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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.

 Maximum output current (Amps) DC Output +3.3V +5V +12V Antec Basiq 350W 20 20 10, 13 Corsair VX450W 450W 20 20 33 Enermax MODU82+ 425W 20 20 22, 22, 22 FSP Blue Storm II 400W 30 30 14, 15 OCZ ModXStream Pro 500W 25 25 18, 18 SolyTech SL-C350ATX 350W 28 28 19 Tagan Silver Power SP-SS400 400W 20 20 17, 17

Only the MODU82+ spreads its 12V power across three lines, with most of the other PSUs opting for dual 12V rails. The Corsair and SolyTech are exceptions, consolidating all their 12V power on a single line. For the latter, the maximum current is only 19A.

Don't get 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.

 Maximum output power (W) DC Output +3.3V +5V +12V Antec Basiq 350W 66 100 120, 156 130 276 350 Corsair VX450W 450W 66 100 396 130 450 Enermax MODU82+ 425W 66 100 264, 264, 264 120 396 425 FSP Blue Storm II 400W 99 150 168, 180 152 348 400 OCZ ModXStream Pro 500W 82.5 125 216, 216 150 432 500 SolyTech SL-C350ATX 350W 92.4 140 228 215 330 Tagan Silver Power SP-SS400 400W 66 100 204, 204 130 360 400

You've probably had enough of tables, but this one's important. You see, while some of these PSUs have lofty amperage aspirations, it's the combined wattage ratings that really matter. For example, despite its single 12V rail, the Corsair VX450W is capable of supplying 396W of 12V power. That nicely matches the 12V output capacity of the MODU82+, but pulls up just shy of the 432W offered by the ModXStream. The Antec and SolyTech units have much lower effective 12V capacities.

Even though it doesn't offer much in the way of 12V power, our generic entry's 3.3 and 5V output capacity is quite beefy. In fact, it has more combined 3.3 and 5V power than any other PSU here. Despite the PSU's 350W total output, it's only rated to supply up to 330W across its 3.3, 5, and 12V lines.

We generally like to see PSUs bias their output toward the 12V line—that's what processors and graphics cards tend to hit the hardest—so the SolyTech's preference for 3.3 and 5V power is a little puzzling. All of the other PSUs we're looking at today favor the 12V line, with rated wattages at least two times the combined output power of their 3.3 and 5V rails. As for actual combined 3.3 and 5V wattages, the rest of the field is pretty tight, with four units offering between 120 and 130W and the remaining two checking in with about 150W.