I have a 4x65w light in my 55 gallon (4.7WPG). Without water, I get 3300 lux at 19 inches where the gravel is.
I have a 27 watt home depot light that's popular here for nano on a 2.5g. That should be 10.8WPG, however when I measure the lux at the gravel level (12 inches), I get 1200 lux.
I can get almost the same lighting in my nano as my big tank if I measure the light 6-7 inches from the bulb. If you had the light resting AT the water level, you could get almost comparable lux ratings, for double the watts per gallon on a 2.5g.
For low carpets in a nano, I would even possibly consider BIGGER than the 27 watter, especially if you're going to have the light a reasonable distance away from the water (So you can get access, etc)
For comparison, today, at mid day, it's overcast, cannot see the sun, and it's winter, and I'm measuring 24,000 lux at the ground outside.
The lux that hits the plants is really what matters. Watts per gallon, etc, are just rough estimative guides for lack of measuring equipment. PM me if you want me to measure anything else around the house.