Seasonic is quickly making a name for themselves as one of the highest efficiency active PFC power supply manufacturers out there. The common misconception is that a higher wattage PSU is always better than a lower one. In reality, a quality 300W model is enough for most mid range systems and a solid 400W is sufficient even for high end rigs. The reason why some 500W and even 600W still fall behind is because they use low quality components, combined rails, and low amp ratings that don't come anywhere near stable 500 or 600W power delivery. In the PSU realm, quality > quantity.
To that end Seasonic created the S12 (Silent 120mm fan) series. This is one of their first models that aims to combine near silent operation (thanks to an exponentially load controlled, low spinning 120mm fan), sustained power delivery, an environmentally friendly 0.99PF rating, and on top of all that up to 80% efficiency. Now we've seen passive-PFC power supplies go above and beyond the 80% efficiency mark (Antec's Phantom comes to mind), but active PFC is a lot cleaner and may, in the long run, reduce overall power costs.
Seasonic provides a very complete kit with the S12. Aside from the power supply and obligatory AC cable, you also get a 24 to 20-pin ATX adapter, 3-pin fan adapter, installation screws, Seasonic sticker, and their Dr. Cable management system. It's nice to see manufacturers include other useful add-ons with their models. This is just one aspect that sets the company apart from the rest.
From a design standpoint, the main box does pretty much everything right: there's a 120mm slow spinning, low noise intake fan, meaning good airflow (and cooling) while being almost silent. As mentioned earlier it's also load controlled (rather than temperature controlled), but the speed increase is exponential rather than linear. That is, it spins at the same rate in a certain load range (up to 40% according to their diagram) before spinning up to its maximum speed at 100% load. According to Seasonic it can also adapt to ambient temperature, so the higher the ambient temperature the faster it'll spin at full load (though it still spins at the same rate up to 40% load regardless of ambient).
There are a few ventilation holes in the front and right side, but most of the airflow goes from the fan and out the back. The rear uses a honeycomb mesh, the most air-efficient design possible (circular and grid meshes are both structurally weaker and don't let as much air through). Here you'll also find the AC plug and a power switch. No voltage selector is present and none is needed because the active PFC takes care of that.
Looking at the sticker on the side we see 30A on the 3.3V line, 30A on the 5V line, 14A on the first 12V line, and 15A on the second 12V line. There are a few points of interest here. First off, the dual 12V rails are a nice feature when it comes to stability. They can provide a total of 348 watts. The 3.3V line can supposedly put out 99 watts while the 5V line can put out 150 watts. Seasonic claims dedicated voltage lines for all the rails, but there's still a combined maximum rating of 150 watts on both of these lines.
