power invertor?? just figure out haw many amps it will draw from the alternator first. if you take the number of watts it takes to run the ac and divide that number by 12 you will get a ballpark figure of the amps the alternator must provide. if it's higher than 30 amps you might need to upgrade the alternator. i would wire the invertor directly from the battery with decent sized wire and put in a relay wired to the ignition switch. if you give me the specs on the ac unit you want to use i could draw up a wiring diagram.