I agree, that is sufficient light intensity (with a good full spectrum or daylight tube as you say you have) for the majority of plants in a 15g tank.
Just so you know, watts is not a reliable guide any longer, as so many new types of fluorescent lighting tubes are now being developed that are energy efficient. Watts is just the measurement of energy that it takes for the tube or bulb to produce the light, so a better-made more energy-efficient tube can use fewer watts but still produce comparable light to another tube. As an example, a Compact Fluorescent Bulb of 10w will produce light intensity comparable to the old 25w incandescent bulb. So you can see how "watts per gallon" and such becomes rather meaningless, unless you know exactly what the tube/bulb emits in terms of actual light.
And longer duration does not compensate for less intensity (brightness), just so you know. Plants grow by photosynthesis, and to do this they need sufficient light (intensity) along with 17 nutrients. With respect to just the light, if it is too weak for them to photosynthesize, they cannot, no matter what the duration. But algae will take advantage as it can use any light. Assuming the intensity is sufficient (as yours is) the duration should always be in balance with available nutrients, and light should be the limiting factor to avoid algae issues.
And, welcome to Tropical Fish Keeping forum.