How is a Butterfly Labs Bitforce Single powered?
It uses ~70W @ 12V of external power. The stock power brick is cheaply made, giant, and inefficient (PF=.5,efficiency~.8). Fortunately, the power brick can run on 240V.  Many people make their own cables to use an ATX or bench PSU (with higher powerfactor and efficiency)

source: https://bitcointalk.org/index.php?topic=67819.msg796852#msg796852