to be honest the watt's per XYZ rule means naff all.
What matters is the area the light covers, the depth between the surface and the plants and also the intensity of the lap (lumens, or PAR idealy but not easy to find out).
you can have a really high wattage lamp but the shape of it and the depth of water means you cannot grow most plants, and vice versa you could potentially have a low output lamp but directed over the plants which need it and in a long yet shallow tank and have a high light plant thrive.
My honest opinion is you need to decide what you would like to grow first then look at lighting.
If you would like a whole tank full of demanding plants you need to start looking at a 4 bulb T5HO fixture that covers the full length of the tank.
If you fancy the odd bit of java fern on wood you could get away with your current one, possibly....
If you fancy just growing one nice plant in a set area why not get a 25w grow bulb and put it in an angle poise lamp pointed at the area you wish to grow it????
The possibilities are endless but you need to have some sort of plan.
Used to try and keep track of my fish here.....
Ran out of room and time!!!
Instead I'll tell you the best piece of fishkeeping equipment ever....... Algae Scrubber :)