There is some power source in one type of alien base module, the one with a long coridor at level 0, a lift, a little room on level 1 with a big room on each of its side. The bottom-left room usualy have 2 power source.
and in OpenXCom, there is most of the time an alin engeneer with a blaster -> don't remenber having an alien with blaster each time in this room.