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Rated capacities
We've already discussed the total output wattage rating of each of the PSUs 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, many PSUs also spread 12V power over multiple individual lines.

 Maximum output current (Amps) DC Output +3.3V +5V +12V Corsair HX750W 750W 25 25 62 Enermax Revolution85+ 850W 25 25 30, 30, 30, 30, 30, 30 Seasonic X Series 750W 25 25 62 XFX Black Edition 850W 24 30 70

Of these four models, only the Enermax splits its 12V line between multiple rails—a whopping six of them, to be exact. The rest of the units consolidate their 12V power on single rails.

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 Corsair HX750W 750W 82.5 125 744 150 750 Enermax Revolution85+ 850W 82.5 125 360, 360, 360, 360, 360, 360 160 840 850 Seasonic X Series 750W 82.5 125 744 125 750 XFX Black Edition 850W 79.2 150 840 150 850

Interestingly, the Corsair and Seasonic PSUs have identical output ratings for their individual 3.3, 5, and 12V lines. However, Corsair is more optimistic about how much power can be shared between the 3.3 and 5V rails. The maximum sustained output across all three rails is identical at 750W, though.

The Revolution85+ has a little more combined 3.3 and 5V capacity than the 750W units. About 100W of additional 12V output power is also spread over its half-dozen rails. That total 12V capacity neatly matches the XFX Black Edition, whose single 12V line can likewise handle up to 840W. The Black Edition actually has a less powerful 3.3V rail than even the 750W units, though. It also has less combined 3.3 and 5V power than the Enermax PSU.

As one might expect from a collection of high-end PSUs with loads of PCI Express graphics card connectors, there's a clear bias toward 12V power. Each model is capable of reaching very close to its total output wattage on just the 12V line alone. CPUs and graphics cards demand more from the 12V line than any other component, a fact clearly not lost on these vendors.