Lighting requirements

I'm curious about the "2 watts per gallon" rule.

First, different lighting technologies produce significantly different
amounts of lights (lumens) per watt. So how can there be a simple rule like
"2 watts/ gallon"???

Right now I have 3 x 55 watt CF on a 75 gallon tank. This works out to the
"2 watts/gallon" rule, and in practice the plants grow quite well (what the
Tiger Oscars don't dig up).

I'm setting up a 38 gallon tank for my daughter's classroom. I have some 20
watt flourescent bulbs (the kind that comes with tank setup). I'm interested
in how these compare with CF lights in terms of lumens/watt and how the "2
watts/gallon" rule applies.

/harry