I see so often people talking about wattages and amps and PSUs and needing 5404230420 watt PSUs to supply their computer with enough power. This isn't meant to be comprehensive (including PSU efficiency, design factors, brand-names, etc, would take way too long, but I thought I could shed a little light to debunking some of the common perceptions.
Let's start with something simple.
To convert Amps to Watts:
Watts = Amps x Volts
This is why--when assessing which PSU to get-- it's important to look at the 3.3v, 5.5v, and 12v "Amps". The 12v rail is arguably the most important since your CPU, Video card, hard drives, optical drives, etc, draw power from it.
Here are some max-power draws expressed as Watts:
AMD X2 4200+: 89 watts MAX
Dual 7800GT in SLI: 112 watts MAX
Two Raptor 74GB in RAID 0: 16 watts max
Extra Seagate 7200.9: 12 watts max
TOTAL: 229 watts
Obviously, this doesn't include some other minor things, but it is more than ample to give you an idea of the actual wattages involved here. Also, let's keep in mind this is about maximum PEAK LOAD on every component. This system would draw significantly less during normal non-gaming use.
Based on the formula I stated above, that means an AMD X2 4200+ expressed as "MAX AMPS REQUIRED" on the 12v rail would be:
AMD X2 IN AMPS|
89 watts / 12 volts = 7.41 amps
Single Geforce 7800GT IN AMPS|
56 watts / 12 volts = 4.66 amps
You can factor in 2 amps for the three hard drives. 1 amp for cooling. These are 12v rail items as well.
This puts our test system at ~20 amps (rounding up).
Now let's go ahead and assume 10% overhead. Just to build in a ton of extra breating room. That puts us at ~22 Amps or ~260 Watts for our main hardware. Need another hard drive? Add an amp or 10 watts. Going to add another CPU? 90 watts or 8 AMPS.
So, let's back track a bit and think about this. If our test system has nice overhead @ 22 amps, with 3 hard drives, an AMD X2 and 7800GT SLI setup.... what PSU can we get?
http://www.newegg.com/Product/Product.a ... 6817104154
OCZ Powerstream 450 watt: 12v RAIL? 26 AMPS (that's enough)
http://www.newegg.com/Product/Product.a ... 6817192002
Atrix 500 watt: 12v RAIL? 15 AMPS (no way... nowhere near enough).
So, yeah, Atrix has a "500 watt" power supply, and it's cheap... but it's completely insufficient for our system. There is no way the configuration would run properly on that "500 watt" PSU. The "450 watt" OCZ powerstream, however--SOMEWHAT against conventional wisdom that you need 500 watt or 600 watt PSU to do SLI--would readily run our test configuration.
Here are some other common wattages:
Radeon X850XT = 45 watts @ idle / 68 watts @ peak load
Geforce 7800GT = 39 watts @ idle / 56 watts @ peak load
Radeon X1900XTX = 49 watts @ idle / 120 watts @ peak load
Geforce 7800GTX = 53 watts @ idle / 94 watts @ peak load.
And AMD X2 Manchester (3800+, 4200+) run @ MAX 90 watts. The Toledo cores (4800+, 4400+) run @ MAX 110 watts.
A Raptor 74GB draws 8 watts.
A Seagate 7200.9 draws 12 watts.