specking your power supply is very important as i found out, since i fried my 800W as i was pulling ay to many amps on the 5V rail. melting smoke aint funny. as a rule of thumb always asume psu are only 80% efficient. so with a 550W dont go above 440W and obviously pay not on the amp rating of the psu as 550W not the whole power supply but more likely 470W only for the 12V rail.
RIP odin....
A good psu can deliver near 100% to the componentry. The 80% efficiency means it will be drawing 125% from the wall to deliver that 100% to the computer.
i.e. the rating is supposed to be "to the computer". Not "from the wall".
Note this only applies to quality supplies with real ratings.
YumCha psu's might deliver the approximate power of a bowl of fried rice.