often its done in WPG. 1-1.5 wpg low 2-3 med 3+ high but it was made up for florescent bulbs which now many people use power compacts which give out more light using less watts. plus we are now using better and better reflectors. its prob not the answer you want but really it depends. 2 bulb shop light over that 55 should be able to grow some low light stuff like crypts anubias java ferns.