11 watts over 10g
I have a 10 US gallon tank lit by an 11-watt flourescent tube (exactly the same kind as in my desk lamp) with white pea-gravel and am able to grow red ludwigia, egeria, pygmy chain sword and vallis until there's no space left for fish to swim. Just removed 80% of the plant mass and added some Indian fern, which is also doing well. The lamp has no reflecter, just a white metal casing behind it, and is protected from water by a plastic cover, which must reduce its impact a little, especially beween cleanings.
CO2 is from Hagen, but probably not very effective because 10 times an hour all the water in the tank passes through the filter in the canopy and drops back into the tank via a spray-bar. Also, I don't refresh the Hagen mixture often enough.
My water has a PH of 8.2 before CO2 added. I also use Tetra Easybalance which adds Iron and other micronutrients for plants.
Tank was set up in the summer when my tap water contained 50-100ppm Nitrates. I try to remove Nitrates and phosphates as much as possible to control algae, last time I measured phosphate was 0 and Nitrates 20 ppm.
Plant growth slowed down a lot after first couple of months, which I'm happy about - don't want to be pruning all the time.
My main point is I'm skeptical about the wpg rules. I have much more recently set up a 1 wpg 68 US gallon tank which has every stem plant I could think of growing to the surface, including Rotala Macranda. (To be fair, I planted stems with lots of spacing between them, and anything shaded by the surface stuff stops growing. Three quarters of the surface is covered by plants at the moment.) This tank has eco-complete substrate and pressurised CO2.
The 10 gall. is helped by probably an hour a day of feeble sunlight, once every two weeks when London skies are not completely overcast...