You would probably be better off making sure that your alternator is a 90 amp
one before you get into this.  If it isn't I can get one for you for $50 as I
have offered before.  I just can't believe that the energy transferred from an
engine, to an alternator, to an inverter to an electric motor to an A/C
compressor is more effecient than energy transferred from an engine, to an A/C
compressor.  Could there be something in your numbers that you are
overlooking?  Sounds like perpetual motion to me.  Of course I could be wrong.
Don't wish to be a wet blanket.
Ken Wilford
John 3:16