Rated capacities
We've already discussed the total output wattage rating of each of the power supply units in the spotlight today, but that's only one component of the output rating story. Modern PSUs divide power across three main lines at 3.3, 5, and 12 volts, with each of those lines carrying a maximum current rating. As if that weren't complicated enough, most PSUs also spread 12V power over multiple individual lines.

There are exceptions, of course. Corsair's TX and PC Power & Cooling's Turbo-Cool both consolidate 12V power into a single line rather than spreading it over multiple rails. Most of the others spread 12V power over four rails, usually with identical maximum current ratings for each. Tagan takes things one step further, distributing its 12V power across a whopping six rails, each of which carries a maximum current rating of 20 amps. Ultra's X2 sits at the other end of the spectrum with only two 12V lines.

Don't get too married to these current ratings, though; they're only the tip of the iceberg. To get a handle on each PSU's true capacity, we have to determine the maximum output wattage of each voltage line. The math is easy enough thanks to Ohm's Law, which allows us to calculate wattage given voltage and amperage. However, power supply units with multiple rails are typically limited by how much power can be spread across those multiple lines—a total that's usually less than the sum of each rail's output capacity. Most PSUs also place limitations on how much power can be shared between the 3.3V and 5V lines. And then there's the maximum output wattage across the 3.3, 5, and 12V lines, which often falls short of the PSU's total output rating in order to reserve capacity for lesser-used voltage lines like the -12V and 5V standby rails.

To make sense of it all, we've put together a table showing the maximum output power for each PSU's 3.3, 5, and 12V rails. Where applicable, we've also indicated the maximum combined 3.3 and 5V power, the maximum combined 12V power, and how much wattage the PSU can spread across all three main rails.

Now that's a mean-looking table.

Some PSUs, such as Super Talent's Atomic Juice, don't set maximum combined wattage ratings for their multiple 12V lines. That's why the Atomic Juice's 864W combined 12V output looks so formidable, until you look at the PSU's combined 3.3, 5, and 12V output, which is only rated for 680W.

With a kilowatt of power to distribute, Antec's TruePower Quattro is predictably the most generous when we look at combined output ratings. The Quattro's maximum combined 3.3 and 5V output of 200W is the highest of the lot, and its 840W of combined 12V output is eclipsed only by the Super Talent's lack of a combined 12V output rating.

Interestingly, three of the PSUs we've gathered—the Type R, Turbo-Cool, and BZ—are rated for 768W of combined 12V output. The Odin GT and Corsair TX also offer combined 12V power in the 700W range, with the Ultra X2 falling well short with a maximum 12V output rating of only 600W. The X2 does carry a higher output rating for its combined 3.3 and 5V output than one might expect, however. When it comes to 3.3 and 5V power, the Turbo-Cool finds itself at the back of the pack with Super Talent's Atomic Juice.

Lining up maximum combined wattages is useful for determining how each PSU biases its output capacity. It also gives us a handy reference to draw from when determining test loads that will stress each power supply to its limits.