How many watts on a 20 amp circuit

If it's a 120 volt circuit, the number of watts is

Watts\,=\,Volts\,x\,Amps\,=\,120*20 = 2400

If it's a 240 volt circuit, it's

Watts\,=\,Volts\,x\,Amps\,=\,240*20 = 4800

Of course, those are maximums and, strictly speaking, it only applies to resistive loads.

If a continious load(more than 3 hours) derate to %80. 2400 X .8 = 1920 Watts(16 Amps)

You have to remember, the definition of a continuous load is a load that is expected to be on for three hours or more, NOT a load that is possible to be on for three hours or more.