How is PSU wattage calculated?

[wrugoin]

Hey Dudes,

I've got a question regarding how wattage is calculated on PSUs. Now I know that the formula is: Watts = Voltage x Amps. However, the new PSU I purchased doesn't seem to go by that formula.

I went with the Sunbeam NUUO SUNNU550-US-BK (http://images10.newegg.com/NeweggImage/productimage/17-709-004-03.JPG)

+3.3V@28A, (92.4w)
+5V@36A, (180w)
+12V1@20A, (240w)
+12V2@18A, (216w)
-12V2@0.8A, (9.6w)
+5VSB@2.5A (12.5w) = Total of 750w

By those amperages stated, I calculated that it should put out 750w, however, I'm pretty sure that the Mfg rates it at 550w for a reason. Does it have something to do with Dual 12V rails? What's the deal?

The reason I bring the issue up is because I'm replacing an Ultra 600w powersupply (http://www.moddin.org.uk/reviews/cactushead/ultra%20600W%20silver%20PSU/ultra%20600w%20silver%20psu%20002_full.jpg). If you click the link, you'll see the way they calculate the 600w rating. They follow the formula W = V x A exactly.

+3.3V@28A, (92.4w)
+5V@28A, (140w)
+12V@29A, (348w)
-3.3V@0.6A (1.98w)
-5V@2A (10w)
-12V@.06A (7.2w) = 599.6w (close enough)

To me, it seems like i'm replacing a 600W PSU with a 750W PSU by the Amperage ratings. However, I'm wondering why SunBeam would rate their PSU lower. Any help would be appreciated.

[toxikneedle]

You just add the 3 rails. 3v 5v and 12v rails. And for your PSU, I guess the 2 12v rails have a only a certain max output.

[Q]

Replace it with a PC Power one of any wattage and never worry again.

Really.

[wnd]

Marketing. What a joke.

This is a bit like saying your computer can encode 30 minute audio-clip into Vorbis is three minutes, decode MPEG4-video with resolution of 720*480 at 50 Hz, and render Battlefield 2 at 60 FPS using 1600*1200 with AA on. I wonder if the number are valid if you try to do everything at the same time. Even if the combined power would be what you get all-at-once, you always lose some power in the transformer. As far as I can see the wattage number only tells how much it takes in (or how much it was designed to take before blowing out), not how much it gives out.

Marketing. Here to rip you off.