Without going into too much detail for you DUmmies, the wattage is the max that your adapter will put out; i.e., a 20-watt adapter (assuming it fits your jack on your electrical equipment) at 120 volts will supply 1/6 of an amp. 120 X 1/6 = 20 watts.
Just for your own clue, a 20-watt adapter is typically not enough to power anything past a clock radio, if that. The adapter I have for this laptop is in the 50-60 watt range.
But you keep trying.