Bet he didn't do a Manual J. The Manual D calculation would have been easy; 1 duct, 6". I think he should have installed a 7". He can increase the efficiency if he wraps that duct as well. Wonder if his car hesitates whenever the compressor kicks on and the alternator output soars monentarily. Bet that inverter gets a little warm. Wonder if the inverter load was figured in the Manual J.