I have a 20 gallon, 24" long tank that I simply added another 15W bulb to, bringing it up to 30W or 1.5 WPG. This is not great, but I successfully grow crypts, sagittaria, and vallis. I also have a fairly new Echinodorus ozelot which is growing, although slowly. It will look amazing when a bit bigger.
If I were doing it over again, I would have added a 36W power compact instead of another 15W. That would bring me up to 51W or about 2.5WPG. I have another tank with about 2.4 WPG and the plants in the are doing amazingly well. (Of course having a substrate of soil + gravel helps as well.)
Of course, you could just not use your current 15W light, and just get the 55W power compact. That would be better for the plants, as it's longer and would reach the ends of the tank better.
However, in favor of the two-bulb system, I find that it's nice to have two different tubes on staggered timers. That way I can have at least some lights on for much more time (8:30am til 8:30pm) so I can see the fish for longer. Also I wonder if it's a bit easier on the fish: they don't get hit by all the light coming on at once, they get some of it, and the rest comes on later. It also goes off in two steps.
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