unit with two fans and a blower. And, of course, there are a number of single and dual-fan implementations in between. We'll see how different cooling designs impact system temperatures and noise levels in a moment.

On the warranty front, three years seems to be the consensus. There are a few deviations, though. OCZ deserves extra praise for offering a generous five-year warranty, while Ultra and Zalman should be scolded for their stingy one-year warranties. Since a quality power supply can potentially handle a few years worth of system upgrades, a good warranty is essential.

Finally, we come to price, where we have quite a spread. There's a $115 gap between our cheapest and most expensive PSUs and plenty of models to fill in the gap. It's interesting to note that our two lowest wattage PSUs are so far apart in price. Vantec's 350W Ion 2 retails for only $45, while Silverstone's passive 300W PSU sells for more than three times that.

Wait, there's more. Each of the PSUs we're looking at also has a different mix of power connectors. Rather than go through them all individually, I've listed them in a handy chart below.

PSU Main power 8-pin EPS 6-pin PCI-E 6-pin AUX 4-pin P4 4-pin peripheral SATA 4-pin floppy
Antec NeoPower 24-pin 0 1 0 1 6, 2 2, 2 0*
Enermax EG485P-SFMA24P 24-pin 0 1 0 1 7 4 2
OCZ PowerStream 24-pin 1 0 1 1 6, 2 2 2
SH SH-ATX465P4 20-pin 0 0 1 1 6 2 2
Silverstone SST-ST30NF 20-pin 0 0 1 1 6 2 2
Ultra X-Connect 20-pin 0 0 1 1 8 0** 1
Vantec Ion 2 20-pin 0 0 1 1 6 2 2
Zalman ZM400B-APS 20-pin 0 0 1 1 7 2 2
*The NeoPower comes with a splitter that yields two four-pin floppy connectors, but it will use up one of the PSU's four-pin peripheral connectors
**Ultra X-Connect owners are entitled to a free SATA splitter that serves up two SATA power connectors at the expense of one Molex peripheral connector

Note that the OCZ PowerStream is the only power supply of the lot with an eight-pin EPS power connector for high-end server and workstation boards. The Antec and Enermax PSUs are the only ones with six-pin PCI Express power connectors. The Antec, Enermax, and OCZ power supplies are also the only ones with 24-pin main power connectors. 24-pin power connectors are common on dual-processor motherboards and many new LGA775 boards, but for more common 20-pin boards, the 24-pin power supplies ship with 24-to-20-pin adapters.

I'll explain why the Antec and OCZ PSUs have two different connector counts listed for some of their power connectors when I discuss those power supplies individually.