10 watts because typical aquarium lights are fluorescent, but that's only power consumption, not intensity. Typical housould compact fluorescent can generate more light than an incandescent bulb with the same amount of power, so in your example, the CF bulb can generate 5 times more light than the same 10 watt incandescent equivalent, that's all.
This gets a lot more complicated because it really depends on the kind of bulb, T8s, T12s and T5s have different properties being all fluorescent; the most accurate way to measure light inside the tank is by using a PAR meter, the "watts per gallon" measure is totally deceiving, you can measure light inside your tank by sticking your finger in the water and have similar scientific background.