Bronze MemberUsername: Subwoofaz_rock
Brisbane, Queensland Australia
Post Number: 37
Gold MemberUsername: Chaunb3400
Huntsville, Alabama U.S.
Post Number: 8177
Silver MemberUsername: Van_man
Boston South, MA
Post Number: 773
Gold MemberUsername: Basshead86
Google is your Friend, FL
Post Number: 4035
your formula shows that the amp is 100% efficient...which none are.
a very good generic way to go by is in Chauncey's link.
multiply your 20 amps by 1.4 if it is a Class a/b amp
or multiply your 20 amps by 1.2 if it is a Class d amp.