We need to install a heating/cooling split system inverter (8.0w/9.0w by memory).
We have an exsiting 16amp isolator for the a/c unit and the label on the unit says heating/cooling requires 11amp. However the instruction manual says 30amp fuse is required.
We've spoken to 2 electricians, one says that the 16amp fuse is sufficient while the other one says that we need more.

Any suggestions.. Thanks!

You go by circuit ampacity for the device or unit you are wiring. If it calls for a 30 amp circuit then usually number 10 copper and a 30 amp breaker along with a proper size inverter for the unit.