There is no one-size-fits-all answer to this question, as the best way to use an extension cord for an air conditioner depends on the type of AC unit and the gauge of the cord. Some general tips include using a heavy-duty AC extension cord with a low gauge (12 to 14) and high amps, and avoiding using cords with electric ratings that are lower than the amperage of your AC unit. If you are not an expert in electrical wiring, do not try to figure out the AC ratings on your own.