Watts is not a usable guide, as I'll explain in a moment. To your 10w CFL (compact fluorescent) bulbs, I would get higher wattage. I have two 10w GE daylight CFLs over my 10g and 20g tanks, and they are fine. I have a 29g tank, and I doubt two 10w will provide sufficient intensity. I know there are 13w CFLs, but I would suggest the next higher wattage, whatever it might be.
Now to explain watts. This is simply the measurement used for the amount of energy (= electricity) needed by a bulb/tube to produce the light it emits. It has no direct relation to intensity, except when comparing identical types. As in the above, a 13w GE daylight is obviously more intense (brighter) than a 10w. But that is as far as comparisons can go.
The light needed over a given aquarium varies due to depth, length, width. The type of plants impacts this, since some will do well on less light than others. There is no guide such as the sometimes-seen "watts per gallon" that is reliable, since with all the different types of bulbs and tubes today, intensity can be the same with so many different wattages depending upon the way the tube/bulb is manufactured. For instance, a 48-inch fluorescent tube in the regular T8 used to always be 40w, but many manufacturers now make a 32w tube that outputs exactly the same light intensity. The 32w is simply made differently so it uses less energy to emit the same light. But again, a different type of tube can be much different at the same wattage. A 32w Daylight 6500K tube will emit more intense (brighter) light than a 32w "Aquarium" tube, which will be about half the intensity. So watts is rather meaningless overall.