How is it possible for a 20 watt bulb to have enough power for high light aquatic plant?

It is a matter of how long that light is on, how far it is away from the leaves of the plant, and how much area your tank has (where the plants are planted. A 20 watt bulb burning 16 hours a day could well be enough to grow plants....trial and error at this point. Too much light and you will have an alga mess.