Long story short... It's not that simple as using WPG. But you're closer with the 2.6 WPG estimate.. but the real answer could be significantly lower or higher depending on your setup.
WPG or anything watt-based is a fundamentally stupid way to measure lighting, as it inherently depends on what kind of lighting you are talking about, how efficient it is, and how directional it is. Your plants don't care about how many watts you burn, they care about how many photons hit their leaves in a given period of time.
However, WPG estimations are easy to work with, and can work OK, assuming you are dealing with only one type of lighting.
The WPG rules you are quoting are designed around T12 or T8 bulbs in a "shop light" or similar fixture with white-painted flat reflectors. Assuming you are dealing with this kind of lighting, you can make general estimates of how much light density you are creating per watt consumed.
Your problems trying to apply WPG to your scenario are:
1) CFL bulbs are designed around newer fluorescent technologies than T12 bulbs, and thus produce more photons per watt.
2) your fixture is certainly not a "shop light", so it may be focusing that light more or less efficiently than a shop light does. A 20 watt CFL bulb just hanging bare above the tank sends most of its light out into the room. Put it in a broad white-painted desk light fixture and it will put more into the tank, but probably has a lot of spill... Put it in a polished parabolic reflector, and that light can be focused down into a very small part of your tank. These three scenarios result in very different levels of light hitting your plants.
This is why we measure light in PAR instead of Watts, but that does make things more complicated...
Generalities of PAR vs Watts:
Lighting an Aquarium with PAR instead of Watts
Some par measurements of spiral CFL bulbs (see post 21 in particular, also note that authors idea of what constitutes low/medium/high light are quite different than Hoppy's idea of what those ranges mean.. personally, I think 40-70 PAR for "low light" is absurdly high. Those ranges pretty much require CO2 injection to avoid algae issues)
PAR Data-Spiral Power Saver Bulbs, lighting question
So I should go off the 100 watts and not the 26 watts. Does that mean two 100 watt bulbs equals 200 watts in the aquarium? Thanks for your reply.
No... do NOT go off the "incandescent equivalent"...