Antman,
It depends on how close the light source is to the alga. I think 13 might cut it if it is close but it is certainly less than optimal. Also, the advertising claim that a 13w pc lamp puts out the light of a 65w incandescent lamp doesn't mean anything for photosynthetic organisms. It is still a 13w lamp. I use a 45w screw-in PC lamp in a dome-shaped "shop light" fixture (both availble at wal-mart or a hardware store) on my 'fuge. I have pics in my gallery.
Kevin