Perhaps 'correct', is not the right term to use. I would suggest the lamp with the most UV-A energy that you can afford.
Black light lamps will output more UV-A energy than almost any other fluorescent lamp.
8 watts is much less than 40 watts. 40 watts is the electrical energy in, not out.
I showed a picture of a fluorescent exposure unit in
http://www.t-shirtforums.com/screen-...html#post95549 