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Rated capacities
Each power supply comes with a total wattage rating, but there's more to it than that. Power is divided among three primary lines at 3.3, 5, and 12 volts, and each of those lines carries a maximum current rating. To complicate things further, all but PC Power & Cooling's Silencer spread 12V power over multiple rails. Instead of opting for multiple 12V lines, the Silencer offers a single 12V rail with a massive 60-amp capacity.

 Maximum output current (A) DC Output +3.3V +5V +12V Antec EarthWatts 500W 24 24 17, 17 Antec Neo HE 550W 24 20 18, 18, 18 Antec TruePower Trio 650W 24 24 19, 19, 19 Cooler Master Real Power Pro 550W 25 20 19, 19, 19 Cooler Master Real Power Pro 650W 25 25 19, 19, 19 Corsair HX 620W 24 30 18, 18, 18 Enermax Infiniti 720W 25 30 28, 28, 30 OCZ GameXStream 700W 36 30 18, 18, 18, 18 PC Power & Cooling Silencer 750W 24 30 60 Seasonic S12II 500W 24 24 17, 17 ThermalTake Toughpower 700W 30 28 18, 18, 18, 18

Amperage is only the beginning, though. Through Ohm's Law, we can determine the maximum wattage for each line, giving us an indication of actual output capacity.

Except it's not quite as simple as multiplying a line's voltage by its maximum current. PSUs with multiple 12V rails are limited by how much power can be spread across those multiple lines, and most units also limit how much power can be shared across the 3.3V and 5V lines. Then there's the total output wattage across the 3.3, 5, and 12V lines, which doesn't always add up to the maximum wattage of the power supply. Some PSUs reserve a portion of their total wattage capacity for lesser-used voltage lines like the -12V and 5V standby rails.

To make sense of it all, we've put together a handy 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 EarthWatts 500W 79.2 120 204, 204 130 408 500 Antec Neo HE 550W 79.2 100 216, 216, 216 504 550 Antec TruePower Trio 650W 79.2 120 228, 228, 228 624 650 Cooler Master Real Power Pro 550W 82.5 100 228, 228, 228 141 432 550 Cooler Master Real Power Pro 650W 82.5 125 228, 228, 228 191 540 650 Corsair HX 620W 79.2 150 216, 216, 216 170 600 620 Enermax Infiniti 720W 82.5 150 336, 336, 360 160 672 720 OCZ GameXStream 700W 118.8 150 216, 216, 216, 216 155 680 680 PC Power & Cooling Silencer 750W 79.2 150 720 170 750 Seasonic S12II 500W 79.2 120 204, 204 130 408 500 ThermalTake Toughpower 700W 99 140 216, 216, 216, 216 180 672 700

These are the power delivery stats that really matter, and as you can see, the units vary quite a bit. Interestingly, only Antec's Neo HE and TruePower Trio make no mention of limits on combined 3.3 and 5V power. Among the others, CoolerMaster's 650W Real Power Pro is surprisingly the most generous, boasting 191W of combined 3.3 and 5V output. ThermalTake's Toughpower comes a close second at 180W, followed by units from Corsair and PC Power & Cooling. The 500W units from Antec and Seasonic predictably round out the low end of the spectrum with combined 3.3 and 5V output ratings of 130W.

Of course, 12 volts is where it's really at. There, PC Power & Cooling's Silencer tops the charts with a whopping 720W rating. The GameXStream and Toughpower PSUs aren't far behind at 680 and 672W, respectively.

Obviously, the higher-wattage PSUs are equipped to deliver more 12V power than the others. What's more interesting here is how each PSU balances the power it can provide across its 3.3, 5, and 12V lines. In a moment, we'll see just how well these PSUs react to loads that push their maximum output ratings.