A 20W bulb over a 20 gallon tank will give you exactly 1wpg. I don't know about the effectiveness of bulbs other than fluorescent for planted tanks so I can't help you with that. But a 20W fluorescent bulb for a 20 gallon tank would be OK I suppose, if the plants are low-light and slow-growing. Though on Plant Geek I have read that the smaller the tank, the more wpg's it actually needs.
Oh, and if the bulb you're getting is incandescent, be careful not to overheat your tank with it.