Question: how much can 20 amps power?
Answer: So, to rephrase your question, how many watts can a circuit deliver before it's overloaded? Most modern residential circuits are 15 or 20 amps, so we're looking at a max load of either (15A x 120V =) 1800 watts or (20A x 120V =) 2400 watts before the breaker trips. The breaker will be labeled either 15 or 20.