Residential- 20 amp circuit breakerwith 12ga wire = 20x120 = 2400 volt/amps aka watts. The Nec has a minimum of 3 VA per square foot or 800 sq.ft. of general lighting area per circuit(600 sq.ft. with 15 amp & 14ga wire). Less square footage per circuit is better. This doesn't include receptacles. Residential uses a square footage minimum as opposed to an actual load computation but it is good practice to not go over the load factor 2400 VA.
Commercial- 20 amp circuit breaker with 12ga wire = 20x120 = 2400 volt/amps less 20% safety factor so 2400 - 480 = 1920 volt/amps aka watts. This is based on computation of actual load.
Reduce your load for voltage drop from longer than normal wire runs.