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Rail by rail
Although a power supply's total wattage can be helpful, it's more interesting to look at how current and wattage is spread across each voltage rail. I've summarized current and power maximums for each power supply below.

Unfortunately, manufacturers don't necessarily provide max current and power ratings for the same AC input voltage. Antec, Enermax, OCZ, and Zalman's numbers correspond to the full range of AC input, but Vantec only provides numbers for 115V AC, and Ultra only for 230V AC input. Silverstone provides numbers for both, and there's no telling what our generic SH power supply's maximums correspond to.

Maximum output current (A)

DC Output +3.3V +5V +12V
Antec NeoPower 30 38 18, 15
32
Enermax EG485P-SFMA24P 34 40 16, 15
OCZ PowerStream 28 40 33
SH SH-ATX465P4 38.8 44.5 22
Silverstone SST-ST30NF (100-120V) 21 20 17
30
Silverstone SST-ST30NF (200-240V) 25 23 18
35
Ultra X-Connect (230V) 28 30 34
Vantec Ion 2 (115V) 14 30 15
Zalman ZM400B-APS 28 40 18

With some of the PSUs listing max output current for different input voltages, it's hard to compare numbers across the board. There are a few things worth noting, though. First, both the Antec and Enermax PSUs sport dual 12V rails. Antec's dual 12V rails combine for a max output current of 32A—one amp lower than the sum of their individual max currents.

Also note that current across the Silverstone PSU's +5 and +12V rails is capped between 30 and 35A, depending on the input voltage. Components won't be able to draw maximum current from both rails at the same time. Overall, the Silverstone's +5V max currents are also pretty low. Notice, also, the relatively low +3.3V max current on the Vantec Ion 2. That PSU's 12V rail tops out at a low 15A, too. The Zalman PSU's +12V rail looks a little weak, as well, in comparison with its relatively strong +3.3 and +5V rails.

Finally, check out the monster +3.3V and +5V rails on our generic SH PSU. Perhaps they should have diverted some of that current to the +12V line, which looks weak by comparison.

With a little help from Ohm's law, it's easy to translate maximum currents to maximum output power. However, there are some additional limits on what the PSUs can handle with combined +3.3V and +5V loads. Let's look at those limits.

Maximum output power (W)

DC Output +3.3V +5V +12V
Antec NeoPower 100 190 216, 180
384
460
Enermax EG485P-SFMA24P 113 200 192, 180
280
480
OCZ PowerStream 92.4 200 396
500
SH SH-ATX465P4 128 222.5 264
350
465
Silverstone SST-ST30NF (100-120V) 70 100 204
140
250
Silverstone SST-ST30NF (200-240V) 83 115 216
180
300
Ultra X-Connect (230V) 93 150 408
200
480
Vantec Ion 2 (115V) 47 150 180
180
323
Zalman ZM400B-APS 93 200 216
235
380

The table above may be a little confusing, so allow to me illustrate how to read it with an example. All the numbers you see are max wattages, and some of the cells in the table extend across multiple columns because of shared maximum capacities. The Enermax unit can supply a maximum of 113W on the +3.3V rail and 200W on the +5V rail, as the table shows. However, the two rails cannot together exceed 280W of total output. The Enermax also has two +12V rails, one of which can output 192W while the other peaks at 180W. Combined, the four rails of the Enermax PSU can supply a maximum total of 480W. The bottom, shared cell for the Enermax that extends across all three columns delineates this total shared max capacity.

Clear as mud?

You should be able to read the specs for the other PSUs using the same basic methods. Shared cells mean shared max capacities, and not every PSU is arranged the same, so not every entry in the table is organized the same. Note that the Antec and OCZ PSUs are distinguished by their lack of shared capacity between the +3.3V and +12V rails; both can max out their +3.3V and +5V rails simultaneously.