If you're looking at DC power, each blade needs around 400W and the controller and fans burn another 60 or so.

The chips cache work and cgminer updates it, so if cgminer stops sending work the chips will keep hashing on the last thing they were given. Even if they don't, there's still some power being burned in there while the chips idle (maybe on the order of 1W per chip), and if it's not allowed to escape things will get toasty. One guy on here has reported losing a couple blades due to overheats from that very problem.