IMHO, I'm having a very hard time understanding why the freshwater community is still so bent on the "WPG" watts-per-gallon rule.
Not all light is created equal and thus, not all watts are equally useful to our inhabitants. The saltwater reef guys picked up on this several years ago but the freshwater community still seems to stick with the old WPG rules.
As an example:
I have 60 watts (4x15W T8) of normal output fluorescent lighting over my 22G tank. Each of the 15W bulbs has a lumen output of ~600 lumens for a total of 2,400 lumens. These are mounted two each in the standard two bulb all-glass/Perfecto style lighting hoods that we are all familiar with. Suffice it to say, these fixtures do not exactly act as a well-engineered reflector by any means so the amount of lumens actually penetrating the water is reduced substantially.
A 55W PC fluorescent bulb (much like those 55W kits from AHSupply) is actually 5 watts less in output than my set-up described above. Despite the lower wattage output, however, the bulb has a lumen rating of ~4,000 lumens... approx. 1.6 times the lumen output of the above set-up. Add to that a good quality parabolic reflector that punches most of the light to the water and you are actually getting much higher lumens into the tank but with less watts (and thus less WPG).
Now lets look at a 24" length 24W HO T5 fluorescent bulb. Each 24W HO T5 bulb has ~2,000 lumens of output, meaning that with only 48 watts of output, we are getting the same 4,000 lumens as we would get from the 55W PC bulb. The big difference here is that each T5 lamp has it's own parabolic reflector. This individual lamp reflector, combined with the smaller diameter of the T5 bulb (smaller diameter = more efficient light (more like a point source... light travels farther from smaller point source)), creates a situation where there is a very high amount of available lumens making it into the tanks.
Now, an even better way of measuring the usefulness is by measuring the PAR (photosynthetically active radiation) value the light emits. PAR is what plants (and the symbiotic photosynthetic algae in corals in the saltwater world) use to do their "job." HO T5 fluorescent lighting, when used with a high-quality engineered parabolic reflector, is absolutely and unquestionably the highest efficiency lighting available, providing the greatest amount of PAR to the surface fo the gravelk bed (or sandbed in reef aquaria). Based upon measurements taken at the sandbed surface, a single 54W T5 lamp has been shown to put out more PAR than a 150W double-ended MH lamp.
Sorry for the rant... I'm just surprised that so many in the freshwater circles still stick to the WPG rule. We abandoned that rule close to 10 years ago in the reef aquaria world.
-Jimbob