How much power does an AC compressor use?

The AC compressor runs off of a 30 amp fuse. Belt driven is probably going to be smartest here. It might take 5 hp to turn a compressor and only 1-2 to spin an alternator to generate the juice you need, but if you need power, just turn off the A/C.

How many Btu should a car AC compressor be?

Another thing to consider is a lot of the generic "hot rod" style compressors are only 5000 BTU, whereas a OEM electric compressor is way more. Most car ac systems need to be 20000 BTU or higher due to overcoming the green house effect and to actually cool the car in a reasonable amount of time when you get in it.

Why are electric car compressors so oversized?

They are way oversized for the space they condition vs what you'd put in a house, because unlike a house system, it needs to cool quickly and won't run all the time. Most hybrid or electric car compressors factory now run off the high voltage system which is as stated 300 to 900 volts, three phase ac voltage.