How many watts can you put on a 20 amp breaker?

The calculation for power is:

W=EI
W=Watts ( or power)
E = Volts
I= Amperes

W= 120 volts times 20 amps= 2400 watts

If the circuit or load will operate more than 3 hours it is considered as a continuous load and the rating of the circuit is lilimited to ,or derated, to 80%or 1920 watts max on a 20 amp circuit.

Thou this is rare for residential wiring, many electricians still use this as a guideline for home wiring as a reasonable safety factor, but is not mandatory.