I am putting together my own computer and was wondering how to determine how much power the end product will need. I see that when buying a Processor that in it's specifications the power needed will be stated, but on some other products I was wondering if this was the same(video card, HD, Motherboard, DVD-Drive, etc).
If you are building a gaming/video/3D rig, the starting point will be 600watts primarily due to the gaming videocards being such an amp hog (requiring over 40amps peak power on a single 12v rail).
Make sure that the 12v rails can share amps (not all allow it, especially the cheaper PSUs that have a set limit on each rail). Respected PSUs are PC Power and Cooling and Seasonic, they are means tested to withstand extreme power usage and offer more watts than advertized (for cold boots). Too often PSU manufacturers list their power supplies as 400/600/800 but really they're like 350/550/750 and have no peak power. It's an advertizing gimmick.
Due to the trend of replacing 80mm fans for quieter 120mm fans that require more wattage, also make sure the 5v rail can withstand plugging in 7x120mm @ 5watts fans (like it's possible with the Centurion case). Plus, also enough wattage left over for installing hard drives; powering the videocard (for older models); DVD, etc.. To give you an example, this Seasonic SS-600HT PSU is rated at 30A for the 5v rail with 180watt max between it and the +3.3v. 7x5=35watts just for the serious air cooling alone.
Processors take anywhere from around 110watts (PresHOTTS and Smithfields), to around 90watts for Williamette/Northwoods/Wolfdales.
New hard SATA 3.0g drives now take about 15watts to power them (Raptors take more). A SATA 3.0g RAID-0 setup will take around 30watts, plus some extra for seeks.
Motherboards, depending on features and phase, takes around 40watts for itself.
DVD drives will take anywhere from 25 to 35watts.
Addon cards have to be rated individually to your specs.
So, when you add up all the watts and amps required, a new rig will need 600watt PSU even if you're going to use only 400, because of the videocard requiring high amps and the added usage of the 5v rail.
Another thing to consider is the usage of all these USB devices, and the power requirement to wake-on LAN (it uses the +5Vsb rail).
Always better to get a more wattage than less, and usually the more powerful PSUs are the more efficient they are, too. Seasonics can come with a 89 energy efficiency rating (highest in the industry). That also means it runs cooler as less heat is wasted. A 120mm 800rpm fan is more than enough to keep the PSU cool even in an oven of a room.
One more thing: get a PSU with active (not passive) PFC. It'll maintain the power level within the case and eliminate hardware damaging spikes constantly, instead of "on demand" (which maybe too late, much like GFI circuits may not save you from electrocution).