Power per harddrive

Ok I just won 12 47 gig harddrives on ebay. Basically I am building my own box to put them in and will probably just run the cable from my computer to my rack of harddrives. I am going to have to get an independent power supply for them. So I was wondering how much power a harddrive really uses. They are 12 5400rpm scsi drives. They ebay site listed them as using 22 watts when reading or writing, but my external hd enclosure uses 50watts. So in reality how much per harddrive should I be looking at? because only having to use 264 watts to power them seems a little off.
Duck

The actual usage varies from model to model. What is the brand and model of those hard drives, and have you tried looking for a spec sheet on the manufacturer's web site?

The amperage can be found on the HD's I believe.