current needed for 800w car amplifier?

I'm trying to hook up some car subs to my home theater. I have some 120V AC to 12V DC converters lying around. The current output on some of the converters are adjustable so I just need to know the current output from the converter that is necessary to run an 800W car amplifier. I understand that I=P/E, but I'm not sure how to apply the numbers in this situation. I need the current necessary from the converter to the amplifier, not from the amplifier to the speakers.

It would help if we knew the brand of the amp. How many amps is the power supply rated at?  You will need at least a 35 amp supply, and this may not be enough to get max power from the amp. 

Its a Boss SE550 400Wx2 @ 2 ohms. I can't find the current rating online and I don't have the amp with me to see if its on there anywhere.

Whatever the fuses in the end of the amp total. If you have two 20A fuses, (pretty likely, as it is a Boss amp, and SPECTACULARLY over-rated) then you would need 40A worth of current. If not planning on running it WFO, ever, then you would simply buy as big a power supply as your pocketbook can support.It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

i am an idiot wrote: How many amps is the power supply rated at?

35 and above amp power supplies are not the norm.