i asked the same question and got something like....
up to 500wrms without big 3
up to 1000wrms with big 3
up to 2000-2500wrms with big 3 and one extra battery
up to 3000wrms+ with big 3, extra battery, and upgraded alternator.
my car has only a 90A alt. How many watts could I get away with, with only big 3 and an extra battery? Could I do 2000wrms daily without killing the alt?