I picked a 'standard' CFL grow log at random as an example below. (Post your own or a grower you like and we shall examine it.) This grower is using over 30 watts per plant, but notice how only about 3% (I am guessing) of the light is actually hitting the leaves.

Attachment 280040

The cone of light on my plant is just slightly larger than the width of the leaves so the utilization is probably close to 75-80%. Again a guess.